PREPARATION OF COLLODION. 
211 
is, however, not permanent, generally disappearing in a few 
minutes. 
When applied to the skin, this preparation forms a per- 
fectly smooth transparent pellicle, more difficult to remove 
than that of ordinary collodion. Being more pliab le, it 
yields to the motion of the skin and will not crack even after 
several days' application. It might be supposed that the 
turpentine would render it more irritating : but this is not 
the case owing to the absence of that mechanical stimulus, 
so powerfully displayed in the former solutions. 
The addition of two drachms of mastic to the above may 
be at times advisable, if the pellicle be required of great 
toughness and strength ; but it dries more slowly, and re- 
mains opalescent longer than that containing Venice turpen- 
tine alone. This preparation is much more suitable for the 
purpose of a varnish, than as an application to the skin. 
The label of a small vial was coated with it, and exposed 
thirty-six hours to the action of cold water which was 
afterwards raised to the boiling point — without any effect? 
except a temporary destruction of transparency. Cold and 
boiling alcohol were alike powerless. 
Myrrh, balsam of tolu and gum benzoin were productive 
of similar results. Solutions of these in collodion, spread 
upon plates of glass, and permitted to dry spontaneously, or 
by very gentle heat, form pellicles, easily detached while yet 
slightly moist, and which when dry are very convenient for 
experimental purposes. If a film of perfect transparency is 
desired, certain precautions must be observed, or failure will 
result. The solution, after being poured upon the glass, 
must be partially protected from the atmosphere by an in- 
verted capsule or similar vessel ; a gentle heat applied be- 
neath being advisable. 
This mode of procedure has almost invariably resulted in 
the formation of a perfectly transparent pellicle ■ while 
portions of the same liquid evaporated in the open air, 
became cloudy, white, and sometimes perfectly opaque ; an 
