INFLUENCE OF LIGHT. 135 



merit, the vitality of bacteria is destroyed by protracted 

 and violent shaking, which causes a molecular disinte- 

 gration of the cells. 



D' Arson val and Charrin submitted a culture of bacil- 

 lus pyocyaneus to a pressure of fifty atmospheres under 

 carbonic acid. At the end of four hours cultures could 

 still be obtained, but the bacillus had lost its power of 

 pigment production. A few colonies were developed 

 after six hours 7 exposure to this pressure, but after 

 twenty-four hours no development occurred. 



Influence of Light. A large number perhaps the 

 majority of bacteria are inhibited in growth by the 

 action of diffuse daylight, still more by that of direct 

 sunlight, and when the action is prolonged they lose 

 their power of developing when later placed in the dark. 



In order to test the susceptibility of bacteria to 

 light, it is best, according to Buchner, to suspend a 

 large number of bacteria in nutrient gelatin or agar 

 and pour the media while still fluid in Petri dishes, 

 upon which has been pasted a strip of black paper on 

 the side upon which the light is to act. The action of 

 heat may be shut off by allowing the ray of light to 

 pass first through a layer of water or alum of several 

 centimetres' thickness. After the plates have been 

 exposed to the light for one-half, one, one and a half, 

 two hours, etc., they are taken into a dark room and 

 allowed to stand at 20 or 35 C., a sufficient length 

 of time to allow of growth, and then examined, to see 

 whether there are colonies anywhere except on the spot 

 covered by the paper; when the colonies exposed to the 

 light have been completely destroyed there is a sharply 

 defined region of the shape of the paper strip crowded 

 with colonies lying in a clear sterile field. 



