70 PHYSICAL SCIENCE 



If we cool the solution of some substance 

 such as sodium chloride, that is, common salt, 

 the ice which freezes out is the solid form of pure 

 water. The process can be illustrated in a very 

 striking manner by using the solution of a 

 coloured salt. If, for example, a dilute solution 

 of the purple-coloured potassium permanganate 

 be placed in a glass bottle and be surrounded for 

 some hours by a freezing mixture, most of the 

 water solidifies to form a hollow cylinder of 

 perfectly colourless ice, while the permanganate 

 is concentrated in an intensely coloured liquid 

 core along the axis of the cylinder. 



Similar phenomena occur in other cases where 

 the separation is not so clearly visible. 



If the ice be frozen rapidly, some trace of salt 

 may be deposited also ; but experiment has shown 

 that it does not enter into the composition of the 

 crystals, and is entangled merely mechanically 

 in their interstices. Essentially, then, the salt 

 remains in the liquid solution, and, as the solvent 

 is gradually frozen out, the concentration of that 

 solution must increase. The stronger the solution 

 becomes, the lower is its freezing-point ; but, if 

 the temperature at our disposal be low enough, 

 we can go on freezing out water till the residual 

 solution is saturated with salt at the temperature 

 of its freezing-point. Any further abstraction of 

 heat, by removing some of the necessary solvent, 

 must then be accompanied by the simultaneous 

 deposition of salt ; ice and salt will be precipitated 

 together, and the residual solution will retain the 

 constant composition of saturation. 



Since, as the process of freezing goes on in 

 these conditions, there is no change in the 



