Fulminating Compounds

Medieval alchemists knew that gold could be dissolved in a mixture of nitric and hydrochloric acids (known as aqua regia). One of the goals of alchemy, after all, was the production of gold from cheaper materials, and divining the properties of gold was an obvious first step. They also knew that a basic substance like lye or ammonia could neutralize acids. Thus, it was almost inevitable that one of them would eventually dissolve gold in aqua regia, and then try to neutralize the acid with ammonia or potassium carbonate.


Imagine their surprise when it exploded in their faces.

In 1585, an alchemist in Germany named Sebald Schwaertzer wrote out a formula for making what was then known as aurum fulminans, a compound we today call fulminating gold. It calls for adding eight parts of gold to concentrated nitric acid, warming it in a bath of sand, then adding 8 parts ammonium chloride. This dissolves the gold (the nitric acid allows the chloride ions to combine with the gold to make hydrogen tetrachloroaurate(III)). By making the aqua regia with ammonium chloride instead of hydrogen chloride, the resulting liquid contained quite a few ammonium ions.

The recipe then calls for adding a mixture of iron and copper sulfate solutions to the mix, to get a precipitate, which is filtered out, washed, and concentrated by evaporation into a thick solution. Finally, concentrate potassium carbonate (a strong alkali) is added carefully, so that it does not cause heavy foaming and overflow, or worse, explode. The precipitate is purple crystals of fulminating gold. The recipe says the whole process takes 4 days, but no more than 5 days.

The word fulminate means to explode violently (it comes from the Latin word for lightning). First applied to this compound of gold, it was later used to describe other compounds that exploded violently, such as compounds of silver and mercury. The latter compound gives name to a class of compounds called fulminates. Fulminating gold and fulminating silver are not fulminates — they just fulminate.

Chemists tried to analyze fulminating gold for centuries. It seems that no two batches of it are quite the same, and even in a single batch, there are compounds with different properties. The conclusion at this time is that each gold atom is connected to a nitrogen atom, and that oxygen atoms and chlorine atoms connect these into a non-crystalline large three-dimensional polymer.

The explosive nature of fulminating gold was noteworthy and chemists experimented with it, but it was clearly too expensive to use in weapons or mining. However, it remains the first high-explosive compound discovered. A high explosive is a chemical compound where the oxidizer and the fuel are combined in the same molecule. You may recall from the discussion of the manufacture of gunpowder that many of the improvements were due to the finer and finer mixing of the nitrates with the sulfur and charcoal. The closer the fuel is to the oxidizer, the faster the reaction can happen.

Black powder merely burns if it is not contained. In high explosives, the reaction takes place faster than the speed of sound. This means that the air cannot move out of the way fast enough to let the exploding gases out, and the air itself (and the inertia of the initial ingredients) acts like a container. We hear a sharp report as the material disintegrates into gas so fast that it generates very high pressures.

In 1786, the prolific French chemist Clause-Louis Berthollet added ammonia to precipitated silver oxide and produced fulminating silver. It was highly sensitive, exploding easily by touch or slight heating. It was also quite powerful, and, like fulminating gold, it was a high explosive.

Berthollet is known for many chemical discoveries, demonstrating the bleaching effects of chlorine gas, creating sodium hypochlorite (the bleaching agent in Clorox bleach), and (of particular interest here) producing potassium chlorate, a strong oxidizing agent that became known as Berthollet's Salt.

In 1789, another French chemist (and contemporary of Berthollet) Antoine Francois, comte de Fourcroy, described another fulminating mixture, called fulminating powder. It was a mixture of three parts potassium nitrate, two parts potassium carbonate, and one part sulfur. The ingredients were ground together in a hot marble mortar with a wooden pestle, and the powder then placed on a metal ladle and warmed until the sulfur melted. After a short while, the mixture then exploded, denting or perforating the iron ladle. In further experiments, he notes that a mixture of one part potassium sulfide to two parts potassium nitrate "fulminates more rapidly", leading him to conclude that the first mixture created potassium sulfide shortly before detonating.

