16 



is harmless because it is absorbed and desti-oyed by the blood- 

 cells, and injurious because this does not happen; or at least that 

 it becomes harmless if the destruction by the blood-cells takes 

 place more rapidly and to a greater extent than the growth and 

 multiplication of the bacillus, the converse being also time." If in 

 the case of frogs, the temperature be raised, "thus favouring the 

 growth of the bacilli and at the same time lowering the vitality of 

 the cells, the bacilli grow and penetrate into the circulating blood." 

 If, however, the bacilli when introduced into the blood or 

 into the tissues are not destroyed by the colourless blood-cells or 

 otherwise, but are such as are able to live in the blood or tissues 

 of living animals, what is the nature of the change which they 

 produce 1 It appears that among the products which result from 

 the decomposition of organised matter and are associated with the 

 growth of bacteria, certain chemical poisons appear, which are 

 capable of destroying animal life. We have long been familiar 

 with the fact that the micrococcus or bacillus lacticus is the 

 agent by which the lactic acid fermentation of the sugar con- 

 tained in milk takes place, and that this lactic acid being 

 neutralised with lime or other substance, will, after a time, 

 through the agency of another micro-organism, probably the 

 bacillus amylobacter, undergo a further change or fermentation, 

 and be converted into butyric acid, with the simultaneous evolu- 

 tion of carbonic acid and hydrogen. 



(OH 

 2C,H^-^ = C3H, . COOH + 2C0, + 2H,. 



(COOH butyric acid 

 lactic acid 



And there are other fermentations of a similar character. 

 But it is only in recent times that it has been discovered that, as 



