50 THE VOLATILITY AND SOLUBILITY OF CANTHARIDIN. 
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 
replaced, speedy vesication will occur. These experiments 
prove that cantharidin must be in solution to have its vesi- 
cating 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 imme- 
diately as was formerly directed, but little of the cantharidin 
is dissolved by the fatty matter, and when applied to the 
skin the process of vesication 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 principle 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 contains but four 
thousandths of their weight of cantharidin, the quantity con- 
tained in a pound of cerate is about eight grains, whilst the 
lard in the same weight of cerate is 1600 grains, or two hun- 
dred 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 can- 
tharidin may be dissolved by the vehicle. Another advan- 
tage of employing a continued heat in digestion is the re- 
moval of the hygrometric water from the flies, which is the 
source of the mouldiness to which the cerate is prone in cer- 
tain conditions. 
66 In a former essay ( Amer . Journ. Pharm ., vol. xiii, p. 302), I 
have advocated digestion in making this cerate (a recom- 
mendation also made by Mr. Donovan, of Dublin, about the 
same time), and also the use of a portion of the oil of tur- 
pentine to facilitate the solution of the cantharidin, but the 
foregoing experiments prove that fatty matter is quite as 
good, if not a better solvent alone, than with turpentine. 
“ c. It has been asserted long ago by Beaupoil, Robiquet, 
and others, that water will perfectly extract the active matter 
from Spanish flies, which these experiments corroborate. 
Hence it is easy to understand how the condensed perspira- 
tion may facilitate the action of a blister, especially when, as 
was formerly much the case, its surface is coated with the 
dust of the flies and the skin moistened. 
u It is also clear why the Unguentum Cantharidis of the 
U. S. Pharmacopoeia is active although made with a decoc- 
tion of flies, yet, in this preparation, care should be observed 