Fourcroy also described another fulminating mixture, made by replacing the potassium nitrate in gunpowder with Berthollet's new discovery, potassium chlorate. This new mixture would explode when hit with a hammer.

In 1797, Pierre Bayen published Volume I of his Opuscules Chimiques, where he describes fulminating mixtures of mercury compounds with sulfur, that explode when heated.

Around this time, Luigi Brugnatelli (who later would become famous for gold electroplating) experimented with nitrates of gold, silver, and mercury, and noted that compressing them with sulfur or phosphorus could produce detonations. Belgian chemist Jean-Baptiste Van Mons also worked with oxides of noble metals combined with sulfur or phosphorous to make explosive mixtures, and notes that such mixtures produce a more uniform explosive effect than the mixtures containing potassium chlorate.

Edward Charles Howard was the son of a Catholic wine merchant in Sheffield, England. Feeling that he could not get a proper Catholic education in Protestant England, at the age of nine, he was sent to study in France, at the Catholic English College in Douay. In 1788, when he reached the age of fourteen, he returned to England, despite having finished only half of his schooling. His father had died six months earlier, and the troubles that were about to lead to the French Revolution were already apparent. Over the next twelve years, he became a highly skilled chemist, and was elected to be a Fellow of the Royal Society. His election was supported by his third cousin, the 11th Duke of Norfolk, and several other prominent scientists, including Peter Woulfe, who had discovered picric acid a few years earlier.

In 1800, at the age of 26, Howard published an account of his experiments with fulminating mercury in the Philosophical Transactions of the Royal Society. He describes what we now call mercury fulminate this way:

The mercurial preparations which fulminate, when mixed with sulphur, and gradually exposed to a gentle heat, are well known to chemists: they were discovered, and have been fully described, by Mr. Bayen.

MM. Brugnatelli and Van Mons have likewise produced fulminations by concussion, as well with nitrate of mercury and phosphorus, as with phosphorus and most other nitrates. Cinnabar likewise is amongst the substances which, according to MM. Fourcroy and Vauquelin, detonate by concussion with oxymuriate of potash [potassium chlorate].

But mercury and most if not all its oxides, may, by treatment with nitric acid and alcohol, be converted into a whitish crystallized powder possessing all the inflammable properties of gunpowder, as well as many peculiar to itself.

Mercury fulminate

He describes the procedure for making it — putting red oxide of mercury into alcohol, and then adding nitric acid. The oxide gradually dissolved, and then the mixture boiled, producing dense white smoke, and a dark precipitate that eventually turned white. He filtered the white crystals and dried them. In testing these crystals, he poured sulfuric acid on them, and was quite surprised at the resulting explosion:

I therefore, for obvious reasons, poured sulphuric acid upon the dried crystalline mass, when a violent effervescence ensued, and, to my great astonishment, an explosion took place.

He then performed more tests:

I first attempted to make the mercurial powder fulminate by concussion; and for that purpose laid about a grain of it upon a cold anvil, and struck it with a hammer, likewise cold: it detonated slightly, not being, as I suppose, struck with a flat blow; for, upon using 3 or 4 grains, a very stunning disagreeable noise was produced, and the faces both of the hammer and the anvil were much indented.

Half a grain or a grain, if quite dry, is as much as ought to be used on such an occasion.

The shock of an electrical battery, sent through 5 or 6 grains of the powder, produces a very similar effect: it seems indeed, that a strong electrical shock, generally acts on fulminating substances like the blow of a hammer. Messrs, Fourcroy and Vauquelin found this to be the case with all their mixtures of oxymuriate of potash.

To ascertain at what temperature the mercurial powder explodes, 2 or 3 grains of it were floated on oil, in a capsule of leaf tin; the bulb of a Fahrenheit's thermometer was made just to touch the surface of the oil, which was then gradually heated till the powder exploded, as the mercury of the thermometer reached the 368th degree.

