i62 CHEMICAL FERTILIZERS 



curve for potassium sulphate is only very slightly curved, 

 being almost a straight line like potassium chloride. 

 Magnesium sulphate, another constituent of the potassium 

 manures, exhibits very slight curvature, although the 

 solubility rises steadily with the temperature. 



The action of water on a solid is, in the first place, to 

 liberate those molecules with which it comes into contact, 

 and allow them to distribute themselves equally throughout 

 the liquid, until the whole of the salt is dissolved. It follows 

 that the maximum solubility depends upon the exact nature 

 of the solid present ; when a salt crystallizes in two different 

 forms, the solubility is almost always different in the two 

 cases ; also, the solubility of an anhydrous salt is different 

 from that of its hydrates. Recognition of the principle, 

 that the solubility depends upon the degree of hydration of 

 the salt, has supplied the explanation of broken curves when 

 solubilities are plotted ; the break in the curve generally being 

 due to a change in the degree of hydration of the salt separating 

 out at that temperature. The examination of the behaviour 

 of a solid to water is of great importance in the study of the 

 various hydrates formed by the salt. In investigations into 

 the conditions of equilibrium of various systems of solids 

 and liquids, each of the chemical substances present is called 

 a " component " of the system, and each uniform solid or 

 liquid present is termed a " phase." The state of equili- 

 brium in such a system is fully defined when the three 

 variable factors, temperature, pressure and concentration, 

 are known. These three are not always independent of one 

 another, since in many systems when one is fixed the others 

 can be calculated. When it is only necessary to fix one of 

 the three factors in order to have a fully defined state of 

 equilibrium, such a system is said to have only one degree of 

 freedom, but some systems have no degrees of freedom, whilst 

 others have more than one. It has been found that the 

 number of degrees of freedom depends entirely on the number 

 of components and phases which are present. 



The general law known as the "Phase Rule," states 

 that in any [system which is in equilibrium, the number 



