147 



a. Covers thrown into stock 577 — 12 tubes. 



Result: All clear to the end of the experiment. 



h. Covers thrown into stock 579 — 10 tubes. 



Result: No clouding in any of the tubes. 



Conclusion: In each case the transfers from potato did better than 

 those from beef broth. The dilute bouillon appeared to be a more 

 favorable medium than the concentrated. Ps. pJiaseoli seems to be 

 less resistant to dry air than Ps. campestris. 



Resistance to Sunlight. 



The writer's experiments have not been very numerous, and the 

 shortness of exposure absolutely necessary for the destruction of the 

 organisms is not known, but probably it is considerably less than 

 the time given below. The tests were made in poured plates of nutri- 

 ent agar, which was inoculated copiously. The exposures were made 

 in very thin-bottomed Petri dishes lying bottom up on larger Petri 

 dishes filled with pounded ice. The exposures were made in Wash- 

 ington in May. One-half of each plate was covered by several folds 

 of thick paper and the other half exposed to the unclouded sun. A 

 good-sized drop of well-clouded fluid was used in making each inocu- 

 lation, i. e., many thousands of the bacteria. The bacteria in the 

 covered portion of the dishes developed normally (except near the 

 margin of the paper covering) as a dense uniform sheet of crowded 

 small colonies. On the exposed part of each plate, and for some milli- 

 meters beyond, nearly all of the bacteria were destroyed. The few 

 that remained were tard}^ in development and undoubtedly owed their 

 escape to the protecting shade of less fortunate individuals. The 

 exposed plates were as follows: 



1. Ps. hyacinthi. — 30 minutes' exposure; all killed. 



2. Ps. hyacinthi. — 45 minutes' exposure; all killed. 



3. Ps. campestris. — 30 minutes' exposure; 95 per cent killed. 



4. Ps. campestris. — 45 minutes' exposure; 98 per cent killed. 



5. Ps. phaseoli. — 30 minutes' exposure; 98 per cent killed. 



6. Ps. jpJiaseoli. — 45 minutes' exposure; all killed. 



The exposure was at midday. The temperature of the plates during 

 the experiment ranged from 24^ to 27^ C, i. e., was held down satis- 

 factorily by the ice. In each case a considerable portion of the bacteria 

 were killed under that part of the cover nearest to the exposed por- 

 tion, i. e., over a width of one-fourth inch or more. On this part the 

 colonies developed slowly at first, and, being fewer, had more room to 

 grow, and became larger than on any other portion of the covered part 

 of the plates. The covered part of each dish was turned south, i. e., 

 toward the sun. 



Stewart found that exposure of Ps. sttvjartl in a portion of an agar 