The next tests were made to test the new crystals in the same way gunpowder was normally tested:

Desirous of comparing the strength of the mercurial compound with that of gunpowder, I made the following experiment, in the presence of my friend Mr. Abernethy.

Finding that the powder could be fired by flint and steel, without a disagreeable noise, a common gunpowder proof, capable of containing eleven grains of fine gunpowder, was filled with it, and fired in the usual way: the report was sharp, but not loud. The person who held the instrument in his hand felt no recoil; but the explosion laid open the upper part of the barrel, nearly from the touch-hole to the muzzle, and struck off the hand of the register, the surface of which was evenly indented, to the depth of 0,1 of an inch, as if it had received the impression of a punch.

The instrument used in this experiment being familiarly known, it is therefore scarcely necessary to describe it ; suffice it to say, that it was of brass, mounted with a spring register, the moveable hand of which closed up the muzzle, to receive and graduate the violence of the explosion. The barrel was half an inch in caliber, and nearly half an inch thick, except where a spring of the lock impaired half its thickness.

At this point, it was time to test the new explosive in a real gun:

A gun belonging to Mr. Keir, an ingenious artist of Camdentown, was next charged with 17 grains of the mercurial powder, and a leaden bullet, A block of wood was placed at about eight yards from the muzzle, to receive the ball, and the gun was fired by a fuse. No recoil seemed to have taken place; as the barrel was not moved from its position, although it was in no ways confined. The report was feeble : the bullet, Mr. Keir conceived, from the impression made upon the wood, had been projected with about half the force it would have been by an ordinary charge, or 68 grains, of the best gunpowder. We therefore recharged the gun with 34 grains of the mercurial powder; and, as the great strength of the piece removed any apprehension of danger, Mr. Keir fired it from his shoulder, aiming at the same block of wood. The report was like the first in section IV, sharp, but not louder than might have been expected from a charge of gunpowder. Fortunately, Mr. Keir was not hurt, but the gun was burst in an extraordinary manner.

The breech was what is called a patent one, of the best forged iron, consisting of a chamber 0,4 of an inch thick all round, and 0,4 of an inch in caliber; it was torn open and flawed in many directions, and the gold touch-hole driven out. The barrel, into which the breech was screwed, was 0,5 of an inch thick; it was split by a single crack three inches long, but this did not appear to me to be the immediate effect of the explosion.

I think the screw of the breech, being suddenly enlarged, acted as a wedge upon the barrel. The ball missed the block of wood, and struck against a wall, which had already been the receptacle of so many bullets, that we could not satisfy ourselves about the impression made by this last.

As it was pretty plain that no gun could confine a quantity of the mercurial powder sufficient to project a bullet, with a greater force than an ordinary charge of gunpowder I determined to try its comparative strength in another way.

I procured two blocks of wood, very nearly of the same size and strength, and bored them with the same instrument to the same depth. The one was charged with half an ounce of the best Dartford gunpowder, and the other with half an ounce of the mercurial powder; both were alike buried in sand, and fired by a train communicating with the powders by a small touch-hole.

The block containing the gunpowder was simply split into three pieces: that charged with the mercurial powder was burst in every direction, and the parts immediately contiguous to the powder were absolutely pounded, yet the whole hung together, whereas the block split by the gunpowder had its parts fairly separated. The sand surrounding the gunpowder was undoubtedly most disturbed: in short, the mercurial powder appeared to have acted with the greatest energy, but only within certain limits.

