24 GENERAL MORPHOLOGY AND BIOLOGY. 



spore-free condition will survive a temperature of 57 C., if 

 long enough applied. Many organisms lose some of their 

 properties when grown at unnatural temperatures. Thus 

 many pathogenic organisms lose their virulence if grown 

 above their optimum temperature, and some chromogenic 

 forms, most of which prefer rather low temperatures, lose 

 their capacity of producing pigment, e.g., spirillum rubrum. 

 Some organisms which can grow at a temperature of 

 from 60 to 70 C. have been isolated from dung, the 

 intestinal tract, etc. These have been called therm op hi/ic 

 bacteria. 



Effect of Light. Of recent years much attention has been 

 paid to this factor in the life of bacteria. Direct sunlight is 

 found to have a very inimical effect. One observer found that 

 an exposure of dry anthrax spores for one and a half hours 

 to sunlight killed them. When they were moist, a much 

 longer exposure was necessary. Typhoid bacilli were killed 

 in about one and a half hours, and similar results have been 

 obtained with many other organisms. In such experiments 

 the thickness of the medium surrounding the growth is an 

 important point. Death takes place more readily if the 

 medium is scanty or if the organisms are suspended in 

 water. Any fallacy which might arise from the effect of 

 heat rays of the sun has been excluded, though light plus 

 heat is more fatal than light alone. In direct sunlight it is 

 chiefly the green, violet, and, it may be, the ultra-violet rays 

 which are fatal. Diffuse daylight has also a bad effect 

 upon bacteria, though it takes a very much longer exposure 

 to do serious harm. A powerful electric light is as fatal as 

 sunlight. Here, as with other factors, the results vary very 

 much with the species under observation, and a distinction 

 must be drawn between a mere cessation of growth and the 

 condition of actual death. 



Conditions affecting the Movements of Bacteria. In 

 some cases differences are observed in the behaviour of motile 

 bacteria, contemporaneous with changes in their life history. 

 Thus, in the case of bacillus subtilis, movement ceases 

 when sporulation is about to take place. On the other 



