PHYSIOLOGY OF THE MICROBES 85 



than bacilli. The spores of Bact. butyricum are scarcely affected 

 by the action of air for 265 days, whereas the growth of the 

 bacillus is inhibited by the action of air for fifteen hours. 



Oxygen is a food which bacteria take from compounds 

 liberating it more or less readily ; anaerobes take it from com- 

 pounds which retain it and resist decompositions. Although it 

 is not absolutely true that fermentation is life without air (there 

 are fermentations which go on in presence of oxygen), it is true 

 that anaerobiosis favours the majority of fermentations and is 

 the usual condition for these. 



In nature, anaerobes occur wherever there is little penetra- 

 tion of air, or where the air is diluted or replaced by other 

 gases, as, for example, in the earth, in mud and slime, in sewage, 

 in the ooze of the sea, in dunghills, and in the intestines and 

 excrements of animals ; and it is in these surroundings that the 

 most important fermentations and putrefactions of organic 

 matter take place. 



Respiration of the Pigmented Bacteria. The 

 purple bacteria (a certain number of which are also sulpho- 

 bacteria) contain a ^igmQn^bacterio-purpurine, quite distinct from 

 the pigment of Bacillus prodigiosus. According to Engelmann, 

 these pigmented bacteria absorb the infra-red rays of the 

 spectrum (of wave-length 0*8 to 0*9 /x) and employ them, as also 

 the red rays, in the decomposition of carbonic acid from the 

 air and the liberation of oxygen, just as plants do with chloro- 

 phyll ; they have a " chromophyll " function analogous to the 

 chlorophyll function of green plants. This opinion is not 

 shared by all observers : according to Molisch, the purple 

 bacteria are not capable of decomposing CO 2 nor of assimila- 

 ting directly inorganic compounds. They certainly differ from 

 other bacteria in their power of using light in their nutritive 

 process, but their foods are still organic food-stuffs ready made 

 and they cannot do without these : they are not capable of the 

 synthetic function of the green plants. They can assimilate 

 organic food-material in the dark like other bacteria, but 

 they have advanced a step by adapting themselves to light 

 and by using it to increase their nutritive resources. But 



