a. Drying oils (such as the oils of poppy and walnut) may be distinguished by their not being converted into solid crystallizable elaidin by hyponitric acid or concentrated solution of nitrate of protoxide of mercury. Olive oil which contains any considerable proportion of one of these oils, no longer solidifies if exposed for a moment to one of the above-mentioned reagents. This test however is not of sufficient delicacy for small amounts of drying oils.
b. Olive oil being one of the lighter oils, the specific gravity may to some degree indicate admixture with a heavier oil. To make use of this fact, Gobley and other chemists have invented an instrument called an elaiometer, for taking the specific gravity of oils.
c. Observation of the Cohesion-figure.—This test, proposed by Tomlinson in 1864,[1554] depends on the forces of cohesion, adhesion, and diffusion. Thus, if a drop of any oil hanging from the end of a glass rod is gently deposited upon the surface of chemically clean water, contained in a clean glass, a contest takes place between the forces in question the moment the drop flattens down by its gravity upon the surface of the water. The adhesion of the liquid surface tends to spread out the drop into a film, the cohesive force of the particles of the drop strives to prevent that extension, and the resultant of these forces is a figure which Mr. Tomlinson believes to be definite for every independent liquid. The figure thus produced is named the cohesion-figure.
So far as our experience goes, the processes hitherto recommended for testing olive oil (and there are several that we have not mentioned) are only available in cases where the adulteration is considerable, and are quite insufficient for discovering a small admixture of other oils. How little they are appreciated, may be inferred from the fact that the Chamber of Commerce of Nice[1555] offered a reward of 15,000 francs (£600) for a simple and easy process for making evident an admixture with olive oil of 5 per cent. at least of any seed-oil.
APOCYNEÆ.
CORTEX ALSTONIÆ.
Cortex Alstoniæ scholaris; Dita Bark;[1556] Alstonia Bark.
Botanical Origin—Alstonia[1557] scholaris R. Brown (Echites scholaris L.), a handsome forest tree, 50 to 90 feet in height, common throughout the Indian Peninsula from the sub-Himalayan region to Ceylon and Burma; found also in the Philippines, Java, Timor and Eastern Australia, likewise in Tropical Africa. It has oblong obovate leaves, in whorls of 5 to 7, and slender pendulous pods a foot or more in length.
History—Saptachhada and saptaparna (literally seven-leaf), occurring in early Sanskrit epic poetry and also in Susruta, are ancient names of Alstonia (Dr. Rice). Rheede[1558] in 1678 and Rumphius[1559] in 1741 described and figured the tree, and mentioned the use made of its bark by the native practitioners. Rumphius also explained the trivial name scholaris as referring to slabs of the close-grained wood which are used as school-slates, the letters being traced upon them in sand. The tonic properties of the bark were favourably spoken of by Graham in his Catalogue of Bombay Plants (1839), and further recommended by Dr. Alexander Gibson in 1853[1560]. The drug has a place in the Pharmacopœia of India, 1868.
Description—The drug, as presented to one of us by the late Dr. Gibson and by Mr. Broughton of Ootacamund, consists of irregular fragments of bark, ⅛ to ½ an inch thick, of a spongy texture, easily breaking with a short, coarse fracture. The external surface is very uneven and rough, dark grey or brownish, sometimes with blackish spots; the interior substance and inner surface (liber) is of a bright buff. A transverse section shows the liber to be finely marked by numerous small medullary rays. The bark is almost inodorous; its taste is purely bitter and neither aromatic nor acrid.