162 TEAGUE AND BARBER. 



That plague bacilli may be unable "to long maintain their 

 unusual or perhaps exalted degree of virulence" by passage 

 from lung to lung, as is suggested by Gill, appears to us to be 

 highly improbable, since the experimental data at hand indicate 

 that passage from lung to lung in susceptible animals is the 

 method of choice and, perhaps, the only method of exalting the 

 virulence of plague bacilli and maintaining the high virulence 

 thus attained. 



The epidemiological observations of Gill possess, however, 

 great interest with regard to the influence of atmospheric tem- 

 perature upon the spread of pneumonic plague. He found that 

 pneumonic plague occurred during cold weather and ceased when 

 the warm weather began, in spite of the fact that the number 

 of bubonic cases was still on the increase. Unfortunately, he 

 did not publish his notes in suiScient detail for us to determine 

 the atmospheric temperature and humidity which existed during 

 his several epidemics, but as far as his observations go, they 

 indicate that the atmospheric temperature was probably a factor 

 of importance in the spread of pneumonic plague and the sup- 

 pression of the epidemic'. 



The only other epidemic of pneumonic plague of recent years 

 of which we find a reliable record is the small one which occurred 

 in Osaka, Japan, also in the cold season of the year. The first 

 patient was taken sick on December 19, 1899. This case was 

 quickly followed by twelve others, the last dying on January 

 13, 1900. 



The above discussion has been confined entirely to pneumonic 

 plague, but obviously the same ideas apply also to other pneu- 

 monias. In other pneumonias, however, it is not unlikely that 

 the dosage and virulence of the inhaled bacilli and the suscep- 

 tibility of the host at the time of exposure are factors of far 

 greater importance than in plague pneumonia; hence, the in- 

 fluence of atmospheric temperature on their spread would be 

 more or less obscured by these other factors. 



We have endeavored to obtain experimental data confirmatory 

 of the ideas advanced in the foregoing discussion. It was, of 

 course, impracticable to perform actual experiments with plague 

 bacilli sprayed into the air on account of the danger of con- 

 tracting pneumonic plague. We, therefore, sprayed harmless 

 bacteria and determined how they behave in the air under dif- 

 ferent conditions, believing that the results obtained would 

 justify us in drawing conclusions as to how plague bacilli would 

 act under similar conditions. We selected for most of the ex- 

 periments B. prodigiosus and a yellow sarcina obtained from the 



