INTRODUCTION 13 



the alimentary tract, and consequently exercises different 

 effects when given by the mouth than when injected. 

 Similarly, zinc is not at all readily absorbed. 



The case of the action of the heavy metal salts is, how- 

 ever, more complex, and attempts have been made to 

 account for their general irritant effect in the light of the 

 theory of ionic dissociation. 



When the salts of the heavy metals — e.g., lead, silver, 

 mercury, copper, zinc — come into contact with the proteins 

 of the cells and cell walls, for the most part they form an 

 albuminate of the metal, which is insoluble, or soluble only 

 in excess of albumin. 



The general reaction for an albumin of feebly acid 

 character would be, taking the case of a sulphate — 



Metal, SO4-1-H2 alb. .^rn?" Metal-albuminate + 2H+ + SO4 ; 



i.e., there would be formed metal-albuminate and sulphuric 

 acid, more or less completely ionised according to concen- 

 tration, the whole being a reversible reaction, since the 

 albuminate is decomposed by excess of acid. The irritant 

 effect is ascribed to the acid thus liberated. 



Irritant effects cannot be held to be solely due to the 

 degree of ionic dissociation. Were this alone responsible, 

 the greater the ionization the greater the irritation, in 

 accordance wherewith the alkali nitrates, chlorides, and 

 sulphates ought to be more irritant than the corresponding 

 salts of copper, which is not the case, although ionic dis- 

 sociation is more complete with the former than the latter. 



A possible explanation of some value is to be found in 

 the fact that, with the heavy metal salts as a class, a 

 different type of dissociation occurs. That is liydrolytic 

 dissociation, which means that water decomposes the salt 

 into oxide and acid, as may be illustrated by reference to 

 zinc chloride ; thus — 



ZnCl2 + 2H20 < — > Zn(OH)2-h2HCl. 



The extent or degree of hydrolytic dissociation is gener- 

 ally small, and the reaction is reversible — i.e., for every 



