INFLUENCE OF ATMOSPHERIC TEMPERATURE. 



163 



air. Those organisms possess the following advantages for these 

 experiments: (1) They are harmless, (2) their colonies on agar 

 are readily recognized on account of the characteristic pigment 

 production, and (3) they differ considerably in their resistance 

 to death by drying, the prodigiosus being killed more readily 

 than the sarcina. In a few experiments the cholera vibrio was 

 used ; this organism is much more readily killed by drying than 

 is B. prodigiosus. The following experiment demonstrates the 

 relative resistance to death by drying of the three varieties of 

 bacteria just mentioned and of the plague bacillus. 



EXPERIMENT NO. 1. 



Suspensions in 0.5 per cent sodium chloride solution were made from 

 fresh cultures of sarcina, B. prodigiosus, plague, and cholera. The sus- 

 pensions were passed through filter paper with the exception of that of the 

 sarcina, which was filtered through cotton. Carefully cleaned slides were 

 sterilized in the hot-air sterilizer and allowed to cool to room temperature. 

 Pledgets of cotton were soaked in the suspensions, squeezed out thoroughly, 

 and quickly rubbed over the surfaces of a series of the sterile slides. The 

 slides were placed at intervals face down upon solidified agar in Petri 

 dishes and brought into close contact with the agar by gentle pressure. 

 After the first few minutes had elapsed, the remaining slides of the series 

 were placed face down upon a sterile wire-netting frame in a box which 

 was covered with a sheet of blotting paper. This was done to reduce the 

 number of contaminating air-organisms upon the slides which were ex- 

 posed for long intervals. 



Each slide was left upon the agar for an hour or longer and then moved 

 back and forth a few times over the surface and finally transferred to a 

 second Petri dish. It remained in the second Petri dish overnight and was 

 then removed. The number of colonies that developed on the first plate 

 gave an indication of the number of bacteria that were alive on the slide, 

 but the second plate merely furnished information as to whether or not 

 living bacteria were present. 



The result of one such experiment will be recorded in full. 



Cholera suspension on slide. 



(1,200,000,000 per cubic centimeter.) 



Time 

 exposed. 



Plate No. 1. 



Plate No. 2. 





Colonies. 





At once. 



1,000 



Positive. 



Min. 







i 



5 



Positive. 



1 







Negative. 



2 







Neg-ative. 



3 







Contaminated. 



4 







Negative. 



6 







Negrative. 



8 







Negative. 



10 







Negrative. 



