xviii.] NON-PATHCGENIC ORGANISMS. 235 



carbonate of ammonium. Solutions containing starch, 

 dextrin, or sugar, infected with the bacillus amylobacter 

 yield, as mentioned in a former chapter, butyric acid. 

 The same bacillus converts glycerine (Fitz) into butyric 

 acid, ethyl-alcohol, &c. Alcohol is oxidised by the 

 presence of a definite micrococcus (Pasteur) into acetic 

 acid. 



A species of minute bacillus subtilis produces from fats 

 butyric acid (Pasteur, Cohn), and many kinds of micro- 

 organisms from pigmentary bodies, e.g. those producing the 

 blue colour of milk. 



What the chemical influence of pathogenic organisms on 

 animal tissues may be is not yet known ; and even when they 

 grow outside the body, i.e. in artificial cultures, it is not yet 

 known what their chemical effect on the nourishing material 

 is, except that, as is the case with all other organisms, putre- 

 factive and pathogenic, they continue to grow and multiply 

 as long as there are present the necessary substances, i.e. 

 until the medium is " exhausted." 



From the enormous number of micro-organisms present 

 in the outer world, it is clear that the role they play in the 

 disintegration of higher organic bodies into lower compounds, 

 as well as in the building up of new compounds, is a very 

 important one ; to mention but one series, to wit, the enor- 

 mous importance they have for the vegetable kingdom in 

 reducing nitrogenous compounds to soluble nitrates of in- 

 organic salts, so essential to the existence and growth of our 

 common field crops (Laws). 



One of the most interesting facts observed in the growth of 

 septic micro-organisms is this, that the products of the 

 decomposition started and maintained by them have a most 

 detrimental influence on themselves, inhibiting their power 

 of multiplication, in fact, after a certain amount of these 



