88 THE ORIGIN AND NATURE OF SOILS [chap. 



of the regular constituents of soil, and is the result of 

 the decay of the vegetation that previously lived on the 

 soil ; it is a dark brown or black compound of carbon 

 containing also nitrogen, and is to some considerable 

 extent soluble in dilute alkalis. It is one of the chief 

 colouring matters of soils, among the particles of which 

 it also acts to some degree as a cement. 



It is now necessary to make a rough mechanical 

 analysis of the mineral part of a soil; lO grammes are 

 weighed out into a dish, a little water is added, and the 

 contents of the dish are rubbed up into a paste by means 

 of a pestle made by sticking a rubber bung on to the 

 end of a glass rod. A beaker about 8 cm. in diameter 

 and 9 cm. high is now clearly marked on the side at a 

 point 7-5 cm. from the bottom, and the pulped-up soil is 

 washed into this beaker through a small sieve made of 

 brass-wire cloth of lOO meshes to the inch. The sieve 

 and its contents are put in the oven; when dry, the 

 material on the sieve is weighed and counted as " coarse 

 sand." The dirty liquid in the beaker is now made up 

 to the 7-5 cm. mark with more water, given a good stir, 

 and left to stand for exactly twelve and a half minutes, 

 at which moment the upper turbid liquid is steadily but 

 quickly poured off from the sediment below into a large 

 jar. The sediment is now rubbed up afresh with more 

 water, the water is brought to the mark, stirred, left to 

 stand for twelve and a half minutes, and poured off as 

 before into the same jar ; all the processes being repeated 

 half a dozen times or more, until at last there is nothing 

 left floating in the clear water at the end of the twelve 

 and a half minutes' wait. The sediment remaining in 

 the beaker is dried and weighed ; the contents of the 

 large jar can also be evaporated down and weighed, but 

 as this constitutes rather a long operation it will be 

 sufficient to evaporate a little in order to examine the 



