ON VESICATING APPLICATIONS. 
309 
As it is probable that the concentrated pyroligneous acid of 
commerce is frequently used for the acidum aceticum of the 
College, and as the pyroligneous acid supplied by different 
manufacturers is of very different strength, it is important 
in the preparation of Acetum Cantharidis, that the specific 
gravity and saturating power of the acid should be tested before 
using it. 
Mr. Garden, of Oxford-street, and, I believe, some others, 
employ acetic acid of about the strength of aromatic vinegar 
for making the Acetum Cantharidis. 100 grains of Mr. Gar- 
den's Acetum Cantharidis required 19S grains of crystallized 
carbonate of soda for its saturation. This is, of course, a much 
more active application than that ordered by the College, and 
will often raise a blister in ten minutes; but although prepared 
with so strong an acid, it is still uncertain in its operation, and 
thus an hour or more is in some cases required to produce the 
same effect, that in others, would result in a few minutes. In 
a preparation of Cantharides this uncertainty appears remark- 
able, for one of the great advantages which the blistering fly 
is considered to possess over all other epispastics, consists in 
the certainty and uniformity of its operation. The action of 
the Acetum Cantharidis seems to be distinguished less by the 
characteristics which belong to Cantharides, than by those of 
acetic acid. So much indeed is this the case, that on my 
attention being called to the variable strength and uncertain 
operation of this remedy, I was soon induced to suspect that 
it owes its vesicating power chiefly, if not entirely, to the 
acetic acid. Subsequent experiments have tended to confirm 
this opinion. 
Dr. A. T. Thomson, in his Dispensatory, describes the 
Acidum Aceticum of the Pharmacopoeia, as "a useful and 
powerful substitute for a blister, when the effect of such an 
application is required to be rapidly obtained," and I believe 
that pure acetic acid will be found to possess all the efficacy of 
Acetum Cantharidis made with acid of the same strength, 
the following are the experiments upon which this opinion is 
founded: 
