CONDITIONS AFFECTING BACTERIAL MOT1L1TY. 25 



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 the 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 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 hand, in the bacillus of symptomatic anthrax, move- 

 ment continues while sporulation is progressing. Under 

 ordinary circumstances motile bacteria appear not to be 

 constantly moving but occasionally to rest. In every case 

 the movements become more active if the temperature be 

 raised. Most interest, however, attaches to movements 

 which, from the use of an unscientific terminology, are 

 often described as if they were purposive, that is when the 

 bacilli are attracted to certain substances and repelled by 

 others. Schenk, for instance, observed that motile bacteria 

 were attracted to a warm point in a way which did not 

 occur when the bacteria were dead and therefore only 

 subject to physical conditions. His method was to intro- 

 duce the upturned point of a copper rod into a drop of 

 fluid containing the bacteria and suspended from the lower 



