BACTERIOLOGICAL TESTING OF DISINFECTANTS 375 



It is unfortunate that, in the majority of investigations 

 which have hitherto been made, the conditions of experi- 

 ment have varied to such an extent as to make it difficult 

 to derive any conclusion from a comparison of their results. 

 In consequence, the most various statements are made as 

 to the disinfectant action of even ordinary substances ; and 

 there can be no doubt that many substances are used as 

 disinfectants in strengths which exercise little or no action. 

 It would, therefore, be of doubtful utility to describe the 

 large amount of experiments which have been made up to 

 now. 



The collation of existing results would involve a minute 

 examination of the differences between the conditions of 

 each set of experiments, and absorb an amount of space 

 and time disproportionate to the result ; and it is probable 

 that in the near future they will undergo substantial 

 revision. It is, however, desirable that the student should 

 realise for himself some method of at least elementary 

 examination which can be applied to any substance which 

 may be suggested as affording disinfectant action. For 

 this purpose it is indispensable to treat separately the 

 various factors which in practice combine to affect the 

 action of the disinfectant. Thus, in practical experience 

 organisms are seldom found without some particles to 

 which they are attached, or by which they are surrounded. 

 The presence of these particles is liable to exercise an 

 important action upon the disinfectant, but their absence 

 still leaves the organism undestroyed. The real question 

 to be determined in the examination of a disinfectant is, 

 therefore, the strength and time of exposure which will 

 enable it to kill organisms in the presence of a relatively 

 definite proportion of standard extraneous matter. Now, 

 the resistance of organisms varies in the way which we 

 have indicated; and if the resistance of organisms from 



