INHIBITORY MEANS. 651 



Of great practical importance also is the cessation of Chemical 

 development as the result of the addition of small quan- p( 

 tities of active chemical and specifically poisonous mate- 

 rials to the nutrient substrata. Here also it is not easy to 

 ascertain precisely the value of these means, because they 

 behave very differently according to the composition of the 

 nutrient substrata. This difference chiefly depends on Difference in 

 the fact that when these chemical materials are added according to 

 to the nutrient substrata, chemical decompositions fre- ]J e na ^ u r e f 



the nutrient 



quently occur, as the result of which a portion of the substratum. 

 disinfecting substance is destroyed or rendered inactive. 

 Hence it is clear that definite values of the inhibitory 

 means can only be laid down in the case of one and the 

 same nutrient solution, while the values will differ where 

 the composition of the nutrient substratum varies. For 

 example, Boillat* ascertained that in albuminous sub- 

 strata chloride of zinc and other metallic salts only 

 produced a disinfecting action after all the albumen was 

 precipitated by them, and when a sufficient excess 

 remained in the solution for purposes of disinfection. 

 Further, as mentioned above, various species of fungi 

 behave differently towards noxious agencies, and in the 

 same species the degree of the action always depends on 

 the other conditions of life which may be present at the 

 same time. Hence a generally applicable scale of the 

 value of disinfecting substances cannot be given ; and in 

 like manner trustworthy results are only obtained from 

 those experiments which have been made with known 

 and well- characterised species of bacteria. 



Of the figures which have been as yet published we Experiments 

 may mention here only a few, some of which, however, ^th bacteria* 



have not been obtained by the employment of all the in mea t in- 

 above-mentioned precautions. De la Croix tested the 

 bacteria of meat infusion (that is to say, not a pure 

 cultivation of a single species), and found that their 

 development in meat infusion ceased when the following 

 substances were present in the degree of concentration 

 mentioned below : 



* Journ.f. prakt, C/iem., N.F., vol. xxv. 



