CHAPTER VIII 



INFLUENCE OF PHYSICAL AGENTS 



Light, Electricity ; Pressure, Temperature, Dryness and Moisture ; 

 Disinfection by means of Physical Agents. 



THE Erythrobacteria are the only forms in which nutrition is influenced 

 by light, as in the higher plants. It is the predilection for light which 

 causes Chromatium and other motile species to seek out the brightest parts 

 of the vessels they are kept in, forming a reddish coating on the side of the 

 glass. 



All other pigment bacteria, most of which are only chromoparous, are 

 quite destitute of the power to assimilate and split up CO 2 , although they 

 naturally absorb certain rays of light. The pigment is in fact only an 

 ' accidental ' by-product of bacterial life ; it is formed just as abundantly in 

 the dark as in the light, and as a result none of the chromogenic bacteria 

 become bleached or ' etiolated ' when light is excluded. This alone shows 

 that the pigment does not play the part of the chlorophyll of higher plants. 

 But whilst bacteria grow just as well in darkness as in weak diffused daylight 

 it is necessary that the illumination should remain within certain limits, the 

 overstepping of which leads, in our cultures at all events, to the injury and 

 death of the cells. Very numerous experiments have been made to show 

 the deleterious effect of light on bacteria (39), but it must not be forgotten 

 that the conditions under which bacteria occur in nature are not those of 

 our artificial cultures, be they on solid media or liquid. In the transparent 

 gelatine and agar it is impossible for them to obtain any shade whatever, 

 whereas in nature, in ponds and rivers for instance, the minutest algal cell 

 or particle of mud offers abundant shelter. It seems therefore certain that 

 diffuse light in nature is innocuous, and cvdh bright sunlight can only 

 destroy a limited number, and drive the motile forms to seek shelter from 



