Whether benzoin owes its original fluidity to a volatile oil holding the resin in solution, and its solidification to the volatilization of this oil, or whether the resin itself hardens by oxidation,—what occasions the remarkable diversity of aspect between the opaque and milk-like, and the completely transparent resin, are questions to be investigated by some future observer.
Description—Benzoin (always termed in English commerce Gum Benjamin) is distinguished as of two kinds, Siam and Sumatra. Each sort occurs in various degrees of purity, and under considerable differences of appearance.
1. Siam Benzoin—The most esteemed sort is that which consists entirely of flattened tears or drops, an inch or two long, of an opaque, milk-like, white resin, loosely agglutinated into a mass. More frequently the mass is quite compact, consisting of a certain proportion of white tears of the size of an almond downwards, imbedded in a deep, rich amber-brown, translucent resin. Occasionally the translucent resin preponderates, and the white tears are almost wanting. In some packages, the tears of white resin are very small, and the whole mass has the aspect of a reddish-brown granite. There is always a certain admixture of bits of wood, bark, and other accidental impurities.
The white tears when broken, display a stratified structure with layers of greater or less translucency. By keeping, the white milky resin becomes brown and transparent on the surface.
Siam benzoin is very brittle, the opaque tears showing a slightly waxy, the transparent a glassy fracture. It easily softens in the mouth and may be kneaded with the teeth like mastich. It has a delicate balsamic, vanilla-like, fragrance, but very little taste. When heated it evolves a more powerful fragrance, together with the irritating fumes of benzoic acid; its fusing point is 75° C. The presence of benzoic acid may be shown by the microscopical examination of splinters of the resin under oil of turpentine.
Siam benzoin is imported in cubic blocks, which takes their form from the wooden cases in which they are packed while the resin is still soft.
2. Sumatra Benzoin—Prior to the renewal of direct commercial intercourse with Siam in 1853, this was the sort of benzoin most commonly found in commerce.
It is imported in cubic blocks exactly like the preceding, from which it differs in its generally greyer tint. The mass however, when the drug is of good quality, contains numerous opaque tears, set in a translucent, greyish-brown resin, mixed with bits of wood and bark. When less good, the white tears are wanting, and the proportion of impurities is greater. We have even seen samples consisting almost wholly of bark. In odour, Sumatra benzoin is both weaker and less agreeable than the Siam drug, and generally falls short of it in purity[1495] and handsome appearance, and hence commands a much lower price. The greyish-brown portion melts at 95°, the tears at 85° C.
A variety of Sumatra benzoin is distinguished by the London drug-brokers as Penang Benjamin or Storax-smelling Benjamin. We have seen it of very fine quality, full of white tears (some of them two inches long), the intervening resin being greyish.[1496] The odour is very agreeable, and perceptibly different from that of Siam benzoin, or the usual Sumatra sort. Whether this drug is produced in Sumatra and by Styrax Benzoin we know not; but it is worthy of note that S. subdenticulata Miq., occurring in Western Sumatra, has the same native name (Kajoe Kĕminjan) as S. Benzoin, and that Miquel remarks of it—“An etiam benzoiferum?”[1497]
Chemical Composition—Benzoin consists mainly of amorphous resins perfectly soluble in alcohol and in potash, having slightly acid properties, and differing in their behaviour to solvents. If two parts of the drug are boiled with one part of caustic lime and 20 parts of water, benzoic acid is removed. From the residue the excess of lime is dissolved by hydrochloric acid, and the remaining resins washed and dried. About one-third of them will be found readily soluble in ether, the prevailing portion dissolves in alcohol, and a small amount remains undissolved.