352 HOW CROPS GROW. 
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 
asolvent. 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 will be- 
gin to move toward the solid body from all points, and 
this motion will 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, 
