Analyses of the Ashes of riants. 
101 
instances the precipitate was abundant. Our experience loads 
to the conclusion that the neutralization by an alkali, previous lo 
burnincf, should never be omitted. 
We think, then, that the evidence we have adduced is sufficient 
to prove — ■ 
1st. That sulphuric acid, as such, is liable to a greater or less 
extent to be driven off in the burning of all silicious plants. 
2nd. That in the combustion of non-silicious plants (as, of 
course, also of those of a silicious character) there is always a loss 
of sulphur not existing as sulphuric acid in such plants. 
3rd. That this loss of sulphur is very variable in different ex- 
periments, depending upon a variety of circumstances over which 
the operator can exercise no distinct control. 
4lh. That as it appears that phosphates have no action 
upon the sulphates of the ash, the sulphuric acid afforded by 
an ordinary burning of a non-silicious plant must represent 
the full quantity of that acid actually existing in the plant ; 
but— 
5th. That inasmuch as the (quasi) sulphur retained in the ash 
burnt in the usual way is not a constant quantity, no inferences 
can be drawn as to the real sulphuric acid of the plant by this 
estimation. 
And lastly, that although combustion with a nitrate gives the 
total sulphur in any substances under examination, we are slill 
unable from either or both of these determinations to state how 
much of the sulphur existed as sulphur and how much as sul- 
phuric acid in the plant. We know, indeed, that in the case of 
non-silicious ashes the excess of sulphuric acid by the nitrate pro- 
cess must have existed as sulphur in the subject of analysis, but 
we cannot say that this excess really represents the whole of that 
element in the plant, for, in point of fact, we know that a variable 
quantity is converted into sulphuric acid in the combustion. We 
entertain little doubt that we may eventually devise some method 
of distinguishing the suIpJmric acid of the plant from its sulpliur, 
but at present we are not in possession of a process adequate to 
that end. In the meanwhile, for many practical purposes the 
knowledge of the total sulphur will, it is hoped, be of consider- 
able value. 
We have next to consider whether the phosphoric acid of 
plants is liable to any loss by the usual methods of burning. 
There can be no doubt that this is by far the most important 
point in the whole inquiry, and as such we have bestowed par- 
ticular attention upon its discussion. 
As in the former cases, loss of phosphorus would be move. 
likely to occur in silicious than non-silicious ashes, but we will 
take the simplest question fust. 
VOL. IX. M 
