SECT. IV. SALTS. 73 



25 parts composed of potass in an uncombined 

 state, and of alkaline sulphates and muriates ; 

 and yet, upon further analysis, it was found that the 

 water had not extracted more than one half of the 

 salts which the ashes contained. The soil, therefore, 

 contains an abundant supply of salts for all the 

 purposes of vegetation. It may even in some cases 

 contain too much ; for it is to be recollected 

 that saline substances are beneficial to vegetation 

 only when applied in very small quantities. If they 

 are administered in great abundance they destroy 

 the plant. 



And the argument against their utility that has 

 been drawn from the small proportion in which 

 they are found to exist in the plant itself, is al- 

 together inadmissible; because it is very well 

 known that some particular ingredient may be 

 essential to the composition of a body, and yet con- 

 stitute but a very small proportion of its mass. 

 Atmospheric air contains only about one part in the 

 lOOth of carbonic acid ; and yet no one will venture 

 to affirm that carbonic acid gas is merely an adven- 

 titious and accidental element existing by chance in 

 the air of the atmosphere, and not an essential 

 ingredient in its composition. Phosphate of lime' 

 constitutes but a very small proportion of animal 

 bodies, perhaps not one part in 500 ; and yet no 

 one doubts that it is essential to the composition of 

 the bones. But the same salt is found in the ashes 



