PROPERTIES OF BONE-DUST. 263 



from the presence of sulphuric acid, they have a special 

 value. On chalky soils, the free phosphoric and sul- 

 phuric acids are immediately neutralised, by which they 

 are deprived of one of their essential properties, viz., 

 their ready diffusibility, which renders them so valuable 

 a manure for other soils. 



Among the neutral phosphates bone-dust holds the 

 first rank. When bones are exposed, under high pres- 

 sure, to the action of steam, they lose their toughness, 

 and swell up into a soft gelatinous mass, which, after 

 drying, may be readily ground to a fine powder. In 

 this form it spreads, with great rapidity, through the 

 soil ; it dissolves in water to a small but perceptible 

 extent, without requiring the presence of any other 

 solvent. What dissolves, under these circumstances, 

 in water, is a combination of gelatine with phosphate 

 of lime, which is not decomposed by the arable earth, 

 and therefore penetrates deep into the ground a prop- 

 erty wanting in the superphosphate. In the moist 

 ground, however, the gelatine speedily putrefies, being 

 converted into ammonia compounds, and the phosphate 

 of lime is then retained by the arable earth. Bone-dust 

 is the agent best adapted to supply phosphate of lime 

 to the deeper layers of the arable soil, for which pur- 

 pose the superphosphates are not suitable. Bone-earth, 

 or bone-ash, is the name applied to bones freed, by cal- 

 cination, from the glue or gelatinous part. The animal 

 charcoal of sugar refineries belongs to this category. 

 It must be reduced to the finest powder to render it 

 fully available for manuring purposes. To effect its 

 more speedy distribution through the soil, the presence 

 of a decaying organic substance is necessary to supply 

 the carbonic acid required for its solution in rain water. 

 An excellent way is to mix the powder with farm-yard 

 manure and let the mixture ferment. Among the phos- 

 phates of commerce, the guano coining from the Baker 

 and Jarvis Islands are distinguished, before others, by 

 their acid reaction and greater solubility. They con- 

 tain only a small quantity of an azotised substance, no 

 uric acid, and small proportions of nitric acid, potash, 