The reason the gun was damaged is that mercury fulminate is a high explosive. Unlike gunpowder, it detonates very rapidly, producing enormous pressures without producing the large quantities of hot gas that gunpowder needs in order to produce the same explosion. This is the reason high explosives do not make good propellants for bullets, cannonballs, or shells -- they produce a shock wave that shatters whatever it is in contact with, but does not have the energy to actually move the material very far. A small amount of energy, released in a very small amount of time, has a lot of power. Power is what breaks things, while energy is what moves things. Howard is well aware of this:

From the experiments related in the 4th and 5th sections, in which the gunpowder proof and the gun were burst, it might be inferred, that the astonishing force of the mercurial powder is to be attributed to the rapidity of its combustion; and, a train of several inches in length being consumed in a single flash, it is evident that its combustion must be rapid. From the experiments of the 6th and 7th sections, it is sufficiently plain that this force is restrained to a narrow limit; both because the block of wood charged with the mercurial powder was more shattered than that charged with the gunpowder, whilst the sand surrounding it was least disturbed; and likewise because the glass globe withstood the explosion of 10 grains of the powder fixed in its centre: a charge I have twice found sufficient to destroy old pistol barrels, which were not injured by being fired when full of the best gunpowder.

Howard went on to try other metals. When he tried silver, he also got an explosive, which he is careful to note is different from the fulminating silver produced by Berthollet (and indeed, they are different compounds). Silver fulminate is even more sensitive than mercury fulminate. It cannot be stored in much quantity, since it detonates under its own weight.

Silver fulminate

The next experiments were done with the assistance of the local military, where several tests were done by exploding small quantities inside cannons, and noting the shattering effects. In one test, a small cannon is destroyed, and in another, a bursting shell is demonstrated:

Finding that the carronade, from the great comparative size of its bore to that of its length, required a larger quantity of mercurial powder to burst it than we were provided with, we charged a half-pounder swivel with an ounce and an half avoirdupois of the mercurial powder, (the service charge of gunpowder being 3 ounces) and a half-pound shot between two wads. The piece was destroyed from the trunnions to the breech, and its fragments thrown thirty or forty yards. The ball penetrated five inches into a block of wood, standing at about a yard from the muzzle of the gun; the part of the swivel not broken, was scarce, if at all, moved from its original position.

One ounce avoirdupois of the mercurial powder, enclosed in paper, was placed in the centre of a shell 4,4 inches in diameter, and the vacant space filled with dry sand.

The shell burst by the explosion of the powder, and the fragments were thrown to a considerable distance. The charge of gunpowder employed to burst shells of this diameter, is 5 ounces avoirdupois.

Howard's experiments with mercury fulminate led to his being awarded the Copley Medal of the Royal Society, (an award his benefactor, Irish chemist Peter Woulfe, had earned a few years earlier). The experiments also gained him fame both at home in England, and across Europe. He went on to study meteorites, and show that they were not of this earth, but had fallen from outer space.

As it turns out, Howard was not the first to make mercury fulminate.

Cornelius Jacobzoon Drebbel was a Dutch engraver who later became famous for his many inventions. He made a clock that wound itself using changes in temperature and atmospheric pressure. He modified that mechanism to control the heat of a furnace (thus inventing the first thermostat). He applied the same mechanism to an incubator for raising chickens. An accident led to the discovery of a tin mordant for a bright scarlet dye (he had dissolved some tin in aqua regia, and it spilled into some cochineal dye he had planned to use in a thermometer). He made compound microscopes, and may have been the first to use two convex lenses in a microscope. He built a submarine, rowed by oars, and built of leather covering an open-bottomed wood frame.

As an alchemist, he introduced England to a method of making sulfuric acid by burning sulfur with saltpeter. He wrote a paper on the transmutation of elements in 1608, and another alchemical paper in 1621 that discusses extracts of plants, minerals, and metals. In his work for the British Navy, he devised torpedoes and sea mines, using a detonator that has caused some confusion on the part of his biographers. In the same sentence, he is said to use "Batavian tears", aurum fulminarum, and fulminating mercury to detonate the mines. Batavian tears are also known as Prince Rupert's Drops. They are teardrop shaped bits of molten glass cooled quickly in water. When the glass "tail" is broken, the entire drop shatters explosively, due to the high compression the cooling gave to the glass. Aurum fulminarum is fulminating gold, which the biographers confuse with mercury fulminate.

