THE ASH 01? PLANTS. 151 



plant potassium exists as phosphate, or sulphate, or car- 

 bonate. We judge, indeed, from the predominauce of 

 potassium and phosphoric acid in the ash of wheat, that 

 potassium phosphate is a large constituent of this grain, 

 but of this we are scarcely certain, though in the absence 

 of evidence to the contrary we are warranted in assuming 

 these two ingredients to be united. On the other hand, 

 calcium carbonate and calcium sulphate have been discov- 

 ered by the microscope in the cells of various plants, in 

 crystals whose characters are unmistakable. 



For most purposes it is unnecessary to know more than 

 that certain elements are present, without paying atten- 

 tion to their mode of combination. And yet there is 

 choice in the manner of "representing the composition of 

 a plant as regards its ash-ingredients. 



We do not indeed so commonly speak of the calcium 

 or the silicon in the plant as of "lime and silica, because 

 these rarely-seen elements are much less familiar than 

 their oxides. 



Again, we do not speak of the sulphates or chlorides, 

 when we desire to make statements which may be com- 

 pared together, because, as has just been remarked, we 

 cannot always, nor often, say what sulphates or what 

 chlorides are present. 



In the paragraphs that follow, which are devoted to a 

 more particular statement of the mode of occurrence, rel- 

 ative abundance, special functions, and indispensability 

 of the fixed- ingredients of plants, will be' indicated the 

 customary methods of defining them. 



§3. 



QUANTITY, DISTEIBUTION, AND VAEIATIONS OF THE ASH- 

 INGREDIENTS. 



The Ash of plants consists of the various acids, oxides, 

 and salts, that have been noticed in § 1, which are fixed 

 or non-volatile at a heat near redness. 



