ON THE VOLATILITY AND SOLUBILITY OF CANTHARIDIN. 
301 
in the insect, we are prepared to consider with some clearness, 
the pharmaceutical preparations of the Spanish fly, and their 
action as vesicants. 
a. If l-30th of a grain of pure cantharidin, in fine powder, be 
placed on the skin of the arm and covered with a piece of warmed 
adhesive plaster, active vesication occurs in eight hours, with 
pain. If the same quantity of cantharidin be put on the other 
arm, a small piece of paper be laid over it, and then a piece of ad- 
hesive plaster with a circular hole in it be applied, so as to hold 
on the paper, no vesication occurs in sixteen hours, the powder 
remaining dry. If then a large piece of plaster be put over the 
whole, at the end of eight hours more no blistering action will 
have taken place. If now a trace of olive oil be applied to the 
back of the paper covering the cantharidin, and the plaster re- 
placed, speedy vesication will occur. These experiments prove 
that cantharidin must be in solution to have its vesicating action, 
and that oily matter is a proper medium. 
b. When powdered flies are stirred into the ordinary vehicle of 
resin, wax, and lard, so as to chill it almost immediately as was 
formerly directed, but little of the cantharidin is dissolved by the 
fatty matter, and when applied to the skin the process of vesica- 
tion is retarded. If, however, the cerate be kept fluid for a length 
of time, say for half an hour, by a water-bath or other regular 
heat, no loss of cantharidin occurs by the heat, the active prin- 
ciple is in great measure dissolved by the fat, and every part is 
impregnated and active. In the foregoing experiments it has 
been shown that twenty parts of olive oil will dissolve one of 
cantharidin when hot. If we admit with Thierry that cantharides 
contain but four thousandths of their weight of cantharidin, the 
quantity contained in a pound of cerate is about eight grains, 
whilst the lard in the same weight, of cerate is 1600 grains, or 
two hundred times the weight of that principle, not to speak of 
the influence of the wax and resin, which, in union with the melted 
lard, act as solvents. Hence the whole of the cantharidin may 
be dissolved by the vehicle. Another advantage of employing a 
continued heat in digestion is the removal of the hygrometric water 
from the flies, which is the source of the mouldiness to which the 
cerate is prone in certain conditions. 
In a former essay (Amer. Journ. Pharm., vol. xiii, p. 302,) I have 
