MR. JOHNSTON ON THE CONSTITUTION OF THE RESINS. 
359 
would be rendered necessary had we not been previously led to it by the resins, of 
which the analysis is given in the present paper. 
I have frequently had occasion in the course of this investigation to advert to the 
difficulty I have experienced in deciding which of two numbers to adopt as repre- 
senting the equivalents of hydrogen in a given resin, and to state how very little re- 
liance I could myself place on the exactitude of the numbers I was induced to pre- 
fer as the most probable. There are, apart from errors of manipulation, difficulties 
in the way of these analyses, arising from the presence of volatile matter in some of 
the resins, and from the ease with which others are decomposed, which make me 
willing to distrust my own formulae to the extent of one atom of hydrogen at least in 
each. I am not surprised, therefore, at the different results obtained by the distin- 
guished chemists above-named in regard to the crystallized elemi resin, nor do I con- 
sider that the error, on whichever it may rest, is to be attributed to any want of 
dexterity in either of these excellent manipulators, but rather to the nature of the resin 
itself, and to the manner or length of time during which it has been previously heated. 
IV. The analyses contained in the present paper exhibit the necessity also of a 
similar change in our second sectional formula, C 40 H 24 - r O y . The resins of ammo- 
niac and opoponax contain twenty-five, that of assafoetida twenty-six, and that of 
retin-asphalt twenty-seven atoms of hydrogen, while the latter alone approaches in any 
of its properties to the resins of the colophony type. Without inquiring closely, 
therefore, where the limits of the two groups are to be placed, I propose at present 
to insert the sign plus in this formula also, with the view of including in it the resins 
above-named, leaving to a future opportunity the discussion of the precise place in 
our groups or sub-groups which any particular resin ought to occupy. 
Our general expressions, therefore, now assume the form of 
C 40 H 32±ir O y for the first group, and of 
C 40 H 24±r for the second group. 
To these irrational expressions I would not be understood as annexing any per- 
manent value. They are intended merely to symbolize the present state of our know- 
ledge on this subject. New researches will necessarily introduce modifications, and 
make us acquainted at length with expressions much more general, and assuming 
perhaps a different form. 
V. In reference to the points of theory, with a view to the determination of which 
the present inquiry was undertaken, as stated at the commencement of this series of 
papers, we are already prepared I think to infer, that the resins are not all derivable 
from one common radical. Whether we are to consider the members of each of our 
great groups as compounds of a common radical, or the members of those sub-groups 
only, in which the atoms of hydrogen are constant while those of the oxygen vary, 
— whether the radical they contain be a binary compound of carbon and hydrogen, or 
ternary, and contain oxygen ; whether the resins in general be simply oxides of such 
