THE ENVIRONMENT OF BACTERIA. 75 



istics, such as pigment production, thickness of the growth, 

 etc. A constant low temperature of 16 18 is as desirable 

 as a constant high temperature in an incubator. It can 

 readily be obtained by the apparatus shown in Fig. 33, 

 Chapter VII, or by a properly constructed ice-chest. 



Light. 



The higher plants, in the presence of sun-light, are 

 able to assimilate carbonic acid. The relatively few, 

 colored bacteria are likewise favored by an exposure to 

 sun-light. Apart from these few organisms it may be said 

 that direct sun-light exerts an unfavorable action on all 

 bacteria. Naturally, some species will be less affected 

 than others. Moreover, the vegetating form will, as a 

 rule, be more easily destroyed than the spore form. A few 

 hours' exposure to the direct sun usually destroys vegetat- 

 ing bacteria. 



The action of sun-light may be exerted upon the 

 medium as well as upon the organism. This is seen in the fact 

 that sterile bouillon, or urine, after an insolation of some 

 hours, will not permit a development of certain organisms. 

 The change that has taken place in the soil seems to inhibit 

 the germination of spores more readily than a multiplica- 

 tion of the organism itself. Clearly, therefore, a pro- 

 longed exposure of a given medium to sun-light changes, in 

 some way, its chemical composition. Similar alterations 

 will be met with when strongly alkaline media are heated 

 in an autoclave for some time at 120. The action of sun- 

 light, or of heat, may cause an oxidation of the fats or 

 sugars present giving rise to acid products which change 

 the reaction of the medium and consequently render it use- 

 less. Moreover, oxidation products may form, like formic 

 acid, or formaldehyde, which exert a marked antiseptic 

 action. Apparently the most important antiseptic that 



