294 ON THE VOLATILITY AND SOLUBILITY OF CANTHARIDIN. 
rect, then by a series of analyses, quantitative as regards that 
principle, determine whether its proportion varies, and to what 
extent, in different specimens of cantharides of fair quality; and 
finally to test the preparations derived from the same samples and 
see how far they correspond with the inferences drawn from the 
ascertained properties and proportions of the active principle. I 
have at present undertaken to resolve but a part of these queries — 
yet by far the most important ones — as will be seen. 
Cantharidin is a white, neutral substance, of which the formula 
according to Regnault is C 10 H 4 . Gmelin considers it of the 
nature of a solid volatile oil. As usually seen it has the 
form of minute flattened four-sided prisms (c,) much broken up, so 
as to appear like scales. When deposited from an ethereal solution 
of cantharides by slow evaporation, or from its solution in hot 
acetic acid by cooling, it assumes the form of flattened oblique 
four-sided prisms with dihedral summits, derived from the rectan- 
gular prism by the bevelment of its 
edges (see fig. a and b from c.) The 
crystals by slow sublimation are four- 
sided rectangular prisms of great bril- 
liance and sometimes iridescent, c&d, 
Solubility. Pure cantharidin is insoluble in water hot or cold. 
It is slightly soluble in cold alcohol, readily so when hot. Ether 
dissolves it to a greater extent, yet much more easily hot than 
cold. Chloroform is its best solvent, cold or hot, as shown in a 
former essay (Am. Jour. Pharm. vol. xxiii.124,) and will remove it 
from the aqueous infusion of the flies. Acetic ether dissolves 
cantharidin, especially when hot, but does not retain much on 
cooling. When one part of cantharides is mixed with 20 parts of 
olive oil and heated to 250° Fahr. it is completely dissolved. As 
the solution cools, the cantharidin rapidly separates in shining 
needles in such quantity as at first to give the oil a pulpy consist- 
ence. The clear cold oil retains sufficient to act as an efficient 
rubefacient but not as an epispastic. One part of cantharidin 
requires 70 parts of oil of turpentine to dissolve it at the boiling 
temperature, the greater part separating, as the solution cools, in 
long asbestos-like needles. A piece of paper saturated with the 
cold solution and applied to the skin under adhesive plaster did 
not vesicate. Acetone (from the distillation of acetate of lime) 
