PRINCIPLES OF ITS USE. 187 



springs in Savoy., by which the inhabitants cured 

 their goitre ; it was shown, that they contain small 

 quantities of iodine. In burnt sponges used for the 

 same purpose, the same element was also detected. 

 The extraordinary efficacy of Peruvian bark was 

 found to depend on a small quantity of a crystalline 

 body existing in it, viz. quinine ; and the causes of 

 the various effects of opium were detected in as 

 many different ingredients of that drug. 



Calico-printers used for a long time the solid 

 excrements of the cow, in order to brighten and 

 fasten colours on cotton goods ; this material 

 appeared quite indispensable, and its action was 

 ascribed 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 constituent is phosphate of 

 soda. 



Now all such actions depend on a definite cause, 

 by ascertaining which, we place the actions them- 

 selves at our command. 



It must be admitted as a principle of agriculture, 

 that those substances which 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 

 (silicate of potash), with the ashes of burnt straw, 



