Chlorpicrin is a liquid and is commonly loaded in a shell or bomb with 20 per cent. of tin chloride, which produces dense white fumes that go through gas masks.  It is made from picric acid (trinitrophenol), one of the best known of the high explosives, by treatment with chlorine.  The chlorine is obtained, as it is in the household, from common bleaching powder, or “chloride of lime.”  This is mixed with water to form a cream in a steel still 18 feet high and 8 feet in diameter.  A solution of calcium picrate, that is, the lime salt of picric acid, is pumped in and as the reaction begins the mixture heats up and the chlorpicrin distils over with the steam.  When the distillate is condensed the chlorpicrin, being the heavier liquid, settles out under the layer of water and may be drawn off to fill the shell.

Much of what a student learns in the chemical laboratory he is apt to forget in later life if he does not follow it up.  But there are two gases that he always remembers, chlorine and hydrogen sulfide.  He is lucky if he has escaped being choked by the former or sickened by the latter.  He can imagine what the effect would be if two offensive fumes could be combined without losing their offensive features.  Now a combination something like this is the so-called mustard gas, which is not a gas and is not made from mustard.  But it is easily gasified, and oil of mustard is about as near as Nature dare come to making such sinful stuff.  It was first made by Guthrie, an Englishman, in 1860, and rediscovered by a German chemist, Victor Meyer, in 1886, but he found it so dangerous to work with that he abandoned the investigation.  Nobody else cared to take it up, for nobody could see any use for it.  So it remained in innocuous desuetude, a mere name in “Beilstein’s Dictionary,” together with the thousands of other organic compounds that have been invented and never utilized.  But on July 12, 1917, the British holding the line at Ypres were besprinkled with this villainous substance.  Its success was so great that the Germans henceforth made it their main reliance and soon the Allies followed suit.  In one offensive of ten days the Germans are said to have used a million shells containing 2500 tons of mustard gas.

The making of so dangerous a compound on a large scale was one of the most difficult tasks set before the chemists of this and other countries, yet it was successfully solved.  The raw materials are chlorine, alcohol and sulfur.  The alcohol is passed with steam through a vertical iron tube filled with kaolin and heated.  This converts the alcohol into a gas known as ethylene (C_{2}H_{4}).  Passing a stream of chlorine gas into a tank of melted sulfur produces sulfur monochloride and this treated with the ethylene makes the “mustard.”  The final reaction was carried on at the Edgewood Arsenal in seven airtight tanks or “reactors,” each having a capacity of 30,000 pounds.  The ethylene gas being led into the tank and distributed through the liquid sulfur chloride by porous blocks or fine nozzles, the two chemicals combined to form what is officially named “di-chlor-di-ethyl-sulfide” (ClC_{2}H_{4}SC_{2}H_{4}Cl).  This, however, is too big a mouthful, so even the chemists were glad to fall in with the commonalty and call it “mustard gas.”