XVIIL] NON-PATHOGENIC ORGANISMS. 171 



tion, i.e. oxidation and splitting up of sugar into alcohol and 

 carbonic acid. 



The bacterium lactis introduced into substances containing 

 lactic sugar, moist, or grape sugar, produces by oxidation a con- 

 version of the sugar into lactic acid and carbonic acid (?). A 

 micrococcus (see a former chapter) produces, according to 

 Pasteur, the conversion of dextrose into a sort of gum, called 

 by Bechamp viscose, and recognised by Pasteur as the cause of 

 the viscous change of wine and beer. The urea in the urine 

 is converted by the micrococcus urese (Pasteur) into 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 form 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 enormous 

 importance they have for the vegetable kingdom in reducing 

 nitrogenous compounds to soluble nitrates of inorganic salts, 

 so essential to the existence and growth of our common field 

 crops (Lawes). 



One of the most interesting facts observed in the growth of 

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

 composition 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 

 products has accumulated, the organisms become arrested in 

 their growth, and finally may be altogether killed. Thus the 

 substances belonging to the aromatic series, e.g. indol, skatol, 



