TEXT-BOOK OF BACTERIOLOGY. 21 



tion of litmus to some test-tubes containing 1 nutrient gelatin, and 

 then planting- them with different micro-organisms. 



It will be seen by comparison with a tube containing litmus 

 alone, without any bacteria, that the coloring matter will be de- 

 cidedly altered in nearly all of them by the growth of the bac- 

 teria. 



In this way, and also by other more accurate proceedings, it can 

 be shown that some micro-organisms produce very considerable 

 quantities of acid, others of alkali. Ammonia and the higher bases, 

 such as trimethylamin, further, also, sulphuretted hydrogen, etc., 

 have been detected as immediate products of the vital action of 

 bacteria. We are particularly indebted to the successful re- 

 searches of Brieg-er for the knowledge of some alkaloidal substances 

 of very complicated combination (but chemically well defined) 

 which are reg-ularly produced by certain bacteria. This gives us 

 an important element for judging- the pathogenic or disease-pro- 

 ducing importance of these same micro-organisms. 



We know that many bacteria, especially those of a saprophytic 

 nature, and some parasitic ones also, have a very considerable 

 power of reduction: they change, for instance, nitric into nitrous 

 combinations, or even into ammonia. Others are said to have an 

 opposite action and to produce oxidation. 



In what has just been said, the action of the bacteria has been 

 presented in certain details, but it is rendered more tangible and 

 interesting if we consider it from a more general point of view. 



It has already been stated that we study the bacteria princi- 

 pally and almost entirely because it has been proved that they are 

 the cause of a whole series of phenomena which are of deep and 

 wide-spread importance in the economy of nature. The micro- 

 organisms are, in the first place, the exciters of fermentation. 



It was Pasteur who, in opposition to the prevailing 1 opinion of 

 his time, first maintained the doctrine of the vital principle of fer- 

 mentation, and proved that this important and for us, in some re- 

 spects, indispensable process is brought about by the action of cer- 

 tain micro-org-anisms. Many of them, especially those chiefly con- 

 cerned in alcoholic fermentation, do not exactly belong to the bac- 

 teria; but there are genuine exciters of fermentation among the 

 bacteria, as, for instance, those which cause the formation of lactic 

 and butyric acid. 



It is further to be noted that the known varieties of fermenta- 

 tive action which lead to different final results g-enerally owe their 

 origin to different species of micro-organisms. 



Still more important, perhaps, is the service rendered by bac- 



