254 ARTIFICIAL MANURES [chap. 



absorption of the sun's rays, and on sunny days will raise 

 the temperature of the soil by one or two degrees. 

 Soot is very variable in composition, and cannot, as a 

 rule, be purchased with any guarantee as to the amount 

 of nitrogen it contains. The best guide is its lightness. 

 A good sample should be free from all admixture of 

 cinders and similar refuse, and should not weigh more 

 than 28 lb. per bushel. 



The second of the great groups of fertilisers is made 

 up of those which contain phosphoric acid as their 

 valuable constituent. Of these, bones in various forms 

 constitute the oldest and still among the most widely 

 used of fertilisers. A bone consists of a mineral frame- 

 work containing phosphate of lime mixed with a little 

 carbonate. This mineral framework we can see left 

 behind if we put an ordinary bone into the fire or in a 

 muffle furnace. On the other hand, if we immerse a 

 similar bone in dilute hydrochloric acid, after a day or 

 two all the mineral matter will become dissolved in the 

 acid, and there will be left behind the cartilaginous 

 framework of the bone, consisting of material contain- 

 ing nitrogen, which will be converted into gelatine when 

 heated up with water at high temperatures. At one 

 time the bones were only roughly broken up, and then 

 proved to be only a slow-acting, if effective, fertiliser. 

 Nowadays it is customary to remove as much of the fat 

 as possible, and then grind the bones to a fine powder, 

 which is sold as " bone meal." Owing to the toughness 

 of the organic matter contained in the bones, the bone 

 meal is never really fine, and though it is highly valued 

 on pastures and on some of the lighter arable soils, it is 

 comparatively slow in its action and unremunerative in 

 its results. 



In addition to bone meal, various other manures are 

 prepared from bones ; sometimes they are treated with 



