The various mixtures of gold and silver, called electrum,* those of the Corinthian metal, the varieties of bronze, the compound of copper and zinc now called brass, the metal for specula, || the metal called argentarium, ¶ (in some measure answering to our pewter,) the art of plating and of tinning,** and the process of amalgamation,†† evince how great a progress had been made by the ancients in the mixing and working of metals. Much, however, remains to be done, and much may be expected, from a regular and systematical series of experiments on the properties of compound metals. For, exclusively of the immediate application of many of the alloys to economical purposes, it cannot be doubted that science will derive other considerable advantages; our ideas concerning the properties of the metals, whether simple or mixed, will be much enlarged, and clouds of errors, with the traditionary prejudices which as yet shade this branch of human knowledge, will be dispersed. * PLINIUS, lib. xxx111. cap. iv. Ibid. and cap. ix. + PLIN. lib. XXIV. cap. ii. PLIN. lib. xxx111. cap. ix. and lib. xxxiv. cap. xvii. .. Ibid. †† VITRUVIUS, lib. v11. cap. viii. V. Observations on the chemical Nature of the Humours of the Eye. By Richard Chenevix, Esq. F. R. S. and M. R. I. A. Read November 5, 1802.. THE functions of the eye, so far as they are physical, have been found subject to the common laws of optics. It cannot be expected that chemistry should clear up such obscure points of physiology, as all the operations of vision appear to be; but, some acquaintance with the intimate nature of the substances which produce the effects, cannot fail to be a useful appendage to a knowledge of the mechanical structure of the organ. The chemical history of the humours of the eye, is not of much extent. The aqueous humour had been examined by BERTRANDI; who said, that its specific gravity was 975, and therefore less than that of distilled water. FOURCROY, in his Système des Connoissances chimiques, tells us, that it has a saltish taste; that it evaporates without leaving a residuum; but that it contains some animal matter, with some alkaline phosphate and muriate. These contradictions only prove, that we have no accurate knowledge upon the subject. The vitreous humour is not better known. WINTRINGHAM has given its specific gravity (taking water at 10000) as equal to 10024; but I am not acquainted with any experiments to investigate its chemical nature. We are told by CHROUET, that the crystalline lens affords, by destructive distillation, fetid oil, carbonate of ammonia, and water, leaving some carbon in the retort. But, destructive distillation, although it has given us much knowledge as to animal matter in general, is too vague a method for investigating particular animal substances. I shall now proceed to mention the experiments I have made upon all the humours. I shall first relate those which were made upon the eyes of sheep, (they being the most easily procured,) and shall afterwards speak of those of the human body, and other eyes. I think it right to observe, that all these eyes were as fresh as they could be obtained. SHEEPS' EYES. Aqueous Humour. The aqueous humour is a clear transparent liquid, of the specific gravity of 10090,* at 60° of FAHRENHEIT. When fresh, it has very little smell, or taste. It causes very little change in the vegetable reactive colours; and this little would not, I believe, be produced immediately after death. I imagine it to be owing to a generation of ammonia, some traces of which I discovered. When exposed to the air, at a moderate temperature, it evaporates slowly, and becomes slightly putrid. When made to boil, a coagulum is formed, but so small as hardly to be perceptible. Evaporated to dryness, a residuum remains, weighing not more than 8 per cent. of the original liquor. Tannin causes a precipitate in the fresh aqueous humour, All these specific gravities are mean proportionals of several experiments. The eyes of the same species of animal, do not differ much in the specific gravity of their humours. |