Dr. Stenhouse, to whom I have given a sample of it, is now engaged in its investigation; and in a letter which I have received from him, he says, that though he has not been able to get the aloin ready for analysis, yet from the experiments he has already made with it, he has scarcely a doubt that it will be found identical with that formerly obtained from Barbados aloes. It forms, he adds, a precisely similar combination with bromine, and, in short, agrees with it in every particular; I shall, therefore, provisionally term this crystalline principle the aloin of Socotrine aloes. On comparing it with a fine specimen of aloin, kindly presented to me by Messrs. Smith, I find its crystals smaller and more tapering—the summits of the crystals being more acute.
[21] See New York Journal of Pharmacy, No. vi. page [177.]
In drying, the crystals of the Socotrine aloin have a strong {238} tendency to break up; so that crystals which in the moist state are moderately large and regular, become small and pulverulent when dry. Like the aloin crystals of Messrs. Smith, the aloin crystals of Socotrine aloes, strongly doubly refract and depolarize light, and are, therefore, beautiful objects when viewed by the polarizing microscope.
The crystals of aloin contained in Socotrine aloe juice cannot be confounded with the crystals of oxalate and phosphate of lime found in the juices of various plants, and which are called by botanists raphides. The appearance under the microscope of the former is very different from that of the latter. Moreover, the ready fusibility, solubility, and complete combustibility of aloin crystals easily distinguish them from the calcareous salts just referred to. On platinum foil the aloin burns without leaving any residue, except such as may arise from the presence of traces of some foreign matter.
Aloin may be readily obtained from the juice by mixing the latter with spirit (either rectified or proof,) and collecting and drying the precipitate. When procured in this way it appears to the naked eye like a yellow powder; but when examined by the microscope it is found to consist of minute fragments of crystals.
The tincture from which the aloin has been separated, yields by distillation a spirit having the fragrant odor of the juice; showing that the latter contains some volatile odorous principle. By evaporation the tincture yields a resiniform extract.
In the first edition of my Elements of Materia Medica, published 1840, I have stated, that by digesting hepatic aloes in rectified spirit of wine, a yellowish granular powder is obtained which is insoluble in [cold] water, alcohol, ether, and dilute sulphuric acid, but is readily soluble in a solution of caustic potash, forming a red colored liquid. The powder like residue here referred to, is identical with the aloin of Socotrine aloes. When examined by the microscope, it is perceived to consist of very minute prismatic crystals, which depolarize polarized light like the larger crystals of aloin above referred to. I {239} think, therefore, that it may be safely inferred that hepatic aloes has been prepared without the employment of artificial heat, and that its opacity is due to the presence of minute crystals of aloin.
When Socotrine aloes is digested in rectified spirit, an insoluble portion is also obtained; but its color, instead of being yellow, as in hepatic aloes, is dark brown. On submitting this dark brown insoluble portion to microscopic examination, I find that it contains depolarizing crystals.
Artificial Socotrine aloes (prepared by evaporating this aloe juice) also yields, when digested in rectified spirit, a dark brown insoluble portion.
I think, therefore, that Socotrine aloes differs from hepatic aloes in the circumstance of its having been prepared by the aid of artificial heat; by which its aloin constituent has become altered. This inference is further substantiated by the fact, that after it has been melted, hepatic aloes is found to have acquired the clearness and transparency of the Socotrine sort.