§ VI.] Conditions of vegetation. Food. 5 7 



slightly alkaline or at most with a slightly acid reaction ; should 

 the reaction be strongly acid, vegetative processes are hindered 

 or wholly stopped. According to Brefeld (19) the development 

 of Bacillus subtilis, for example, is impeded, if o'o5 per cent, of 

 sulphuric or tartaric acid or o'2 per cent, of lactic or butyric 

 acid is added to a good nutrient solution. But this, too, is 

 only a rule which has its exceptions ; the Bacterium of kefir 

 vegetates well, and, as far as our experience goes, best in milk 

 which has been rendered strongly acid by lactic and even acetic 

 acid ; the Micrococcus of vinegar vegetates in the same way in 

 an acid fluid. 



Other soluble bodies also impede or destroy the vegetative 

 process when mixed with the food-material. This is of course 

 the case with substances which always act as poisons upon living 

 cells, such as corrosive sublimate, iodine, &c., when present in 

 sufficient quantity. But other bodies have a similar at least re- 

 tarding poisonous effect on Bacteria. Fitz, for instance, found 

 that the vegetation of his Bacillus of butyl-alcohol in a solution 

 of glycerine and under conditions otherwise most favourable was 

 impeded by the addition of 2-7-3-3 per cent, by weight of ethyl- 

 alcohol, o'9-i'o5 per cent, of butyl-alcohol, or o'l per cent, of 

 butyric acid. Since these prejudicial compounds are often 

 formed by the vegetative process itself, the latter may even be 

 stopped by the accumulation of its own products, as, for instance, 

 in lactic acid fermentation in sugars by the accumulation of 

 lactic acid ; if this is fixed, as by addition of chalk or zinc- 

 white, the vegetation of the Bacterium which causes the fer- 

 mentation continues. These phenomena are also found muialis 

 mutandis, in other plants beside Bacteria, especially in Fungi, 

 and they vary in the individuals of different species. That 

 which disturbs one species may be of advantage to others, and 

 hence a change in the composition of the substratum may 

 favour the supplanting of one species by another, which was 

 previously perhaps present in the very smallest quantity. In 

 such a case the first species has prepared the ground for the 



