BACTERIA. 79 



barrel-shaped during the formation of spores. In nutrient 

 gelatine it forms colonies which, when seen in transmitted 

 light, appear blackish-brown or black. The second species 

 consists of strongly-curved vegetative rods, at the end of which 

 the spores are formed ; it forms yellowish or yellowish-brown 

 colonies. The third species is also capable of growth and 

 of causing fermentation in the absence of oxygen ; its develop- 

 ment is, however, decidedly assisted by the presence of oxygen, 

 and it is only then able to produce spores. The vegetative rods 

 are cylindrical ; with the formation of spores the rods become 

 spindle-shaped, and in the centre of the spindle the large spore 

 is formed. The colonies in nutritive gelatine are of a yellow 

 colour. All three species form butyric acid and butyl alcohol. 



A very interesting butyric acid bacterium is the Clostridium 

 Pasteurianum, closely studied by WINOGRADSKY, which pos- 

 sesses the power of directly absorbing and causing the combina- 

 tion of the free nitrogen of the air, whereas combined nitrogen 

 has no nutritive value for this bacterium. 



In imperfectly sterilised milk BAIER and WEIGMANN dis- 

 covered two species of butyric acid bacteria, which have their 

 optimum at 30C., and are not strictly aerobic. They dissolve 

 the caseine of milk in the course of some days, and impart to 

 the liquid an objectionable, putrid odour. 



According to FITZ the spores of butyric acid bacteria can 

 withstand the temperature of boiling water for a period of time 

 which is naturally dependent, as in all cases, on their condition 

 and on the nature of the substratum ; FITZ gives three to 

 twenty minutes as the limits. They can, however, be killed by 

 a lower temperature, if continued long enough ; thus they are 

 killed by being heated for six hours at 90 C. in a solution of 

 grape-sugar ; but in glycerine, at the same temperature, only 

 after six to eleven hours. 



Thus butyric acid fermentation, like lactic acid fermentation, 

 is not produced exclusively by one species. When butyric acid 

 fermentation occurs in distilleries, breweries, and pressed-yeast 

 factories, bacteria are frequently found which are entirely 

 different from those described above. 



Clostridium butyricum, and various other species, are capable 

 of dissolving cellulose, and therefore play an important part in 