While Drebbel's claim to have created mercury fulminate is suspect, the German chemist Baron Johann von Löwenstern-Kunckel describes making it in 1690. In his book Laboratorium Chymicum, he tells of the vigorous reaction that mercury nitrate has with "spiritus vini" (ethanol). He describes the explosion that resulted, but was not known to have used the substance for anything. He also wrote about fulminating gold and fulminating silver, both of which were well known to alchemists of the time. His experiments with gold produced a brilliant red glass, called "ruby-glass", by incorporating gold nanoparticles, produced by precipitation from solution. The son of a glassmaker, with a long family history of glassmaking, he built several glass factories under the patronage of Frederick William, Elector of Brandenburg.

By 1807, fulminate of mercury was well known, due to the prestige of the journal where Edward Howard had published. In that year, a Scottish minister named Alexander John Forsyth patented a new way to fire a gun, using the new compound. He was an ardent duck hunter, and one of his main complaints was that the noise of the flintlock hammer striking the spark, and the subsequent flash and smoke from the primer pan, frightened the ducks into flight before the powder charge in the gun had time to fire the bird shot.

His invention involved putting a mixture of two fulminating powders, mercury fulminate and the potassium chlorate gunpowder, next to the touchhole of the gun, and modifying the flintlock mechanism to act like a hammer, striking the fulminating powder, and igniting the main charge in the gun. The fulminating powder acted very quickly on the gunpowder charge, and the ducks no longer had any warning.

Not much happened with the invention until after the patent ran out. Forsyth turned down an offer by Napoleon of 20,000 pounds sterling to bring his invention to France, and in general seemed to lack the ability to do much in the way of marketing.

After the patent expired, however, many improvements came in rapid succession by a number of different inventors in France, Switzerland, and Britain. The tube-lock held the fulminating powder in a metal tube that was crushed by the falling hammer. It was quickly succeeded by the percussion cap, a metal cup that fit over a nipple at the gun's touchhole, and was likewise crushed by the falling hammer.

Percussion Caps

In Prussia, the Dreyse needle gun used a long needle to pierce a paper cartridge to hit the enclosed percussion cap.

Black Powder Percussion Cap Rifle

By the 1850's, the idea of putting the percussion cap, the powder charge, and the bullet into a single metallic cartridge made loading and firing a gun much faster and easier. The soft brass casing made breech-loading weapons finally practical, as it would expand in the chamber and seal in the exploding gases.

The fulminating mixture still had some problems, however. The chlorate in it, and to some extent the mercury fulminate, was corrosive, and the steel barrels of the guns were prone to rust and jam. Later primers used just the mercury fulminate, until the discovery of even better primary explosives (the name given to sensitive compounds that explode on concussion, and are used to set off less sensitive, secondary explosives).

In 1858, German chemist Peter Griess discovered diazonitrophenol, a powerful and sensitive explosive, while working with organic compounds rich in nitrogen. He is also thought to be the discoverer (in 1874) of lead styphnate, a now widely used primary explosive for primers in ammunition. The method currently used to produce lead styphnate is due to the work in 1919 by Edmund Herz. Because it is non-corrosive and less sensitive to shock than mercury fulminate, lead styphnate replaced that compound in ammunition primers.

Black Powder Percussion Cap Revolver

In 1891, the German chemist Julius Wilhelm Theodor Curtius discovered lead azide. While it reacts with copper, cadmium, and zinc, and alloys of those (such as brass and bronze), it does not corrode steel. It is used as a primer in the same way as lead styphnate, and is also used to make bullets that explode on impact. The year before (1890), he had discovered another primary explosive, hydrazoic acid (hydrogen azide).

It is not surprising that the first high explosives were the ones that were very sensitive, and would detonate when slightly heated or stirred. An explosion tends to get one's attention. More stable high explosives, such as picric acid, discovered by Irish chemist Peter Woulff in 1771, and trinitrotoluene (TNT), discovered in 1863 by German chemist Julius Wilbrand, and were used as dyes until their explosive nature was discovered much later.

