380 
MR. JOHNSTON ON THE CONSTITUTION OF THE RESINS. 
analyses were made*. The entire subject of the action of alkalies and oxides on this 
resin is deserving- of a complete revision, which it is my intention hereafter to under- 
take. The course and method of research, and the precautions to be adopted are 
clearly marked out by the experiments above-described, and I only leave the subject 
in its present incomplete state, because having made already nearly fifty analyses of 
this resin and its products, I maybe permitted to turn my attention for a short time 
to some of the other members of this interesting natural family, on the constitution 
of which the above researches, though incomplete, tend to throw much light. 
V. On the rational Constitution of Resin of Benzoin. 
Benzoic acid is generally supposed-^ to exist ready formed in the resin of benzoin. 
It is obtained from the crude resin by sublimation, a gentle and uniform temperature 
being applied for several hours, when the acid forms beautiful crystals, amounting to 
about four per cent, of the weight of the resin employed. A quantity of water and of 
a volatile oil, having the odour of benzoin, is always (?) given off along with the acid, 
even when the operation is conducted in the most careful maimer. 
There are several circumstances which, though perhaps not decisive on the point, 
yet induce me to doubt whether the acid really exists ready formed, in any quantity 
at least, in the natural resin. A few of these I shall briefly state. 
1. Heated alone at a temperature of 212° Fahr. benzoic acid is given off by the 
resin, it is also extracted by simple treatment with boiling water. But during this 
treatment the colourless resin always becomes coloured, and less fusible, and the 
brown shade deepens and the melting point rises, the longer it is heated even at this 
low temperature, till at length it becomes of a reddish-brown colour, and does not 
soften at a heat much above 212° Fahr. The mere removal of benzoic acid already 
existing in the resin, should not produce such a change in its physical characters. 
It has been observed by Berzelius and other chemists, that when the resin of ben- 
zoin is heated, a volatile oil is given off, which, by the action of caustic potash, is 
transformed into benzoic acid. The removal of such an oil will account for the in- 
creased colour and diminished fusibility of the resin, while the agency of the atmo- 
sphere, when it is heated in the air, or of the elements of water when it is boiled in 
that liquid, or of the elements of the resin itself when it is heated alone, may produce 
to a certain extent the same effect as is caused more completely by the treatment with 
caustic potash j;. As a general observation, I have found that a deepening of theco- 
* A mixture of the resin C 40 H 26 O 10 with benzoic acid (C 40 H 14 . 3 0 8 .;, 7 ), would give a formula approaching 
that of the resin contained in our first salt of lead, C 40 H 32 o 9 . 
f See Liebig’s Chimie Organique, I. p. 241. 
J This result of the action of caustic potash is not confined to the oil given off by benzoin. Mulder has 
observed, that the oil of cinnamon also (C 20 H n 0 2 ) is transformed into benzoic acid by repeated distillation 
with caustic potash, and hence he has supposed that this oil may be a compound of benzyle (C 14 H 5 O a ) with 
a carbo-hydrogen (C H), since oil of cinnamon, C 20 H n 0 2 = C 14 H 5 0 2 + 6 C H. He has not satisfactorily 
