THE ASH OF PLANTS. 137 
_ Sarts or Awmonia 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, 