The sensitive primary explosives are a diverse group. There are many ways to make a chemical that comes apart easily, or is made of things that bind much stronger to one another in a different arrangement. An example of a molecule that comes apart easily is benzvalene:


Notice how four of the six carbon atoms are connected by acute bond angles. This causes strain, and the molecule relieves that strain by reconfiguring into simpler smaller molecules explosively.

An example of a molecule whose parts bind tighter when rearranged is lead azide:

Lead azide

Those two groups of three nitrogen atoms could relax into three groups of two nitrogen atoms, with a strong triple bond joining each pair, thus releasing a lot of energy. Many azides are explosive, and usually quite sensitive to heat, friction, and concussion.

Sodium azide

Sodium azide is another high explosive that you might find in your garage. It is the explosive used to inflate airbags in cars. A mixture of sodium azide with potassium nitrate and silicon dioxide (silica) is detonated inside the bag when the electronics detect a collision. The highly toxic sodium azide is converted to sodium metal and nitrogen gas. The gas expands the bag. The sodium metal reacts with the potassium nitrate, producing potassium oxide and sodium oxide (and a little extra nitrogen gas). The metal oxides combine with the silica to produce silicate glass, a harmless by-product.

The amount of explosive in airbags is substantial. In a driver's side front airbag, there is about 50 grams of sodium azide. The passenger side airbag is bigger (the passenger is farther away) and contains 200 to 250 grams of explosive. Together, they have about 20 shotgun-shells worth of explosive.

Most primary explosives have both features of an explosive — they come apart easily, and they recombine into smaller parts that are bound more tightly to one another than they were in the original molecule.

Another feature of many explosive compounds is the presence of an oxidizer on the same molecule as a fuel. We saw with gunpowder that getting the fuel and the oxidizer close together made the product burn faster. By using a better oxidizer, Fourcroy's gunpowder made with potassium chlorate instead of potassium nitrate would explode by concussion. But what if we put the fuel on the same molecule as the chlorate oxidizer? We get something like ammonium chlorate:

Ammonium chlorate

The chlorine atom has three oxygen atoms to lose, and the nitrogen atom has three hydrogen atoms to lose. When the molecule is heated or hit with a hammer, we get nitrogen gas, water vapor, hydrogen chloride gas, and a loud bang.

Another example is the class of explosives that combine peroxides with hydrocarbons. A peroxide is two oxygen atoms connected by a weak single bond. That bond is easily broken, and the oxygen atoms are then free to rearrange with the carbon atoms and hydrogen atoms to form carbon dioxide and water vapor. An example is triacetone triperoxide, also known as TATP:

Triacetone triperoxide

Triacetone triperoxide was discovered by German chemist Richard Wolffenstein in 1895. Because it is easy to make from household products, it was used by Palestinian suicide bombers and in several bombings in Europe. It is a sensitive primary explosive, and often detonates during manufacture, taking with it the bomb makers.

You can see in the image that there are three acetone molecules connected by three peroxide links. Those links break, and the oxygen atoms combine with the carbon atoms and hydrogen atoms violently.

Another explosive peroxide is one you might have in your medicine cabinet. Benzoyl peroxide is used to bleach flour (peroxides make good bleaching agents), and to clear up acne. In pure form, it is a primary explosive, detonating by heat or shock. In dilute form, it can be applied to the skin, where it breaks down into oxygen, which kills germs by its bleaching action, and benzoic acid, which is a topical antiseptic:

Benzoyl peroxide

Yet another explosive peroxide that you might find in a household cleaning product or in a swimming pool disinfectant is peroxymonosulfuric acid:

Peroxymonosulfuric acid

Produced by mixing sulfuric acid with hydrogen peroxide, it is one of the strongest oxidizing agents known. An explosive by itself in pure form, it is also capable of adding a peroxide group to compounds of carbon and hydrogen that it touches, making other explosives, such as acetone peroxides. It was first discovered in 1898 by German chemist Heinrich Caro, and is sometimes called Caro's acid.

