This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol2", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This is obtained from Abies Canadensis, or the common hemlock spruce of this country, inhabiting the British Provinces, our own Northern States, and the hilly regions of the interior further south. It is prepared by removing the concrete exuded juice, which is found upon the bark of the older trees, and purifying it by heating in water, and straining. As brought to the shops, it generally requires, in consequence of its impurities, to be again melted in water and strained.
In cold weather it is hard and brittle, but in summer is so soft as to take the shape of the drawer in which it may be placed. Its colour is a yellowish-brown, which becomes almost black on exposure. It has a slight peculiar odour, and very little taste. Its constituents are resm, water, and a minute proportion of volatile oil.
Its effects and applications are precisely the same as those of Burgundy pitch. The only point in which it is inferior is, that it melts somewhat more readily, and, at the heat of the body, is generally a little too soft, so that the plaster does not well retain its place. in this respect it would probably improve with age. in the officinal plaster (Emplastrum Picis Canadensis, U. S.), this difficulty is obviated by the addition of one-twelfth of wax, which is even more needed by the Canada than by the Burgundy pitch.