158 TEAGUE AND BARBER. 



a patient in coughing throws out droplets of sputum which must 

 contain plague bacilli. Strong and Teague ^ demonstrated that 

 this does in fact occur. Petri dishes, containing solidified agar- 

 culture-medium, were held before the mouths of coughing plague 

 patients, and, even when no visible particles of sputum appeared, 

 colonies of plague bacilli developed on the plates. Granted that 

 infection is due to the inhalation of droplets of sputum con- 

 taining plague bacilli, it follows that the longer these droplets 

 remain suspended in the air, the greater the danger of infection. 



These droplets may disappear from the air in the immediate 

 neighborhood of the patient in three ways; namely, (1) by 

 evaporation, (2) by settling, and (3) by being borne away by 

 currents of air. 



The rate of evaporation depends chiefly upon the water deficit 

 of the atmosphere. Under ordinary conditions this is far greater 

 in warm weather than in cold and hence, ordinarily, evaporation 

 of droplets of moisture in the air will take place far more rapidly 

 in warm weather than in cold. At 4° C, with a maximum of 

 moisture in the air, the water vapor has a pressure of only 

 6.0 millimeters of mercury; hence, even if the atmosphere were 

 absolutely dry at this temperature, the water deficit would be 

 small and evaporation would take place very slowly. 



At 30° C, with a maximum of moisture in the air, the pressure 

 of the water vapor amounts to 31.5 millimeters. With 70 per 

 cent of moisture in the air, there would still be a greater water 

 deficit (9.4 millimeters of mercury) than in a perfectly dry 

 atmosphere at 4° C. In a cold climate, with snow on the ground 

 and a rise of several degrees in temperature during the middle 

 of the day, the water deficit of the air would be approximately 

 zero during the greater part of the twenty-four hours. These 

 were the conditions in Manchuria during the recent epidemic 

 of pneumonic plague; hence there must have existed a very low 

 water deficit in the air and little tendency for the droplets of 

 sputum to disappear by evaporation. In India, on the contrary, 

 with a temperature ranging around 30° C, there is usually a 

 large water deficit in the air and hence the droplets of sputum 

 would tend to disappear quickly by evaporation, thus leading 

 to the death of the contained plague bacilli by drying. 



According to curves given in the Report of the International 

 Plague Conference, the temperature at Harbin during the course 

 of the epidemic ranged between —9° C. and —32° C. and the 

 humidity between 61 and 92. At —10° C, the vapor tension 



^ See II, p. 137 of this report. 



