[Canada Pitch; Hemlock Pitch; the prepared concrete juice of the Abies Canadensis, Mich.—As a substitute for Burgundy Pitch, this article is employed in the United States, over which it has the advantage of being in a state of purity. It is the product of the Abies Canadensis, or Hemlock Spruce, a large tree, attaining a height of seventy or eighty feet, with a circumference of six or nine feet. The leaves are six or eight lines long, very narrow, flat, and downy at the time of their expansion. The cones are a little longer than the leaves, oval, pendulous, and situated at the extremity of the branches.
This species of Abies is solely a native of North America, and belongs to the coldest regions of the continent, beginning to appear about Hudson's Bay. In the vicinity of Lake St. John and near Quebec, the forests are filled with it, and it is found in all the Northern States. It prefers high situations, and those the most humid and gloomy.
The wood of this tree is of little value; the bark contains a large amount of tannin, and is used in the tanneries where the oak is scarce.
Hemlock resin does not flow from the bark by incision, but is invariably the result of spontaneous exudation from knots or excrescences, the heat of the sun bringing it to the surface; and it is always obtained from old trees or those approaching decay. The proportion of trees from which any resin can be procured is not more than one in a hundred. Mr. Ellis (Journ. of Pharm., vol. ii. p. 20) informs us that the mode of obtaining it is as follows: "Trees are selected upon whose bark the resin is incrusted, which are easily designated by a streak of a dark brown colour on one side of the tree, from near the top to the bottom. These are cut down, and the bark, upon which the resin has hardened, stripped off and thrown into a kettle containing water, with weights placed upon it to prevent its floating. By boiling the water, the resin is melted and rises to the surface, is skimmed off, and thrown into cold water. It is then put into a coarse linen bag and submitted to a second ebullition, treating it as in the former instance, which deprives it of many of its impurities."
The quantity from good-sized trees is from six to ten pounds, the average from four to five. The colour of it as it exudes is nearly white; it hardens immediately, and changes to yellow, brown, and sometimes nearly black. Hemlock resin is in masses, very brittle. It is a resin in combination with a small quantity of volatile oil. It is heavier than water, sp. gr. 1.034. The odour is peculiar, and unlike turpentine. To purify it, it should be melted and strained. From its adhesiveness and stimulating properties, it affords a plaster which is equal to that made with Burgundy Pitch, if not superior. It may be employed for the same purposes.]
The Elements of Materia Medica and Therapeutics, Vol. II, 3th American ed., was written by Jonathan Pereira in 1853.