CAPILLARITY AND OSMOSIS 115 



kept in moisture-proof receptacles. Sulphuric acid rapidly 

 gains in weight by absorbing moisture from the air, and quick- 

 lime slacks in the same way. When a substance thus dissolves 

 it is said to deliquesce or to be deliquescent. If it only takes on 

 additional moisture, it is said to be hygroscopic. Most mosses 

 and lichens are hygroscopic. The lichens, especially, get 

 much of the moisture used in their life processes from the 

 moisture in the air. There are also certain crystalline sub- 

 stances which act in a manner exactly opposed to deliques- 

 cence when exposed to the air. Instead of taking on more 

 moisture they give up what they have and fall into a fine pow- 

 der. Such substances are said to be efflorescent. 



101. Shrinking and Warping. Many vegetable and animal 

 fibers have the property of becoming shorter and thicker when 

 wet. This is taken advantage of in shrinking cloth to make 

 the threads thicker and closer together. The same thing 

 happens when boards warp. If one side becomes moist the 

 swelling fibers cause that side to become larger and curve 

 toward the dry side. One of the chief reasons for painting and 

 varnishing woodwork is to prevent the absorption of water and 

 the consequent warping. 



102. Osmosis. There is still another way in which water 

 moves in substances against the force of gravity. If two 

 liquids of different density, that ordinarily mix, be separated 

 by a membrane such as parchment paper, hog's bladder, or the 

 cell walls of animals or plants, they usually begin to flow 

 through the separating membrane and continue to do so until 

 the liquids on each side of it have the same density. This is 

 known as osmosis. The liquid of less density will, of course, 

 go through the membrane more rapidly than the denser liquid 

 and for a time may increase the bulk of the latter very con- 

 siderably, but as the density of the two liquids becomes more 

 nearly uniform, the level of the liquids becomes the same. An 

 interesting illustration of osmosis may be had by filling a this- 



