Analyses of Ashes of Pkmts. 
669 
nished it be potash or soda, why should the quantity of the latter 
seldom exceed a sixth part of the former ? Again, in guano we 
have always abundance of common salt and other salts of soda, 
and yet in Spec. 2G, where guano was applied, the proportion of 
soda does not exceed the mean, which is 2 •72. 
Chlorine in combination with sodium as common salt, will be 
seen to be present in some two or three specimens of wheat, but 
only in very minute quantity; and it is singular that in two of 
the three instances where it occurs, an unusually large proportion 
of oxide of iron is also present, as if the same cnxumstances had 
led to the peculiarity in both cases. Possibly in these instances the 
wheat was not fully matured, and these bodies would come under 
the title of " accidental ingredients." 
The absence of soda in any quantity, either as soda or as com- 
mon salt, from both the grain and straw, would seem hicompatible 
with the statement to which nmch attention has lately been drawn, 
that common salt is a manure for wheat ; or rather perhaps we 
should say, that it is an argument in favour of the theory which 
supposes the existence of two distinct classes of manures — one 
serving as food for plants, the other assisting in preparing that 
food, or in effecting some other desirable object in the amelioration 
of the soil. In the first of these ways common salt certainly can 
have no influence on wlieat : it cannot serve as food for the wheat 
crop, because it is not required, and the little soda existing in the 
ash, if essential, is always abundantly supplied by the soil. Com- 
mon salt probably owes its eiEcacy in part to the power which it 
possesses of absorbing and retaining moisture — a tendency which 
would ensure a certain though small supply of moisture to the 
roots in the driest seasons : it is also poisonous to the wire-worm 
and other depredators on this crop. 
We find, then, that in the ash of different specimens of wheat 
deviations in the proportions of its several ingredients occur, which, 
although they do not destroy the principle of uniformity of com- 
position upon which the whole interest of the subject depends, yet 
tend very materially to interfere with its simplicity. 
If wheat requires certain mineral matteis for its growth, why 
should it not always take up these bodies in the same proportion 
and to the same amount ? ^^ hy should one specimen of wheat 
give an ash containing 40 and another an ash containing 50 per 
cent, of phosphoric acid ? V\Tiy should the potash differ to such 
an extent in two samples? We should certainly not have antici- 
pated such an amount of latitude in the composition of the ash 
from the same kind of plants, — or at all events we should have 
looked for some evident connection between the mineral matter 
and the variety of the particular sample. I'lie variety would ap- 
pear, in the case of wheat grain, to be absolutely without influence 
on the composition of the ash. This will be seen at once by 
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