Canada balsam

Summary

Canada balsam is secreted by cells in the bark of the balsam fir Abies balsamea, a conifer widely distributed across Canada and northern U.S.A. Collected by an extraordinarily labour-intensive process, it was initially exported to Europe for use in pharmacy. However, it found a much wider application after Brewster showed that the refractive index of 1·54–1·55 characterizing the vitreous mass remaining after heating the natural product exceeded that of most resins, and therefore closely matched the indices of ‘crown’ and ‘light flint’ optical glasses. ‘Cooked’ Canada balsam was therefore very suitable for cementing achromatic lenses for telescopes and microscopes, as well as for mounting specimens for microscopical examination. Canada balsam remained pre-eminent for over a century, but the rise of synthetic adhesives and other substitutes has now relegated the natural balsam to a few specialized niches.