58 RELATIONS OF SOIL TO WATER 



atmosphere. Such water is known as hygroscopic water. It 

 is parted with slowly in a perfectly dry atmosphere, or on 

 a sufficient rise in temperature. 



The amount of hygroscopic water belonging to any given 

 weight of a substance depends on its extent of surface ; a mass 

 of glass becomes far more hygroscopic when reduced to powder. 

 Porous bodies are thus especially hygroscopic. Wood charcoal 

 recently burnt will increase about 20 per cent, in weight in 

 a moist atmosphere. 



In comparing the power of different bodies to absorb water- 

 vapour, the question is often complicated by the existence 

 of affinities for water which are quite distinct from the purely 

 surface action we have just described. Water- vapour may 

 be absorbed in large quantities by bodies which have a 

 chemical affinity for water, and form hydrates with it, as 

 in the case of calcium chloride, potassium carbonate, and'* 

 many other salts. It may also be absorbed to a considerable 

 extent by colloid bodies, as silk or gelatine ; the water in this 

 case is not held simply at the surface, but penetrates the 

 whole mass. 



Bemmelen (Rec. Trav. Chim., vii. 37) has made a long inves- 

 tigation on the amount of water held by the dried gelatinous 

 precipitates of silica, ferric oxide, alumina, and other similar 

 colloid bodies, and on the gain or loss of water which they suffer 

 at various temperatures, both in dry and moist air. These 

 precipitates when dried by long exposure to ordinary air 

 contain a large quantity of water. Placed in perfectly dry 

 air (over strong sulphuric acid) a portion of this water is lost l . 



1 The precipitated ferric oxide used in some of my own experiments still 

 contained 15-5 per cent, of water after drying over sulphuric acid, and 

 precipitated alumina 33-1 per cent. 



