72 FOOD OF THE VEGETATING PLANT. CHAP. II. 



accelerating putrefaction, why is its beneficial effect 

 confined but to a small number of plants ? 



Lastly, some writers have contended that the 

 salts which are found in vegetables are merely ac- 

 cidental in their occurrence, and not necessary to the 

 health or perfection of the individual ; because they 

 are found to exist in but a very small proportion, 

 both in the soil and plant : but as there are many 

 species in which some salts are to be met with con- 

 stantly and uniformly, at least if they have vegetated 

 in a soil in which they are found to thrive,, we can 

 scarcely regard their occurrence as being merely ac- 

 cidental, or as producing no beneficial eifect upon 

 the plant. But the proportion of salts lodged in 

 the soil is not so small as is generally believed. 

 Re-agents do not indeed detect a great quantity in 

 general ; but that is because the alkaline salts of 

 mould, like the alkaline salts of vegetables, are to 

 be discovered chiefly in the remains of combustion ; 

 and because the ashes of the greater part of vege- 

 table moulds do not readily part with their salts in 

 boiling water. This difficulty of solution is thought 

 by Saussure to be owing to a semivitrifaction that 

 takes place in the mould when the ashes are 

 abundant. An hundred parts of mould furnished by 

 combustion 50 parts of ashes which did not give 

 out their salts to boiling water. But 10O parts only 

 of dried extract from the same mould yielded only 

 14 parts of ashes ; and 10O parts of the ashes 

 formed with boiling water a ley which contained 



