312 
ON VESICATING APPLICATIONS. 
its efficacy, by making, in this case, a somewhat similar inves- 
tigation to that which we have just been pursuing. The Can- 
tharides ointment is intended as a mild stimulating application 
for keeping up the discharge from a blistered surface. In 
many cases, however, it is found to be entirely void of acri- 
mony, and, therefore, inapplicable for the purpose designed; 
whilst, in other instances, it proves too stimulating. This 
ointment is made by boiling the flies in water, and making an 
ointment from the inspissated decoction. Now, boiling water 
is capable of extracting the whole of the acrimony from the 
flies, and if, therefore, in the preparation of Unguentum Can- 
tharidis, the decoction be carefully evaporated, without using 
much heat, the product will be sufficiently active to raise a 
blister. This I have proved by experiment to be the case. 
But the heat generally employed in effecting the evaporation, 
destroys all or nearly all the acrid principle of the fly, and 
often renders the ointment of little use. 
Thus the strong blistering fluid is made by extracting from 
the fly, and preserving for use, an inert viscous matter, to- 
gether with some salts of lime and magnesia, which the insect 
contains, and throwing away the cantharidin or blistering 
principle; while in the process for making the mild blister 
dressing, the active principle is retained, and the more inert 
matter rejected. In the former case, the efficacy of the pre- 
paration depends upon the acetic acid, which is the menstruum 
used; and, in the latter case, the ultimate attainment of the 
object intended, that is, the.production of a mild ointment, is 
due to the decomposition or dissipation of the cantharidin 
during the evaporation ; and as this will take place in propor- 
tion to the heat employed, the result must necessarily be 
uncertain and variable. 
The celebrated Boerhaave proposed an amendment upon 
the Unguentum Cantharidis, by reversing the process. He 
recommended that the flies should be powdered, boiled, in 
water, and strained; that the aqueous decoction be thrown 
away, and the residue of the flies be dried, finely powdered, 
and formed into a pommade, in the proportion of twelve to 
