90 THE ORIGIN AND NATURE OF SOILS [chap. 



that when it dries it forms a tough cake difficult to 

 break, whereas both the coarse and fine sand fall to a 

 powder as soon as they are dry. If we make this partial 

 separation for all the examples of soil we shall find the 

 greater part of each of them to be composed of sand and 

 clay in varying proportions ; however, even the coarsest 

 soil is not free from clay, while even the heavy clay 

 soil probably contains 50 per cent of the sand fractions. 



Before examining the nature of the sand and clay 

 fractions there is still one other regular constituent of 

 soil which must be identified, and this we may do by 

 putting a few grammes of each soil in a dish and 

 covering it with dilute hydrochloric acid. In several 

 cases, especially with the chalky soil, there will be a 

 visible effervescence, due to the escape of carbon dioxide 

 from carbonate of lime in the soil, which is being decom- 

 posed by the hydrochloric acid. Even the soils which do 

 not effervesce probably contain some carbonate of lime, 

 small amounts of which (less than i per cent or so) 

 show no visible evolution of carbon dioxide gas, because 

 it dissolves in the liquid as fast as it is set free. The 

 peaty soil is pretty sure to show no effervescence, and as 

 a rule this is due to the entire absence of carbonate of 

 lime from such soils. A further test which should be 

 applied is to put a little of each soil in a wettish condition 

 on a strip of blue litmus paper ; the peaty soil, and 

 perhaps some of the others, will show themselves after 

 standing to be acid, and from acid soils carbonate of 

 lime is absent except in accidental fragments. These 

 four substances — humus, sand, clay, carbonate of lime, 

 constitute the solid matter of all soils ; but only in the 

 exceptional cases of peats on the one hand or chalky 

 soils on the other do the sand and clay fail to pre- 

 dominate greatly over the other two constituents. 



To study the properties of sand any ordinary clean 



