Bromobenzyl cyanide 0.0003 Martonite 0.0012 Ethyl iodoacetate 0.0014 Bromoacetone 0.0015 Xylyl bromide 0.0018 Benzyl bromide 0.0040 Bromo ketone 0.011 Choroacetone 0.018 Chloropicrin 0.019 The figures give the concentration (milligram per liter of air) necessary to produce lachrymation. The method used in obtaining these figures is given in Chapter XXI. CHAPTER VIII CHLOROPICRIN During the spring of 1917, strange reports came from the Italian front that the Germans were using a new war gas. This gas, while it did not seem to be very poisonous, had the combined property of being a lachrymator and also of causing vomiting. Large number of casualties resulted through the men being forced to remove their masks in an atmosphere filled with lethal gases. The gas had the additional and serious disadvantage of being a very difficult one to remove completely in the gas mask. The first American masks were very good when chlorine or phosgene was considered but were of no value when chloropicrin was used. One of the interesting facts of chemical warfare is that few if any new substances were discovered and utilized during the three years of this form of fighting. Chlorine and phosgene were well known compounds. And likewise, chloropicrin was an old friend of the organic chemist. So much so, indeed, that several organic laboratories prepared the compound in their elementary courses. Chloropicrin was first prepared by the English chemist, Stenhouse, in 1848, by the action of bleaching powder upon a solution of picric acid. This was followed by a careful study of its physical and chemical properties, few of which have any connection with its use as a poison gas. The use of picric acid as an explosive made it very desirable that other raw materials should be used. Chloroform, which is the ideal source theoretically (since chloropicrin is nitro-chloroform, Cl₃CNO₂), gave very poor yields. While it may be prepared from acetone, in fair yields, acetone was about as valuable during the war as was picric acid. Practically all the chloropicrin used was prepared from this acid as the raw material. MANUFACTURE In the manufacture of chloropicrin the laboratory method was adopted. This consisted simply in passing live steam through a mixture of picric acid and bleaching powder. The resulting chloropicrin passes out of the still with the steam. There was a question at first whether a steam jacketed reaction vessel should be used, and whether stirrers should be introduced. Both types were tested, of which the simpler form, without steam jacket or stirrer, proved the more efficient. [Illustration: FIG. 27.—Interior of Chloropicrin Plant.]