THEIR FORMATION 15 



ions being formed. Salts of a strong acid and a weak 

 base are so decomposed, and the solution contains 

 a free acid and a residual salt, which may be a basic 

 or a normal salt. Mercuric sulphate is so decom- 

 posed.* 



Dissociation takes place in colloidal solutions just 

 as it does in watery solutions. Levif has shown that 

 potassium iodide is dissociated to the same extent 

 in solution of gelatine, agar-agar or silicic acid, as 

 in aqueous solution. Dissociation also occurs in 

 the salts of the blood, lymph, serum, and other 

 animal or vegetable fluids. Dissociation is produced 

 by heat and in some bodies by a low temperature. 

 Nitrogen tetroxide may become completely disso- 

 ciated at 27 C. Regnault showed by experiment 

 that some bodies which are decomposed by heat into 

 one or more solid bodies and a gas, give off the gas 

 more freely when in the presence of a foreign gas 

 than when exposed to the products of their own 

 decomposition. Chalk loses its C0 2 more freely in 

 air than in an atmosphere of C0 2 ; and hydrated 

 salts give off their water of hydration more freely 

 in an atmosphere of dry air than in one of watery 

 vapour. Similarly, venous blood gives up C0 2 more 

 freely in pure air than in an atmosphere containing 

 much C0 2 , according to the laws of diffusion. 



The formation of ions depends somewhat upon the 

 constitution of the molecules, for many substances 

 which consist mainly of polymerised molecules do 

 not readily become ionised. In order that a sub- 

 stance shall possess electrolytic conductivity its 



* Guinchant : Bull, de Soc. Chim., 1896, iii., 555. 

 f Gazetta, 1900, ii., 64-70. 



