198 



curie chloride (1-1000) than when they are suspended in 

 water, and 250 times as much when they are distributed 

 in blood serum. 



(3). The temperature at which the disinfection is 

 made. The higher the temperature at which the experi- 

 ments are made the more rapid and energetic will be the 

 action of the disinfectant. Cholera bacteria are not de- 

 stroyed bymercuric chloride (1-1000) in one hour at 3, 

 whereas at 36 C. they are killed in a few minutes. 



(4). Immediate and thorough contact of all the or- 

 ganisms present with the disinfectant. This can be done 

 perfectly only with bacterial suspensions in which each or- 

 ganism is entirely free and separate from others. To ob- 

 tain such a suspension it is necessary first to filter through 

 glass wool and then to agitate the liquid thoroughly at a 

 temperature of about 37 until microscopical examination 

 shows no aggregations of bacteria. Silk threads which have 

 been soaked in bacterial suspensions and then dried are 

 open to the objection that on exposure to the disinfectant 

 organisms are unequally exposed and some even protected 

 by their position and hence when transplanted soon de- 

 velop. The same objection, to a less degree, applies to 

 cover-glasses on which a thin film of the suspension has 

 been deposited. 



(5). The number of bacteria in a given experiment. 

 It can readily be shown that the greater the number of 

 bacteria present the more slowly does the disinfection 

 take place. In order therefore that results may be com- 

 parable, approximately the same number of organisms 

 should be present in each experiment. This is readily as- 

 certained by diluting a small portion of the bacterial sus- 

 pension witli 1-2000 parts of sterilized water and then 

 making a gelatin plate with one drop of this dilution. 



(6). The amount of the disinfectant which is carried 

 over in each trial inoculation. Thus, when the disinfect- 

 ant is applied to the bacterial suspension and at the end 

 of stated intervals transfers of 1-3 loopfuls of the mixture 



