THE ASH OF PLANTS. 137 



SALTS OF AMMONIA exist in minute amount in some 

 plants. What particular salts thus occur is uncertain, and 

 special notice of them is unnecessary in this chapter. 



Since it is possible for each of the acids above described 

 to unite with each of the bases in one or several propor- 

 tions, and since we have as many oxides and chlorides as 

 there are. metals, and even more, the question at once 

 arises which of the 60 or more compounds that may thus 

 be formed outside the plant, do actually exist within it ? 

 In answer, we must remark that all of them may exist in 

 the plant. Of these, however, but few have been proved 

 to exist as such in the vegetable organism. As to the 

 state in which iron and manganese occur, we know little or 

 nothing, and we cannot assert positively that in a given 

 plant potash exists as phosphate, or sulphate, or carbonate. 

 We judge, indeed, from the predominance of potash and 

 phosphoric acid in the ash of wheat, that phosphate of pot- 

 ash is a large constituent of the grain, but of this we are 

 not sure, though in the absence of evidence to the contrary 

 we are warranted in assuming these two ingredients to be 

 united. On the other hand, carbonate of lime and sul- 

 phate of lime have been discovered by the microscope in 

 the cells of various plants, in crystals whose characters 

 are unmistakeable. 



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, speak of the calcium or the silicon in 

 the plant, but of lime and silica, because the idea of these 

 rarely seen elements is much more vague, except to the 

 chemist, than that of their oxides, with which every one 

 is familiar. 



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



