SOLUTION 109 



stand, salt never settles out. Only by evaporating off all the 

 water can a complete separation be effected. 



Practically speaking, there is no limit to the amount of dissipa- 

 tion which may thus be produced. Thus a single small crystal 

 of potassium permanganate, a common disinfectant which gives 

 a very deep purple solution in water, may be dissolved in a liter 

 or even in a hundred liters of water, and the purple tinge which it 

 imparts to the liquid will still be perfectly perceptible in every 

 portion of the solution. We may note here the distinguishing 

 characteristic of a solution as opposed to a compound. Com- 

 pounds contain definite proportions by weight (p. 19) and simple 

 atomic ratios (p. 86) of their constituent elements. The com- 

 position of a solution, on the other hand, can within certain limits 

 (see p. 110) be varied continuously. 



Sometimes, when we shake up a finely-divided solid with a 

 liquid, the latter becomes dull, or cloudy, or muddy. The solid 

 particles are here simply suspended in the liquid, not dissolved, 

 and will eventually settle out. Sand, shaken with water, settles at 

 once. Flour, mixed with water, settles more slowly. The parti- 

 cles of flour can be readily separated from the water, however, 

 by filtration (p. 13), the flour remaining on the paper while the 

 water runs through. Such mixtures are called suspensions. 



In exceptional cases, the subdivision of a suspended substance 

 in a liquid, while not approaching molecular magnitudes, is so 

 minute as to make its retention by filter-paper impossible, or 

 even to prohibit it from settling out in any reasonable time. So- 

 lutions of soap, starch and gelatine in water are of this nature. 

 Such suspensions are known as colloidal suspensions. To the 

 unaided eye, they appear to be true solutions. Their main prop- 

 erties, however, are essentially different from those of true solu- 

 tions, as will be seen later (pp. 440-1). 



Milk owes its cloudy, white appearance largely to droplets of 

 oily matter, which reflect much light from their surfaces. They 

 pass easily through filter paper. But when milk is allowed to 



