DISSOCIATION OF SALTS 85 



sight to be only a special case of the toxicity of all mercury salts, and formerly 

 it was thought that they were equally poisonous if only equal amounts of the 

 metal were present. Equimolecular solutions therefore were supposed to be 

 necessarily of equal disinfectant power. Recent investigations, however (55), 

 based upon the modern theory of solutions, have proved this to be wrong, 

 and rendered it probable that the toxicity of a poisonous salt varies with 

 the degree of dissociation. The dissociation theory (56) has shown that 

 in a solution of a salt there are present not only the unaltered molecules of 

 the dissolved substance, but also a certain number of disintegrated or ' dis- 

 sociated ' molecules. In a solution of HgCl 2 , for instance, a certain propor- 

 tion of the HgCl 2 molecules are split up into their electrically active 

 components or ' ions,' the positive metallic ions (kations) Hg, and the 

 negative ions (anions) Cl. The degree of dissociation, that is to say, 

 the proportion of dissociated molecules to unchanged HgCl 2 molecules, 

 changes with the concentration of the solution, the temperature, and 

 other conditions, and different salts of the same metal are dissociated in 

 different degrees. Now many of the physical properties of a solu- 

 tion, such as electrical conductivity, freezing-point, boiling-point, and 

 osmotic pressure, depend upon the degree of dissociation, and it seems very 

 probable that toxicity does also. Since different salts of mercury are very 

 differently dissociated in watery solutions, we might expect that their toxic 

 powers would be different also, and as a matter of fact we find this to be 

 the case. A sixteen-litre solution (1-58 per cent.) of mercury cyanide, 

 a salt which is but very slightly dissociated, does not kill staphylococci in 

 three minutes, whilst a solution of corrosive sublimate only a quarter the 

 strength (i. e. sixty-four litres, 0-4 per cent.) does. Anthrax spores left for 

 twenty minutes in this solution of sublimate were almost all killed (seven 

 colonies grew), whilst after a sixteen-litre solution of the cyanide, acting for 

 the same time, innumerable colonies sprang up. 



A comparison of differently dissociated mercury salts shows clearly, 

 therefore, the relation between dissociation and toxicity. The connexion 

 becomes more striking still when we compare the same salt in different 

 degrees of dissociation. 



The proportion of the dissociated molecules to the unaltered molecules 

 in a given solution of a salt is constant. For example in a solution of HgCl 2 

 the proportion of Cl ions to unchanged Hg C1 2 molecules is constant. By 

 adding other chlorine ions, for instance by adding the more strongly disso- 

 sociated NaCl, the number of dissociated Hg C1 2 molecules can be reduced, 

 the reduction being dependent on the ratio between the amounts of disso- 

 ciation of HgCl 2 and of NaCl. Let us suppose a sixteen-litre solution 

 of Hg C1 2 to contain x Cl ions and y unchanged molecules, then, broadly 



3C 



speaking, - = c (a constant). If, now, there be added enough solid NaCl, 



