154 
Anah/ses of the Ashes of Plants. 
lost, or 5 "83 altogether : further heating did not appear to alter 
the weight. If the pyrophosphate of soda decomposed an entire 
equivalent of carbonate of soda, the loss for 40*64 grains should 
have been 6*70 grains. The loss fell somewhat short of the 
calculated quantity, but it is plain that a bibasic phosphate will 
at a high temperature become tribasic at the expense of a car- 
bonate. Of course the argument applies with greater force to 
mono-basic phosphates, and we must conclude that phosphates 
with less than three atoms of base will, to a greater or less extent, 
act upon the carbonates of the ash, and thus vitiate the results as 
far as this acid is concerned. That such phosphates really exist 
in plants there can be little or no doubt. The method proposed 
by Professor Rose, of treating the charred vegetable by water and 
estimating the alkaline carbonate in the solution, can but partially 
remedy this defect, for it leaves the insoluble earthy carbonates 
quite undetermined, since no chemist would attempt to estimate 
in the usual way the small amount of these latter carbonates 
which may be distributed through many thousand grains of 
charcoal. 
It is easy also to see that the quantity of alkaline carbonates 
thus dissolved would in no way indicate the proportion actually 
present in the ash, since the character of the solution would be 
entirely regulated by the proportion of other salts present. Thus, 
suppose the ash to contain sulphate of lime and carbonate of 
potash, the aqueous solution will yield sulphate and not carbonate 
of potash, and so on. 
In fact, we know of no existing methods of so burning plants as 
to obtain estimations of the carbonic acid of their ashes which 
shall at all be indicative of the organic acids present in the vege- 
table substance examined. 
It is so far fortunate that, for all practical purposes, a know- 
ledge of the proportion of carbonic acid is not needed, and, 
except in a general way, its estimation must be looked upon as a 
necessary but useless part of plant-ash analysis. 
The next acid which might be subject to loss in the combustion 
of a plant is the sulphuric. 
It is easy to conceive that the sulphates present in an ash may 
be decomposed at a high temperature by silica, the sulphuric acid 
being driven off in vaj)()ur. 
The following .experiment was made with a view of ascertaining 
the liability to loss from this source : — 
45 ■ 39 grains of pure sulphate of potash well dried were dis- 
solved in 1000 grains of water, and the solution poured over 2000 
grains of barley in a platinum dish ; the barley was allowed to 
digest in the liquid, which was subsequently carefully evaporated 
to dryness in contact with it ; when dry, the barley was burned to 
