288 APPLIED BACTERIOLOGY 



the resistance of organisms varies in the way which we 

 have indicated; and if the resistance of organisms from 

 a particular culture — say of anthrax — were examined, the 

 result would not alone fail to be necessarily true of other 

 organisms, such as typhoid, but would probably not be 

 correct for the other races of anthrax, or for the same 

 culture at a later stage, or in altered conditions of dryness. 

 There is at present no means available for defining an 

 exact standard by which the resistance of organisms to 

 disinfection can be measured. In order, therefore, to 

 obtain some trustworthy datum as to the action of a dis- 

 infectant upon a given species of organisms, it is desirable, 

 at the same time as observations are made upon the dis- 

 infectant under examination, to determine the strength and 

 time of exposure required for the disinfection of the par- 

 ticular race on which the examination is conducted when 

 subjected to other common disinfectants. For this purpose 

 it is most convenient to expose the organisms to 1 in 1,000 

 solution of perchloride of mercury, and to a 5 per cent, 

 solution of carbolic acid, both at a temperature of about 

 15° C, and to water or steam at 100° C. The times of 

 exposure respectively necessary for disinfection by these 

 three means will be serviceable data for estimating the 

 degree of resistance offered by the particular specimens 

 used. In the exposure of 100° C, care must be taken that 

 that temperature is exactly reached, as a part of a mass of 

 water may be at boiling-point while other parts are sub- 

 stantially below it. The method of exposure to the per- 

 chloride and carbolic solutions is the same as that next 

 recommended for use with the disinfectant under examina- 

 tion, except that the strength of solution, and not the time 

 of exposure, is kept constant. 



It is more convenient, however, to determine the degree 

 of concentration in which the disinfectant under examina- 



