86 CHEMO TAXIS AND CHEMICAL DISINFECTION 



which is more readily dissociated than Hg C1 2 , to make a sixteen-litre solu- 

 tion, there will be present x + m Cl ions derived from the Na Cl. For the 

 pure HgCl 2 solution x = cy, but for the HgCl 2 plus NaCl solution 



*t _ ^ / 



x + (x + m] = cy or x -. In other words, the number of Cl ions 



2 



derived from the Hg Cl 2 is reduced, its degree of dissociation is lessened, 

 and some of the free Hg and Cl ions are built up again into complete 

 molecules. The following table shows that as the degree of dissociation 

 is lessened the toxicity is reduced. The figures give the numbers of 

 colonies that arose from approximately equal quantities of spores after 

 these had lain for six minutes in the solutions (55). 



Number of Colonies. 



1 6 litre HgCl. 2 solution .... 8 



plus I NaCl . . 32 



2 



3 



;j )> 4 382 



5 j 4-6 . 410 



j> 6 ,, 803 



jo 1,087 



The gradual diminution in germicidal power is unmistakable and needs 

 no comment. 



These facts must be borne in mind in experiments on the disinfection, 

 by sublimate, of liquids such as serum or bouillon which contain 0-7 percent. 

 (eight litres) of NaCl. It is evident that more sublimate will be necessary 

 than in fluids free from salt. The presence of peptone or albumen will 

 necessitate a still further addition of sublimate, because some of it will form 

 insoluble compounds with the proteids whereby its toxicity will be reduced. 



Since the degree of dissociation is dependent upon the nature of the 

 solvent and also upon the temperature of the solution, these factors are of 

 influence on the disinfecting power. The increase of toxicity which accom- 

 panies increased temperature is not, however, solely the effect of augmented 

 dissociation. 



Although the technique of disinfection has not as yet been affected by 

 these recent discoveries, it does not need much penetration to see their high 

 scientific interest and importance. 



There are many other bodies besides those mentioned that could be 

 used for disinfecting purposes anilin dyes, such as methyl violet, ethereal 

 oils, and many aromatic compounds. New disinfectants spring up every day, 

 and are widely advertised, only to disappear again very soon. 



Gases, such as carbonic acid, carbonic oxide, hydrogen, nitrous oxide, 

 nitric oxide, sulphuretted hydrogen, sulphurous acid, and coal gas, arrest 

 the growth of bacteria in agar cultures exposed to a slow stream of the 

 gas, but the method has no practical value. The ozone in the air, too, 



