v.] COMPOSITION OF SOILS 87 



determine how much of each of these constituents is 

 present in the examples that have been selected. Weigh 

 out a series of basins, add about 10 grammes of soil to 

 each, and weigh again ; put the basins in the water oven 

 for a few hours, and reweigh to find the loss of moisture ; 

 finally, ignite for two hours over a lamp and make a 

 last weighing to obtain the loss on ignition, which 

 includes a little combined water as well as the organic 

 matter. Considerable differences will reveal themselves 

 (see Table XI.); the moisture that is retained by the 

 undried soil will vary from about 2 per cent, in the 

 sandy subsoil to 10 per cent, or more in the peaty soil ; 

 the loss on ignition will similarly vary from 2 to 6, and 

 may even run up 30 per cent, in the peaty soil. We 

 can see clearly enough that the humus present is a 

 compound of carbon, from the manner in which it chars 

 and blackens on heating. The humus also contains 

 nitrogen, as may be seen by heating a little more of 

 the soil mixed with soda-lime, whereupon the smell 

 of ammonia, which at once becomes palpable, may be 

 taken as evidence of the presence of combined nitrogen 

 in the humus, just as in the plant (p. 3). We can 

 further show that humus, or rather the humic acid which 

 can be set free by the action of acids on the humus, is 

 soluble in alkalis like ammonia or caustic soda. Take 

 about 20 grammes of the peaty or alluvial soil, and 

 cover it with dilute (5 per cent.) hydrochloric acid for 

 half an hour, then pour off the acid and get rid of it 

 entirely by one or two washings with pure water. 

 Finally, pour on the soil five or six times its bulk of a 

 weak solution of ammonia and shake up from time to 

 time ; a deep brown or even black solution will result, 

 and the remaining soil will be considerably bleached by 

 the removal thus effected of the greater part of its 

 organic matter. We may conclude that humus is one 



