352 HOW CEOPS GEOW. ' 



ter be colored by indigo or cherry juice, its motion may 

 be followed by the eye, and after a certain lapse of time 

 the water and alcohol will be seen to have become uni- 

 formly mixed throughout the two vessels. This manifesta- 

 tion of adhesive attraction is termed Liquid Diffusion. 



What is true of two liquids likewise holds for two 

 solutions, i. e., for two solids made liquid by the action of 

 a solvent. A vial filled with colored brine, or syrup, and 

 placed in a vessel of water, will discharge its contents in- 

 to the latter, itself receiving water in return ; and this mo- 

 tion of the liquids will not cease until the whole is uni- 

 form in composition, i. e., until every molecule of salt or 

 sugar is equally attracted by all the molecules of water. 



When several or a large number of soluble substances 

 are placed together in water, the diffusion of each one 

 throughout the entire liquid will go on in the same way 

 until the mixture is homogeneous. 



Liquid Diffusion may be a Cause of Continual Move- 

 ment whenever circumstances produce continual disturb- 

 ances in the composition of a solution or in that of a mix- 

 ture of liquids. 



If into a mixture of two liquids we introduce a solid 

 body which is able to combine chemically with, and solid- 

 ify one of the liquids, the molecules of this liquid wUl be- 

 gin to move toward the solid body from all points, and 

 this motion Avill cease only when the solid is able to com- 

 bine with no more of the one liquid, or no more remains 

 for it to unite with. Thus, when quicklime is placed in a 

 mixture of alcohol and water, the water is in time com- 

 pletely condensed in the lime, and the alcohol is rendered 

 anhydrous. 



Rate of Diffusion, — The rate of diffusion varies with 

 the nature of the liquids ; if solutions, with their degree 

 of concentration and with the temperature. 



Colloids and Crystalloids. — There is a class of bodies 

 whose molecules are singularly inactive in many respects, 



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