24 GENERAL MORPHOLOGY AND BIOLOGY. 



sulphuretted hydrogen, carbon dioxide, methane, etc. For an 

 exact knowledge of the destructive capacities of any particular 

 bacterium there must be an accurate chemical examination of its 

 effects when it has been grown in artificial media the nature of 

 which is known. The precise substances it is capable of forming 

 can thus be found out. Many substances, however, are produced 

 by bacteria, of the exact nature of which we are still ignorant, 

 for example, the toxic bodies which play such an important part 

 in the action of many pathogenic species. 



Many of the actions of bacteria depend on the production by 

 them of ferments of a very varied nature and complicated action. 

 Thus the digestive action on albumins probably depends on the 

 production of a peptic ferment analogous to that produced in 

 the animal stomach. Ferments which invert sugar, which split 

 sugars up into alcohols or acids, which coagulate casein, which 

 split up urea into ammonium carbonate, also occur. 



Such ferments may be diffused into the surrounding fluid, or 

 be retained in the cells where they are formed. Sometimes the 

 breaking down of the organic matter appears to take place 

 within, or in the immediate proximity of, the bacteria, sometimes 

 wherever the soluble ferments reach the organic substances. 

 And in certain cases the ferments diffused out into the sur- 

 rounding medium probably break down constituents of the 

 latter to some extent, and prepare them for a further, probably 

 intracellular, disintegration. Thus in certain putrefactions of 

 fibrin, if the process be allowed to go on naturally, the fibrin 

 dissolves and ultimately great gaseous evolution of carbon 

 dioxide and ammonia takes place, but if the bacteria, shortly 

 after the process has begun, are killed or paralysed by chloro- 

 form, then only a peptonisation of the fibrin occurs, without 

 the further splitting up and gaseous production being observed. 

 That a purely intracellular digestion may take place is illus- 

 trated by what has been shown to occur in the case of the micro- 

 coccus ureae, which from urea forms ammonium carbonate by 

 adding water to the urea molecule. Here, if after the action has 

 commenced the bacteria are filtered off, no further production of 

 ammonium carbonate takes place, which shows that no ferment 

 has been dissolved out into the urine. If now the bodies of the 

 bacteria be extracted with absolute alcohol or ether, which of 

 course destroys their vitality, a substance is obtained of the 



