POTASSIUM FERTILIZERS 163 



of degrees of freedom may be calculated by adding 2 to the 

 number of components, and subtracting the number of phases. 

 For example, in the simple case of water being in equilibrium 

 with water vapour, there is one component and two phases, 

 therefore, according to the phase rule, the number of degrees 

 of freedom will be i + 2 2, i.e. there is only one degree 

 of freedom. If any single change takes place, such as a rise 

 in temperature, only one thing can happen, namely, that 

 there is less liquid water and more water vapour. If w r e take 

 the case of a salt in solution, the components are salt and 

 water, and the phases are solid salt, liquid water and water 

 vapour. Again, therefore, there is only one degree of freedom, 

 and by raising the temperature, as a rule solid salt goes into 

 solution, the solution becomes stronger, and the amount of 

 water vapour usually increases. In the special case of such a 

 salt as sodium sulphate, having two different crystallized salts, 

 one anhydrous and one hydrated, there are up to a tem- 

 perature of 32 J C. three phases, solid hydrated sodium 

 sulphate, containing 10 molecules of water of crystallization, 

 the liquid solution, and gaseous water vapour. As there are, 

 therefore, two components and three phases, there is one 

 degree of freedom, and the result of raising the temperature 

 is to increase the amount of salt in solution. When, however, 

 the temperature of 32^ C. is reached, both the hydrated 

 sodium sulphate and the anhydrous sodium sulphate can 

 exist in the presence of water, and therefore there are now 

 four phases, which, according to the phase rule, will not allow 

 any degree of freedom at all. Hence the temperature cannot 

 be altered from 32^ C. without one of the phases disap- 

 pearing, that is to say, above 32 J the hydrated sodium 

 sulphate disappears, and below 32 1 the anhydrous salt 

 disappears. The same principles apply to many of the 

 hydrated salts referred to in the following details of the 

 practical manufacture of pure salts. 



Double Salts. The formation of a double salt from two 

 single salts dissolved in water depends largely upon the 

 temperature. In some cases the double salt can only exist 

 below a certain temperature, but in others it is only formed 



