20 GENERAL MORPHOLOGY AND BIOLOGY 



found to have a very inimical effect. It has been found that 

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

 to sunlight kills them. When they are .moist, a much longer 

 exposure is necessary. Typhoid bacilli are 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 ex- 

 posure 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. Some bacteria, especially occurring 

 on the dead bodies of fresh fish, are phosphorescent. 



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, movement 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 the fact that 

 bacilli may be 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. Most important observations have been 

 made on the attraction and repulsion exercised on bacteria by 

 chemical agents, which have been denominated respectively 

 positive and ' negative chemiotaxis. Pfeffer investigated this 

 subject in many lowly organisms, including bacterium termo 

 and spirillum undula. The method was to fill with the agent 

 a fine capillary tube, closed at one end, to introduce this into 

 a drop of fluid containing the bacteria under a cover-glass, and 

 to watch the effect through the microscope. The general result 

 was to indicate that motile bacteria may be either attracted or 



