In its modern manufacture the catalyzer or instigator of the combination is not sunlight but porous carbon. This is packed in iron boxes eight feet long, through which the mixture of the two gases was forced. Carbon monoxide may be made by burning coke with a supply of air insufficient for complete combustion, but in order to get the pure gas necessary for the phosgene common air was not used, but instead pure oxygen extracted from it by a liquid air plant.
Phosgene is a gas that may be condensed easily to a liquid by cooling it down to 46 degrees Fahrenheit. A mixture of three-quarters chlorine with one-quarter phosgene has been found most effective. By itself phosgene has an inoffensive odor somewhat like green corn and so may fail to arouse apprehension until a toxic concentration is reached. But even small doses have such an effect upon the heart action for days afterward that a slight exertion may prove fatal.
The compound manufactured in largest amount in America was chlorpicrin. This, like the others, is not so unfamiliar as it seems. As may be seen from its formula, CCl3NO2, it is formed by joining the nitric acid radical (NO2), found in all explosives, with the main part of chloroform (HCCl3). This is not quite so poisonous as phosgene, but it has the advantage that it causes nausea and vomiting. The soldier so affected is forced to take off his gas mask and then may fall victim to more toxic gases sent over simultaneously.
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 (C2H4). 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" (ClC2H4SC2H4Cl). This, however, is too big a mouthful, so even the chemists were glad to fall in with the commonalty and call it "mustard gas."
The effectiveness of "mustard" depends upon its persistence. It is a stable liquid, evaporating slowly and not easily decomposed. It lingers about trenches and dugouts and impregnates soil and cloth for days. Gas masks do not afford complete protection, for even if they are impenetrable they must be taken off some time and the gas lies in wait for that time. In some cases the masks were worn continuously for twelve hours after the attack, but when they were removed the soldiers were overpowered by the poison. A place may seem to be free from it but when the sun heats up the ground the liquid volatilizes and the vapor soaks through the clothing. As the men become warmed up by work their skin is blistered, especially under the armpits. The mustard acts like steam, producing burns that range from a mere reddening to serious ulcerations, always painful and incapacitating, but if treated promptly in the hospital rarely causing death or permanent scars. The gas attacks the eyes, throat, nose and lungs and may lead to bronchitis or pneumonia. It was found necessary at the front to put all the clothing of the soldiers into the sterilizing ovens every night to remove all traces of mustard. General Johnson and his staff in the 77th Division were poisoned in their dugouts because they tried to alleviate the discomfort of their camp cots by bedding taken from a neighboring village that had been shelled the day before.
Of the 925 cases requiring medical attention at the Edgewood Arsenal 674 were due to mustard. During the month of August 3-1/2 per cent. of the mustard plant force were sent to the hospital each day on the average. But the record of the Edgewood Arsenal is a striking demonstration of what can be done in the prevention of industrial accidents by the exercise of scientific prudence. In spite of the fact that from three to eleven thousand men were employed at the plant for the year 1918 and turned out some twenty thousand tons of the most poisonous gases known to man, there were only three fatalities and not a single case of blindness.
Besides the four toxic gases previously described, chlorine, phosgene, chlorpicrin and mustard, various other compounds have been and many others might be made. A list of those employed in the present war enumerates thirty, among them compounds of bromine, arsenic and cyanogen that may prove more formidable than any so far used. American chemists kept very mum during the war but occasionally one could not refrain from saying: "If the Kaiser knew what I know he would surrender unconditionally by telegraph." No doubt the science of chemical warfare is in its infancy and every foresighted power has concealed weapons of its own in reserve. One deadly compound, whose identity has not yet been disclosed, is known as "Lewisite," from Professor Lewis of Northwestern, who was manufacturing it at the rate of ten tons a day in the "Mouse Trap" stockade near Cleveland.
Throughout the history of warfare the art of defense has kept pace with the art of offense and the courage of man has never failed, no matter to what new danger he was exposed. As each new gas employed by the enemy was detected it became the business of our chemists to discover some method of absorbing or neutralizing it. Porous charcoal, best made from such dense wood as coconut shells, was packed in the respirator box together with layers of such chemicals as will catch the gases to be expected. Charcoal absorbs large quantities of any gas. Soda lime and potassium permanganate and nickel salts were among the neutralizers used.