EFFECT OF DESICCATION. 



Organisms believed to be non-sporiferous show great differences, some being 

 killed by an exposure of a few minutes or a few hours, while others remain alive for 

 many weeks. For further information see the special chapters on Bacillus trache- 

 ipliilus, B. carotavorus, Bad. hyacinthi, etc. Tests may also be made in air dried over 

 sulfuric acid or calcium chloride. Harding & Pmcha have shown recently that 

 Bacterium campcstre remains alive much longer when dried on cabbage seed than 

 when dried on glass cover-slips. In their experiments this organism was dead on 

 glass at the end of ten days, but alive on seed at the end of thirteen months. 



EFFECT OF DIRECT SUNLIGHT. 



The exposures should be made in a thin stratum of nutrient agar, not sowed 

 too thickly (there may be several hundred colonies on the plate, if properly distrib- 

 uted), in thin-bottomed Petri dishes, to an unclouded sun for 5, 10, 15, 30, 45, and 60 

 minutes, a portion of the bottom of the plate, which is placed uppermost, being 

 covered by some substance impervious to light, such as several folds of Manila paper 





Fig. 6 1* Fi g .62.f 



or of the black paper which comes wrapped around photographic dry plates, covered 

 in turn by white paper. Exposures of several hours are not recommended. If the 

 layer of agar is very deep, or if the sowings are too thick, some organisms will screen 

 others and all will not be killed. Ten cubic centimeters is a proper amount of agar to 

 use for a plate having an area of 60 square centimeters. The latitude, altitude, time of 

 year, time of day, and intensity of the light should also be recorded. In the summer- 

 time it is very important that the exposures should be made on blocks of ice or, 



*Fic. 61. Gelatin culture of Bacillus omylovorus (Burrill) Trev. in a Petri dish. Exposed in 

 1896 to direct sunlight for four hours on ice after covering portions of the plate with pasteboard 

 figures. The bacteria grew only under the protected parts. Drawn from a photograph made after 

 five days incubation of the culture at about 24 C. The temperature of the gelatin during exposure 

 was about 25 C. Three-fifths natural size. 



fFic. 62. Agar culture of Bacterium fhascoliCErw. Sm.) in a Petri dish. Right one-half ex- 

 posed to direct sunlight for thirty minutes, on ice, the other half protected by several folds of Manila 

 paper. Dish then set away in the dark for several days. One-half natural size. The scattering 

 colonies on the right side undoubtedly grew from bacteria which were sheltered from the direct 

 rays of the sun by overlying organisms, i. e., the plate was sown too thickly. 



