EXPLANATION OF ITS ACTION. 187 



peared quite indispensable, and its action was as- 

 cribed to a latent principle which it had obtained 

 from the living organism. But since its action was 

 known to depend on the phosphates contained in it, 

 it has been completely replaced by a mixture of 

 salts, in which the principal constituents are the 

 phosphates of soda and lime.* 



Now all such actions depend on a definite cause, by 

 ascertaining which we place the actions themselves 

 at our command. 



It must be admitted as a principle of agriculture, 

 that those substances w^hich have been removed from 

 a soil must be completely restored to it, and whether 

 this restoration be effected by means of excrements, 

 ashes, or bones, is in a great measure a matter of 

 indifference. A time will come when fields will be 

 manured with a solution of glass f (silicate of pot- 

 ash), with the ashes of burnt straw, and wdth salts 

 of phosphoric acid, prepared in chemical manufac- 

 tories, exactly as at present medicines are given for 

 fever and goitre. 



There are some plants which require humus, and 

 do not restore it to the soil by their excrements ; 

 whilst others can do without it altogether, and add 

 humus to a soil which contains it in small quantity. 

 Hence a rational system of agriculture would employ 

 all the humus at command for the supply of the 

 former, and not expend any of it for the latter ; and 

 would in fact make use of them for supplying the 

 others with humus. 



We have now considered all that is requisite in a 

 soil, in order to furnish its plants with the materials 

 necessary for the formation of the woody fibre, the 



* This mixture of salts is sold to calico-printers in large quantities 

 under. the name of *< dung substitute." It would be well worth experi- 

 ment to try its effects as a manure upon land. Its cost is 3d. or 4d. per 

 pound, and is not, therefore, dearer than nitrate of soda, which is now 

 so extensively used. — Ed. 



t When glass contains a very large proportion of potash, it is soluble 

 in boiling water ; and by combination with other substances, silica 

 becomes soluble in water. According to Dr. Jackson, crenic acid 

 enables water to take it up. 



