Tactical Uses
Because of the high boiling point of chloropicrin it can only be used in shell. The German shell usually contained a mixture of superpalite (trichloromethyl chloroformate) and chloropicrin, the relative proportions being about 75 to 25. These were called Green Cross Shell, from the peculiar marking on the outside of the shell. Mixtures of phosgene and chloropicrin (50-50) have also been used.
The Allies have used a mixture of 80 per cent chloropicrin and 20 per cent stannic chloride (so-called N. C.). This mixture combines the advantages of a gas shell with those of a smoke shell, since the percentage of stannic chloride is sufficiently high to form a very good cloud. In addition to this, it is believed that the presence of the chloride increases the rate of evaporation of the chloropicrin. It has been claimed that the chloride decreases the amount of decomposition of the chloropicrin upon the bursting of the shell, but careful experiments appear to show that this decomposition is negligible and that the stannic chloride plays no part in it. This mixture was being abandoned at the close of the war.
This N. C. mixture has also been used in Liven’s projectors and in hand grenades. The material is particularly fitted for hand grenades, owing to the low vapor pressure of the chloropicrin, and the consequent absence of pressures even on warm days. As a matter of fact, it was the only filling used for this purpose, though later the stannic chloride was changed, owing to the shortage of tin, to a mixture of silicon and titanium chlorides.
While chloropicrin is sufficiently volatile to keep the strata of air above it thoroughly poisonous, it is still persistent enough to be dangerous after five or six hours.
CHAPTER IX
DICHLOROETHYLSULFIDE
“MUSTARD GAS”
The early idea of gas warfare was that a material, to be of value as a war gas, should have a relatively high vapor pressure. This would, of course, provide a concentration sufficiently high to cause casualties through inhalation of the gas-ladened air. The introduction of “mustard gas” (dichloroethylsulfide) was probably the greatest single development of gas warfare, in that it marked a departure from this early idea, for mustard gas is a liquid boiling at about 220° C., and having a very low vapor pressure. But mustard gas has, in addition, a characteristic property which, combined with its high persistency, makes it the most valuable war gas known at the present time. This peculiar property is its blistering effect upon the skin. Very low concentrations of vapor are capable of “burning” the skin and of producing casualties which require from three weeks to three months for recovery. The combination of these properties removed the necessity for a surprise attack, or the building up of a high concentration in the first few bursts of fire. A few shell, fired over a given area, were sufficient to produce casualties hours and even days afterwards.
Mustard gas, chemically, is dichloroethylsulfide (ClCH₂CH₂)₂S. The name originated with the British Tommy because the crude material first used by the Germans was suggestive of mustard or garlic. Various other names were given the compound, such as “Yellow Cross,” from the shell markings of the Germans; “Yperite,” a name used by the French, because the compound was first used at Ypres; and “blistering gas,” because of its peculiar effect upon the skin.