If you work with any of the common two-part plastic resins, you may have a third explosive in your house. Methyl ethyl ketone peroxide (MEKP) is used as a catalyst and hardener for thermosetting polyester plastics. Similar to triacetone triperoxide, the compound is quite explosive, and quite sensitive to shock and heat. Fortunately, in dilute form, it does not explode:

Methyl Ethyl Ketone Peroxide

Because primary explosives are such a diverse group, they include several compounds that are unique or unusual, even for explosives.

One such unique compound is copper acetylide, one of the few explosives that produces no gases when it detonates. It is made by passing acetylene gas through a solution of copper chloride and ammonia. The red precipitate is quite sensitive to heat and shock. Acetylene plants no longer use copper pipes because of the danger of producing this explosive accidentally:

Copper acetylide

Silver acetylide is also explosive and produces no gas. Silver acetylide mixed with silver nitrate (which adds oxygen to combine with the carbon) is used in some commercial explosives. It is a little less explosive than the pure silver acetylide, and does produce gases on detonation.

Another interesting special case molecule is xenon trioxide:

Xenon trioxide

Noble gases like xenon do not combine with other atoms easily. Xenon trioxide easily and explosively breaks down into the gases xenon and oxygen. It is an extremely powerful oxidizing agent, and the dry crystals will explode on contact with cellulose and other organic molecules. It will detonate spontaneously at room temperature. Xenon dioxide is also explosive, and xenon tetroxide explodes if it gets above -35.9° Celsius.

Another curious special case is explosive antimony. Antimony crystals can come in two forms. One is a stable metallic form, with shiny lustrous crystals. This is called β-antimony. Another is a yellow form, called α-antimony, which changes into the metallic form when heated. If exposed to light, α-antimony turns into a third form, which is black and non-crystalline. This black form turns into the metallic β-antimony when heated.

In 1858, the English electrochemist George Gore (one of the many inventors of the safety matches we use today), was electroplating antimony from a solution of antimony trichloride onto a piece of copper. This produced a fourth form of antimony. It is a solid solution of antimony trichloride in α-antimony. It has the appearance of shiny gray graphite.

When he scraped some of it off the copper electrode for analysis, it exploded.

The α-antimony releases a good deal of heat when it transforms back into the shiny β-antimony form. This raises the temperature to 250° Celsius, and vaporizes the antimony trichloride in a puff of toxic white smoke. When crushed in a mortar and pestle, it detonates with a loud crack.

It is perhaps not surprising that the invention of an exploding compound might change history. The subtlety by which is happens, however, is interesting. In 1812, the French chemist Pierre Louis Dulong was experimenting with chlorine gas and ammonium nitrate, and purifying the reaction products. One of those reaction products is nitrogen trichloride. It is a very sensitive high explosive, and in two explosions, he lost two fingers and an eye (one might ask why he wasn't much more careful after the first explosion, but that bit of history doesn't seem to have survived the passage of time).

Nitrogen trichloride

A year later, the Cornish chemist and newly knighted Sir Humphry Davy, former lecturer at the Royal Institution, also damaged his eyesight in a nitrogen trichloride explosion. As the discoverer of the elements potassium, sodium, calcium, magnesium, boron, and barium, he was the person who gave the name chlorine to the gas Swedish chemist Carl William Scheele had discovered in 1774, and insisted it was an element.

Davy describes the new explosive in the Philosophical Transactions of the Royal Society on November 5, 1812:

I immediately exposed a phial, containing about six cubical inches of chlorine, to a saturated solution of nitrate of ammonia, at the temperature of about 50° in common day-light. A diminution of the gas speedily took place; in a few minutes a film, which had the appearance of oil, was seen on the surface of the fluid; by shaking the phial is collected in small globules, and fell to the bottom. I took out one of the globules, and exposed it in contact with water to a gentle heat: long before the water began to boil, it exploded with a very brilliant light, but without any violence of sound.

