TRANSMISSION OF THE INFECTION. 139 



with bacteria from the air were so extensive, that the separate 

 organisms could not be isolated and studied. In a few of these 

 instances a suspension of the whole growth upon the plate- 

 culture was made, and a portion of the suspension either rubbed 

 over the freshly-scarified abdomen of a guinea pig or inoculated 

 subcutaneously into a mouse. On several occasions in which 

 it seemed hopeless to determine whether the plague bacillus 

 was present or not on the medium in the plate, owing to the 

 extensive contamination of the culture with bacteria other than 

 the plague bacillus, the guinea pig so inoculated died of plague. 

 In some instances the plate-cultures were discarded because of 

 very extensive contamination probably from air organisms which 

 covered the whole surface of the medium with a very thick layer 

 of growth. The ideal method would have been to inoculate 

 guinea pigs by the cutaneous method with light scarification 

 of the abdomen, with suspensions of the bacteriological growth 

 on all those plate-cultures in which the separate colonies could 

 not be isolated, and in this manner, perhaps, in others of these 

 plate-cultures the presence of the plague bacillus might have 

 been demonstrated. There is no more delicate a test for the 

 presence of the plague bacillus than this procedure, and its 

 efficacy is very great even when the few plague bacilli present 

 are extensively overgrown by other microorganisms. Unfor- 

 tunately, our supply of guinea pigs was limited to those we 

 brought with us and none could be obtained in Mukden during 

 the winter. Since we had numerous other experiments to per- 

 form, which also required the use of guinea pigs, we could only 

 allow, while in Mukden, a very limited number for the present 

 study. In the case of all of the organisms which suggested 

 in any way the plague bacillus and the colonies of which had 

 been transplanted to agar slants from the plates, inoculations 

 of guinea pigs were made after our return to Manila. 



During the colder weather in Mukden, the plates containing 

 agar, exposed before plague patients during ordinary respira- 

 tion, were frequently entirely sterile. The plates were usually 

 exposed vertically before the mouth and nose of the patient, 

 the time of exposure varying generally between two and five 

 minutes ; usually the shorter period was employed. In the exper- 

 iments performed in the earlier part of the investigation, the 

 plates were held at a distance of from 5 to 7 centimeters to 90 

 centimeters or 1 meter from the mouth of the patient. Later 

 in the experiments, when it became evident that in cases without 

 cough during exposure no plague bacilli were encountered at 



