102 CHEMISTRY FOR AGRICULTURAL STUDENTS 



the water will ultimately rise higher in the tube of soil consisting of the 

 smaller particles. The attraction which sohds have for liquids is mani- 

 fested in the rise of liquids in small tubes, and it is therefore called 

 " capillary attraction." It is, however, simply dependent upon the amount 

 of surface the solid exposes to the liquid ; and as small particles present 

 more surface than large particles in proportion to their bulk, and the inter- 

 spaces between are smaller, the capillarity of a clay soil is greater than 

 that of a sandy soil. It is for this reason that clayey soils have more 

 power than sandy soils of sucking up moisture from below, and therefore 

 remaining moist in dry weather. 



The cooling effect of moisture on soils, and the action of frost on soils, 

 have been referred to on pp. 34, 35. 



XXV. CARBON: MARSH GAS 



Heat strongly in separate test-tubes (i) a piece of wood and 

 (2) a piece of bone. Note that in each case charring occurs 

 owing to the decomposition of the organic matter, and that 

 finally black masses of (i) vegetable charcoal and (2) animal 

 charcoal are left, which, while they have retained the form of 

 the pieces of wood and bone used, are much lighter in weight, 

 having become porous. If the combustible portion of the 

 wood charcoal and " bone-black " be burnt off by kindling and 

 then placing in bottles of oxygen, it will be found that the gas 

 produced in each case renders lime water milky, and is therefore 

 carbonic acid gas, and that in each case also, a white ash, 

 " wood ashes," and " bone ash," is left. Both kinds of char- 

 coal, in fact, consist of the element carbon, together with the 

 mineral substances of the wood or bone from which the char- 

 coal is made. 



Like sulphur and phosphorus, carbon exists in both crystalline and 

 amorphous modifications. Diamond, distinguished by its lustre, and 

 hence valuable as a precious stone, and by its extreme hardness, is a 

 nearly pure and colourless crystalline modification of carbon. Graphite 



