INFLUENCE OF LIGHT. 135 
ment, the vitality of bacteria is destroyed by protracted 
and violent shaking, which causes a molecular disinte- 
gration of the cells. 
D’ Arsonval 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’ 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, ete., 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. 
