ON THE PHENOMENA AND THEORIES OF SOLUTION. 



4o9 



enormously at the melting point, and sometimes at lower temperatures, 

 and the observations of Thorpe and Watts (' Chem. Soc. Journ.' 1880, 

 p. 102) on the specific volume of water of crystallisation in the sulphates 

 of the so-called magnesium group show that, whilst the constitutional 

 water, or water of halhydration, as it was called by Graham, occupies less 

 space than the remaining molecules, each successive additional molecule 

 occupies a gradually increasing volume. 

 The following are mean specific volumes of 



MSOinHaO, where M is Ca, Mg, Zn, Ni, Co, Mn, or Fe. 



which is 



18 

 92 



These last figures approach the specific volume of ice 

 =19*6, that of water being 18. 



So that, when a salt with its water of crystallisation passes into the 

 liquid state, either by melting or solution in water, it requires a very 

 slight relaxation of the bonds which hold the water to the salt for it to 

 acquire the full volume of liquid water, whilst the water of constitution 

 is not so easily released. And this conclusion accords with Nicol's 

 observations on the molecular volumes of the salts when in solution. 



Chemical Constitution. — Fusibility. — Molecular Volume m Relation 



TO Solubility. 



And now comes the question as to what determines the solubility of a 

 substance. Why, for example, is magnesium sulphate very soluble in 

 water whilst barium sulphate is almost totally insoluble ? 



With regard to salts the following propositions seem to be true : — 



1. Nearly all salts which contain water of crystallisation are soluble 

 in water, and for the most part they are easily soluble, calcium sulphate 

 being one of the least soluble, magnesium phosphates and arsenates and 

 some natural silicates (zeolites) being also exceptions. 



2. Insoluble salts are almost always destitute of water of crystallisa- 

 tion, and rarely contain the elements of water. 



3. In a series of salts containing nearly allied metals the solubility 

 and capacity for uniting with water of crystallisation generally diminish 

 as the atomic weight increases, as in the following examples : — 



Na.,SO4l0H2O 

 K.3SO, 



fNaoCrO^lOHoO 

 1 K.,Cr04 



