172 TEAGUE AND BARBER. 



This was shown to be in fact the case by spraying the same 

 cholera suspension into a comparatively dry atmosphere and 

 then, under similar conditions, into an atmosphere nearly satu- 

 rated with moisture; living cholera vibrios remained in the air 

 much longer in the latter instance. A similar experiment was 

 performed with B. prodigiosus with the same result. 



By spraying sarcina and immediately thereafter cholera 

 vibrios, so that the droplets containing these organisms were 

 subjected to identical conditions, living sarcina was found to 

 persist in the air long after the living cholera vibrios had dis- 

 appeared. Since the sarcina is a larger organism than the 

 cholera vibrio, it follows that the disappearance of the latter was 

 not due to settling. 



We believe we are justified in concluding from these experi- 

 ments that were the plague organisms sprayed under similar 

 conditions they would persist longer than cholera vibrios, but 

 a shorter time than prodigiosus bacilli. Hence, it seems probable 

 that the plague bacilli contained in fine droplets of pneumonic- 

 plague sputum would suffer death from drying in a few minutes 

 unless they were suspended in an atmosphere with an extremely 

 small water deficit. Infection in pneumonic plague follows the 

 inhalation of droplets of pneumonic sputum and obviously the 

 longer these droplets remain suspended in the air, the greater is 

 the danger of infection. As has just been stated, these fine 

 droplets disappear very quickly except when they are suspended 

 in an atmosphere with a very small water deficit. Such an 

 atmosphere is under ordinary circumstances of common occur- 

 rence in very cold climates, whereas it is extremely rare in warm 

 ones. Hence, since the droplets of sputum persist longer, the 

 plague bacilli remain alive longer in the air, and there is a 

 greater tendency for the disease to spread in cold climates than 

 in warm ones. 



In harmony with the above ideas, we find that the only great 

 epidemic of pneumonic plague of modern times occurred in 

 Manchuria during the winter of 1910 to 1911, when the atmos- 

 pheric temperature was many degrees below zero Centigrade. 

 The disease spread with amazing rapidity. Furthermore, al- 

 though during the past fifteen years there have been millions 

 of plague cases in India and 2 to 5 per cent of these have been 

 cases of plague pneumonia, yet this form of the disease has not 

 assumed epidemic proportions. The largest epidemic of pneu- 

 monic plague in India (1,400 deaths) occurred in Kashmir in 

 northern India at an elevation of 1,524 meters above the sea level 

 during very cold weather. 



