In addition to its toxicity mustard gas is highly important because of its peculiar irritating effect upon the skin. Its value is seen when we realize that one part in 14,000,000 is capable of causing conjunctivitis of the eye and that one part in 3,000,000 and possibly one part in 5,000,000 will cause a skin burn in a sensitive person on prolonged exposure. According to Warthin, the lesions produced by mustard gas are those of a chemical, not unlike hydrochloric acid, but of much greater intensity. The pathology of these lesions has been carefully studied and fully described by Warthin and Weller in their book on The Pathology of Mustard Gas. Our observations will therefore be confined to certain striking features of the vesicant action of this substance. VARIATION IN SUSCEPTIBILITY OF THE SKIN Every worker who has worked with mustard gas has noticed that some individuals are much more susceptible to skin burns from this substance than are others. Marshall made a study of 1282 men at Edgewood Arsenal, using a 1 per cent and a 0.01 per cent solution of mustard gas in paraffin oil. A small drop of these solutions was applied to the skin of the forearm of the subject and the arm allowed to remain uncovered for about 10 minutes. The presence or absence of a positive reaction is indicated by the appearance or absence of erythema 24 hours later. The results were as follows: 1% 0.01% % of Total Positive Positive 3.3 Positive Negative 55.3 Negative Negative 41.4 The test made on 84 negroes gave the following results: 1% 0.01% % of Total Positive Positive 0.0 Positive Negative 15.0 Negative Negative 78.0 Questionable Negative 7.0 “It is seen from the above tables that negroes as a race, have a much more resistant skin than white men. No negro of the 84 examined reacted to the 0.1 per cent solution, and of course none would react to a more dilute one. About 10 per cent of white men react to the 0.1 per cent solution, while 2 to 3 per cent react to the 0.01 per cent solution or are hypersensitive. About 78 per cent of the negroes fail to react to the 1 per cent solution, while only 20 to 40 per cent of the white race do not show a reaction.” [Illustration: FIG. 37.] The same individual may also show variations in susceptibility and this has also been studied by Marshall.