...An attempt was made to procure the substance in large quantities, by passing chlorine into Wolfe's bottles, containing the different solutions, but a single trial proved the danger of this mode of operating; the compound had scarcely began to form, when, by the action of some ammoniacal vapour on chlorine, heat was produced, which occasioned a violent explosion, and the whole apparatus was destroyed.

I attempted to collect the products of the explosion of the new substance, by applying the heat of a spirit lamp to a globule of it, confined in a curved glass tube over water: a little gas was at first extricated, but long before the water had attained the temperature of ebullition, a violent flash of light was perceived, with a sharp report; the tube and the glass were broken into small fragments, and I received a severe wound in the transparent cornea of the eye, and obliges me to make this communication by an amanuensis. This experiment proves what extreme caution is necessary in operating on this substance, for the quantity I used was scarcely as large as a grain of mustard seed.

...The mechanical force of this compound in detonation, seems to be superior to that of any other known, not even excepting the ammoniacal fulminating silver. The velocity of its action appears to be likewise greater.

The damage to his eyes caused him to hire a valet, who could also assist him in his experiments. That young man was Michael Faraday, who went on to become an even more famous scientist (thus the subtle effects of explosions on history). And, in proof that even brilliant people don't learn from their mistakes, both Faraday and Davy were injured in yet another experiment with nitrogen trichloride, although this time some precautions were taken:

The action of mercury on the compound appeared to offer a more correct and less dangerous mode of attempting its analysis; but on introducing two grains under a glass tube filled with mercury and inverted, a violent detonation occurred, by which I was slightly wounded in the head and hands, and should have been severely wounded, had not my eyes and face been defended by a plate of glass attached to a proper cap, a precaution very necessary in all investigations of this body.

Soon after Humphry Davy hired Faraday, the two were on a trip to France (to receive a medal from Napoleon Bonaparte for Davy's work in electrochemistry). There, the famous French chemist Joseph Louis Gay-Lussac told him about a new substance discovered by a fellow French chemist named Bernard Courtois.

Courtois owned a factory that produced potassium nitrate for Napoleon's war efforts. The gunpowder factories were having trouble finding enough wood ash with which to make the nitrate. Courtois turned to using seaweed ashes, since seaweed was abundant on the French coast. Courtois was trying to find out what was causing the corrosion on the copper pots used to process the seaweed ash. He added sulfuric acid to the ash, and noticed a peculiar purple vapor.

Davy writes about the "peculiar substance" in the Philosophical Transactions of the Royal Society on January 20, 1814:

A new and very curious substance has recently occupied the attention of chemists at Paris.

This substance was accidentally discovered about two years ago by M. Courtois, a manufacturer of saltpeter in Paris. In his processes for procuring soda from the ashes of sea weeds (cendres de vareck) he found the metallic vessels much corroded; and in searching for the cause of this effect, he made the discovery. The substance is procured from the ashes, after the extraction of the carbonate of soda, with great facility, and merely by the action of sulfuric acid: — when the acid is concentrated, so as to produce much heat, the substance appears as a vapour of a beautiful violet color, which condenses in crystals having the colour and the luster of plumbago.

M. Courtois soon after he had discovered it, gave specimens of it to M. M. Desormes and Clement for chemical examination; and those gentlemen read a short memoir upon it, at a meeting of the Imperial Institute of France, on Nov. 29th. In this memoir, these able chemists have described its principal properties; they mentioned that its specific gravity was about four times that of water, that it becomes a violet coloured gas at a temperature below that of boiling water, that it combines with the metals and with phosphorus and sulphur, and likewise with the alkalies and metallic oxides, that it forms a detonating compound with ammonia...

Davy performs his own experiments, and concludes that the "curious substance" is not a compound, but an "undecompounded body" (i.e. an element), which he names iodine.

The detonating compound is nitrogen triiodide.

Nitrogen triiodide

Nitrogen triiodide is the only known explosive that is so sensitive it will detonate when hit by alpha particles (helium nuclei produced by radioactive elements). Chemistry professors love to demonstrate how it can be set off using the touch of a feather.