MOLECULAR PHYSICS. 85 



the other hand, on cooling a very weak solution of a salt ice only 

 separates, and this continues with lowering ternperature until the 

 same ratio and the same temperature as before are reached, where- 

 upon the salt and the water solidify in union. In this sense water 

 at its freezing temperature may be viewed as a saturated solution 

 of ice in water. Water at its maximum density is still simple water. 

 At temperatures between those of its maximum density and its 

 freezing-point, it is a solution of ice, and hence its diminished 

 density. 



Although the quantity of a crystalline solid which a liquid holds 

 in solution increases with the temperature, yet on cooling a warm 

 saturated solution it often occurs that no solidification ensues 

 unless some nucleus or foreign substance affords a point from 

 which the crystallization may commence. Such a solution is 

 super-saturated, and such super-saturation may take place simul- 

 taneously in regard to two or more of the solid constituents of a 

 solution. , , v , 



The gradual automatic diffusion of a salt through water above 

 it may be viewed either as a motion of the molecules of the salt 

 amongst those of the solution above them until they reach the 

 water; or, as a differential motion of the charged water itself: when 

 such diffusion occurs through permeable walls, it is called osmose. 

 There is in most cases an interchange of liquid through the wall, 

 and the less dense exceeds the more dense in its osmotic quantity. 

 The nature of the porous wall is discriminating in regard to the 

 components of a solution in the sense of determining which it 

 shall let pass. A membrane such as parchment being in contact 

 with water on the one side, and a solution of gum and chloride of 

 sodium on the other, will separate the salt from the gum, letting 

 the former pass through, and retaining the gum further diluted by 

 water passing in the opposite direction. Such separation is called 

 dialysis. By its means various bodies before unknown in purely 

 aqueous solution have been procured ; such as soluble alumina by 

 diffusing away hydrochloric acid from a solution of the chloride of 



