HYGROSCOPIC WATER 59 



Heated to 100 C. still more water is lost. Heating to 300 C. 

 fails however to remove the whole of the water, which can 

 only be certainly expelled at a red heat. If these substances 

 have not been over-heated, a considerable part of the water- 

 lost is regained when they are placed in a moist atmosphere. 



In the case of an ordinary soil of mixed constitution, the 

 power of absorbing water- vapour depends in part on the 

 extent of surface which it presents, that is, on the fineness 

 of its particles ; but it depends also to a still greater extent 

 on the amount and character of its colloid constituents. 

 Chemists are at present unable to separate with certainty 

 the portions of water held in these various ways ; all we can 

 say is that the water lost in perfectly dry air is that most 

 feebly held, and that the water expelled by heat is more 

 firmly combined in proportion as it requires a higher tempera- 

 ture for its expulsion. We may indeed be certain that water 

 requiring a higher temperature than 100 C. for its expulsion 

 is not hygroscopic water held on the surface of a non-colloid 

 body ; but, on the other hand, we may be equally certain 

 that the water expelled below that temperature has been 

 derived in part from the colloids present. It is important 

 to bear in mind that soil dried at 100 C. is not dry, but 

 that the colloids still hold water; any further loss obtained 

 on ignition is thus not simply due to the combustion of 

 organic matter, but is occasioned in part by a further loss 

 of water by the cla}^ and other silicates, and by the colloid 

 ferric oxide and alumina which may be present. Loss on 

 ignition is thus no measure of the amount of organic matter 

 present. 



The methods used for determining the hygroscopic water 

 in soils have varied a good deal, and the results obtained 



