VESICATING COLLODIONS. 
415 
be applied for any length of time without producing a sensi¬ 
ble result, if a dry film of plain collodion be interposed 
between the blister and the cuticle. 
First, then, let us examine the texture of a collodion film, 
to mark its applicability in the present case. If we pour 
upon a glass slide some recently prepared collodion, and then 
examine it by the microscope, it will present the following 
appearance:—A pretty homogeneous and smooth ground, but 
running through which are slight ridges, which produce large 
honeycomb markings. These ridges are caused by the quick 
evaporation of the ether; the whole is interspersed with 
filaments of partially disorganized cotton in a semi-gelatinous 
state. However carefully the cotton may be prepared, it is 
next to impossible to get rid of these fibres, some portion 
always escaping the perfect action of the acids. If we can 
add a small quantity of glacial acetic acid to the collodion, 
we shall find the character of the film greatly changed. 
From the slower evaporation the honeycomb ridges are no 
longer palpable, whilst the solubility is so much increased 
that the filaments are found to have disappeared. This film 
is perfectly uniform, but it presents this peculiarity, that it 
gradually dries into a mass of jelly-like globules, which how¬ 
ever possess but little cohesion; when dry, it is very short, 
for if the finger be run up the glass, instead of leaving it as 
a tough skin, it collects as a moist crumbled mass. Having 
so far seen that the glacial acid, besides destroying the con¬ 
tractility, gives it the properties of porosity and slowness in 
drying, it follows that such a collodion is particularly suited 
for the application of any vesicant which we may intend to 
apply, instead of being a varnish which hermetically seals up 
the active matter. Glacial acetic acid is one of the best 
direct solvents of cantharidin with which we are acquainted. 
Pure cantharidin was found to be very soluble in that acid, 
a saturated solution depositing it unchanged on evaporation 
in hard mica-like crystals. The principle then that we 
propose is, to exhaust cantharides by a mixture of ether and 
acetic acid, and to convert these into a collodion by the 
addition of gun-cotton. 
Take Cantharides, 3 V 1 
Ether from methylated spirits, f^xiij, or q. s.; 
Glacial acetic acid, f^ij; 
Gun cotton, ^ss; 
Methylated spirits of wine, f^vij, or q. s. 
* Two ounces more cantharides to the above proportions, make a very 
powerful collodion. 
