EFFECT OF DESICCATION. 71 
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- 
iphilus, B. carotovorus, Bact. hyacinthi, etc. ‘Tests may also be made in air dried over 
sulfuric acid or calcium chloride. Harding & Prucha have shown recently that 
Bacterium campestre 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. 61.* Fig. 62.+ 
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 amylovorus (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. : 
+ Fic. 62.—Agar culture of Bacterium phaseoli(Erw. 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. ¢., the plate was sown too thickly. 
