The proportion of hydrocyanic acid in the distilled water of the leaves has been the subject of many researches. Among the later are those of Broeker (1867), who distilled a given weight of the leaves grown in Holland under precisely similar circumstances, in each month of the year. The results proved that the product obtained during the winter and early spring was weaker in the acid in the proportion of 17 to 24, 28, or 30, the strongest water being that distilled in July and August. This chemist found that a stronger product was got when the leaves were chopped fine, than when they were used whole. According to Christison,[975] the buds and very young leaves yield ten times as much essential oil as the leaves one year old. We have ascertained that leaves collected in January when they were thoroughly frozen yielded a distillate containing about ten times less of hydrocyanic acid than in summer. The product obtained from the leaves collected in January, but previously dried for several days at 100° C. (212° F.), still proved to contain both essential oil and hydrocyanic acid.
The unwounded leaves of the cherry-laurel in vigorous vegetation have been shown by our friend Prof. Schaer, not to evolve naturally a trace of hydrocyanic acid, though they yield it on the slightest puncture. We are ignorant of the mode of distribution in the living tissue of the laurocerasin, and of the substances causing its decomposition, and how these two bodies are packed so as to prevent the slightest mutual reaction. The leaves may be even dried at 100° C. and powdered without the evolution of any odour of hydrocyanic acid, but the latter is at once developed by the addition of a little water; on distilling its presence is proved by means of all the usual tests in the first drops of the product.
Besides the substances concerned in the production of the essential oil, the leaves contain sugar which reduces cupric oxide in the cold, a small quantity of an iron-greening tannin, and a fatty or waxy substance.
Schoonbroodt (1868) treated the aqueous extract of the fresh leaves with alcoholic ether, which yielded ¼ per mille of bitter, acicular crystals; these quickly reduced cupric oxide, losing their bitterness.
Bougarel (1877) isolated from the leaves under notice and several others, Phyllinic acid, a crystalline powder melting at 170° C.
Uses—The leaves are only employed for making cherry-laurel water (Aqua Lauro-cerasi), the use of which in England is generally superseded by that of the more definite hydrocyanic acid.
FLORES KOSO.
Flores Brayeræ, Cusso, Kousso, Kosso.
Botanical Origin—Hagenia abyssinica Willd. (Brayera anthelminthica Kunth), a handsome tree growing to a height of 60 feet, found throughout the entire table-land of Abyssinia at an elevation of 3,000 to 8,000 feet above the sea-level.[976] We have never noticed it growing in any botanic garden. The tree[977] is remarkable for its abundant foliage and fine panicles of flowers, and is generally planted about the Abyssinian villages.
History—The celebrated Bruce[978] during his journey to discover the source of the Nile, 1768-1773, found the koso tree in Abyssinia, observed the uses made of it by the natives, and published a figure of it in the narrative of his travels. It was also described in 1799 by Willdenow who called it Hagenia in honour of Dr. K. G. Hagen of Königsberg.