Histed was the first to assert that the crystalline matter of Socotrine or Zanzibar aloes is a peculiar substance, according neither with barbaloïn nor with nataloïn. This observation was fully corroborated by our own experiments,[2577] made chiefly on the Zanzibar aloes just described, and we shall call the substance thus discovered Socaloïn. In this drug, the crystals are prisms of comparatively large size, such as we have never observed in Natal aloes. They cannot be so easily isolated as nataloïn, since they are nearly as soluble as the amorphous matter surrounding them. Histed recommends treating the powdered crude drug with a little alcohol, sp. gr. 0·960, and strongly pressing the pasty mass between several thicknesses of calico; then dissolving the yellow crystalline cake in warm weak alcohol, and collecting the crystals which are formed by cooling and repose.
Socaloïn forms tufted acicular prisms, which by solution in methylic alcohol may be got 2 to 3 millimetres long. It is much more soluble than nataloïn. At ordinary temperatures, 30 parts of alcohol, 9 of acetic ether, 380 of ether, 90 of water are capable of dissolving respectively one part of socaloïn; while in methylic alcohol, it is most abundantly soluble. Socaloïn is a hydrate, losing when dried over oil of vitriol 11 to 12 per cent. of water, but slowly regaining it if afterwards exposed to the air. Its elementary composition according to the analysis made by one of us (F.) is C₃₄H₃₈O₁₅ + 5 H₂O. We have not succeeded in obtaining any well-defined bromine compound of socaloïn.
The three aloïns, Barbaloïn, Nataloïn, and Socaloïn, are easily distinguished by the following beautiful reaction first noticed by Histed:—a drop of nitric acid on a porcelain slab gives with a few particles of barbaloïn or nataloïn, a vivid crimson,[2578] but produces little effect with socaloïn. To distinguish barbaloïn from nataloïn, test each by adding a minute quantity to a drop or two of oil of vitriol, then allowing the vapour from a rod touched with nitric acid to pass over the surface. Barbaloïn (and socaloïn) will undergo no change, but nataloïn will assume a fine blue.[2579]
The researches of E. von Sommaruga and Egger in Vienna (1874) have been directed in particular to the aloïn of Socotrine aloes. The melting point of this aloïn was found to be between 118° and 120° C., that of barbaloïn being much higher. The authors conclude that the three form an homologous series, that their composition may probably be represented thus:—
| Barbaloïn | C₁₇H₂₀O₇ |
| Nataloïn | C₁₆H₁₈O₇ |
| Socaloïn | C₁₅H₁₆O₇ |
They derive in all probability from anthracene, C₁₄H₁₀.
The portion of aloes insoluble in cold water was formerly distinguished as Resin of Aloes, from the soluble portion which was called Bitter of aloes or Aloëtin. From the labours of Kossmann (1863), these portions appear to have nearly the same composition. The soluble portions treated with dilute sulphuric acid, is said to yield Aloëresic and Aloëretic Acids, both crystallizable, besides the indifferent substance Aloëretin. These observations have not to our knowledge been confirmed.
It has been shown by Tilden and Rammell[2580] that the Resin of Aloes may by prolonged treatment with boiling water be separated into two bodies, which they distinguish as Soluble Resin A. and Insoluble Resin B. With the first it is possible to form a brominated compound, which though non-crystalline is apparently of definite composition. In the view of these chemists the Resin A. is a kind of anhydride of barbaloïn—Barbaloïn, 2(C₃₄H₃₆O₁₄) less H₂O = Aloe Resin A., C₆₈H₇₀O₂₇. The resin boiled with nitric acid yields a large amount of chrysammic acid, together with picric and oxalic acids, and carbonic anhydride. Insoluble Resin B. was found to have nearly the same composition as Resin A.
Aloes treated with various reagents affords a number of remarkable products. Thus, according to Rochleder and Czumpelick (1861) it yields, when boiled with soda-lye, colourless crystals an inch long, which appear to consist of a salt of Paracumaric Acid, together with small quantities of fragrant essential oils and volatile fatty acids.
When boiled with dilute sulphuric acid, aloes yields paracumaric acid, from which by fusion with caustic potash, as also directly from aloes, Hlasiwetz (1865) obtained Para-oxybenzoic Acid ([p. 408]). Weselsky (1872-73) has shown that accompanying the last two products, there is a peculiar, crystallizable acid, C₉H₁₀O₃, which he has named Alorcinic Acid.