The tragacanth of Kurdistan and Persia shipped from Bagdad, which sometimes appears in the London drug sales under the incorrect name of Syrian Tragacanth, is in very fine and large pieces which are rather more translucent and ribbon-like than the selected tragacanth imported from Smyrna: in fact, the two varieties when seen in bulk are easily distinguishable.
The inferior kinds of tragacanth have more or less of colour, and are contaminated with bark, earth and other foreign substances. They used formerly to be much imported into Europe, and were frequently mentioned during the past centuries as black tragacanth.
Microscopic Structure—The transformation of the cells into tragacanth is usually not so complete, that every trace of the original tissue or its contents has disappeared. In the ordinary drug, the remains of cell-walls as well as starch granules may be seen, especially if thin slices are examined under oil or any other liquid not acting on the gum. Polarized light will then distinctly show the starch and the cell-walls. If a thin section is imbued with a solution of iodine in iodide of potassium and then moistened with concentrated sulphuric acid, the cell-walls will assume a blue colour as well as the starch.
Chemical Composition—When tragacanth is immersed in water it swells, and in the course of some hours disintegrates so that it can be diffused through the liquid. So great is its power of absorbing water that even with 50 times its weight, it forms a thick mucilage. If one part of tragacanth is shaken with 100 parts of water and the liquid filtered, a neutral solution may be obtained which yields an abundant precipitate with acetate of lead, and mixes clearly with a concentrated solution of ferric chloride or of borax,—in these respects differing from a solution of gum arabic. On the other hand, it agrees with the latter in that it is thrown down as a transparent jelly by alcohol, and rendered turbid by oxalate of ammonium. The residue on the filter is a slightly turbid, slimy, non-adhesive mucilage, which when dried forms a very coherent mass. It has received the name of Bassorin, Traganthin or Adraganthin, and agrees with the formula C₁₂H₂₀O₁₀.
Tragacanth is readily soluble in alkaline liquids, even in ammonia water and at the same time assumes a yellow colour; heated with ammonia in a sealed tube at 90° C. it blackens.
The drug loses by drying about 14 per cent. of water, which it absorbs again on exposure to the air. Pure flake tragacanth incinerated leaves 3 per cent. of ash.
Commerce—Tragacanth is shipped from Constantinople, Smyrna and the Persian Gulf. The annual export of the gum from Smyrna has been recently stated[714] to be 4,500 quintals, value 675,000 Austrian florins (£67,500); and the demand to be always increasing.
Uses—Though tragacanth is devoid of active properties, it is a very useful addition to many medicines. Diffused in water it acts as a demulcent, and is also convenient for the suspension of a heavy powder in a mixture. It is an important ingredient for imparting firmness to lozenges and pill-masses.
Adulteration—The fine quantities consisting of large distinct pieces are not liable to adulteration, but the small and the inferior kinds are often sophisticated. At Smyrna, tragacanth is mixed with gums termed respectively Mosul and Caramania Gum. The former appears to be simply very inferior tragacanth; the latter which is sometimes called in the London market Hog Gum Tragacanth or Bassora Gum,[715] is said to be the exudation of almond and plum trees. It occurs in nodular masses of a waxy lustre and dull brown hue, which immersed in water gradually swells into a voluminous white mass. To render this gum available for adulteration, the lumps are broken into small angular fragments, the size of which is adjusted to the sort of tragacanth with which they are to be mixed. As the Caramania Gum is somewhat dark, it is usual to whiten it by white lead, previous to mixing it with Small Leaf or Flake, or with the Vermicelli gum.
By careful examination the fraud is easily detected, angular fragments not being proper to any true tragacanth. The presence of lead may be readily proved by shaking suspected fragments for a moment with dilute nitric acid, which will dissolve any carbonate present, and afford a solution which may be tested by the ordinary reagents.