This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Good opium is of a reddish-brown or deep-fawn colour in mass, and when dry yields a yellowish-brown powder, which becomes adhesive at a slight elevation of temperature. When drawn over paper, it leaves an interrupted trace of a light-brown colour. Its odour is strong, narcotic, and peculiar; its taste bitter, somewhat acrid, and nauseous. When long chewed it irritates the mouth, and may even vesicate. It is inflammable. Water, alcohol, and the diluted acids extract its virtues, which, however, it will not yield to ether. The liquids impregnated with it have a deep-brown colour.
Besides several principles found in other vegetable products, as gummy and extractive matter, resin, a substance resembling caoutchouc, fixed oil, albumen, and various mineral substances in very small proportion, opium contains morphia, narcotina, codeia, narcein or narceia, thebaina or paramorphia, papaverina and opiania, having alkaline properties, meconin, and porphyroxin, which are neuter, an acid denominated meconic acid, and a characteristic odorous principle, all of which are peculiar to opium. Morphia exists in it combined with meconic acid, and probably in small proportions with sulphuric acid. Of these principles the only one hitherto much employed in medicine is morphia. Narcotina, codeia, and narceia have been recommended for special purposes. Of the remainder, though some of them may be and probably are efficient in their action on the system, so little is positively known, that it will not be worth while to burden the memory of the reader with an account of their properties. The four particularly mentioned will be sufficiently described among the preparations of opium.
Incompatibles. Many substances produce precipitates with opium, which, as they do not affect its active principles, are not medically incompatible, so far as the opium itself is concerned. With the infusion, the alkalies throw down its alkaloids, and the astringents containing gallo-tannie acid, as well as kino, catechu, and rhatany, precipitate insoluble tannates of the same alkaloids; but alcohol, inconsiderable proportion, or an excess of acid, will redissolve the precipitate in both instances. Tincture of galls, notwithstanding the alcohol it contains, throws down a copious precipitate.
Tests of Opium. An infusion of opium reddens litmus paper, becomes turbid with solution of ammonia, assumes a deep-red colour on the addition of sesquichloride of iron in consequence of the formation of meconate of iron, is reddened by nitric acid, and is copiously precipitated by infusion of galls. But the only satisfactory test of its value is the proportion of morphia contained in it. Good opium, treated as directed in the U. S. Pharmacopoeia for the preparation of morphia, should afford from 10 to 12 per cent. of the impure morphia precipitated from its infusion by a mixture of alcohol and solution of ammonia; and ether should not dissolve more than from 2 to 4 parts of this impure product.