panied by the lens, which usually forms a part of it. The crossing of the lines, at the point of distinct vision, did not seem so essential to the powers of adjustment as to be depended on; for the appearance varied with different eyes, and was not, except in particular circumstances, exactly as Dr. Young asserts it to have been found in his own trials. From comparing the different facts and the conclusions, which are, occasionally, on each side subject to a little uncertainty, we suspect that the distinctness of vision, at different distances, depends on more than one cause. II. The Bakerian Lecture. On the Theory of Light and Colours. By Thomas Young, M.D. F.R.S. Professor of Natural Philosophy in the Royal Institution.' • The object of the present dissertation is not so much to propose any opinions which are absolutely new, as to refer some theories, which have been already advanced, to their original inventors, to support them by additional evidence, and to apply them to a great number of diversified facts, which have hitherto been buried in obscurity. Nor is it absolutely necessary in this instance to produce a single new experiment; for of experiments there is already an ample store, which are so much the more unexceptionable, as they must have been conducted without the least partiality for the system by which they will be explained; yet some facts, hitherto unobserved, will be brought forwards, in order to show the perfect agreement of that system with the multifarious phenomena of nature. The optical observations of Newton are yet unrivalled; and, excepting some casual inaccuracies, they only rise in our estimation, as we compare them with later attempts to improve on them. A further consideration of the colours of thin plates, as they are described in the second book of Newton's Optics, has converted that prepossession which I before entertained for the undulatory system of light, into a very strong conviction of its truth and sufficiency; a conviction which has been since most strikingly confirmed, by an analysis of the colours of striated substances. The phenomena of thin plates are indeed so singular, that their general complexion is not without great difficulty reconcileable to any theory, however complicated, that has hitherto been applied to them; and some of the principal circumstances have never been explained by the most gratuitous assump tions; but it will appear, that the minutest particulars of these phenomena, are not only perfectly consistent with the theory which will now be detailed, but that they are all the necessary consequences of that theory, without any auxiliary suppositions; and this by inferences so simple, that they become particular corollaries, which scarcely require a distinct enumeration." P. 12. We acknowledge that a hypothesis is often necessary to connect the numerous facts in a science; and that when such facts support, without any force, the hypothesis connecting them, the latter may assume a different name. After, however, considering maturely-and it is to us by no means a new sub. ject-all the facts and arguments, so far from appearances con firming the doctrine before us, they seem, in their whole tendency, to be adverse to it. The hypothesis is the existence of a luminiferous æther, and that light depends on its undulations. In our various philosophical disquisitions we have admitted of an æthereal fluid pervading all bodies; but Dr. Young is by no means explicit in his information, whether light be a modification of this general æther, or the undulations of a peculiar one. From various incidental expressions, however, he considers Newton as the advocate of the opinion, that light is owing to undulations, though he has so pointedly opposed it in his Optics; and his chief arguments rest on this being really the opinion of Newton. The author must have been strongly pressed when he supports his system by the opposition of the Newtonian hypothesis-contending, that, as both are equally probable, the opposition is merely accidental.' We had intended to have pursued our author's explanations at length, and pointed out much fallacious reasoning; but think it unnecessary. We have lately endeavoured to lead the views of philosophers to light as a chemical substance; and have at times adduced various and striking facts which can only be accounted for on this supposition. We can trace light as a component part of many bodies, and can again separate it. Chemical changes of very different kinds are produced by it; and in the preparation of argentum fulminans it is an essential ingredient; for this substance does not explode, unless, in the preparation, it have been exposed to a strong solar light. We mean not to contend that a hypothesis so diametrically opposite to our author's supports either. They are inconsistent with each other. One decisive argument against Dr. Young's system is, that light is propagated in straight lines; whereas undulatory motions must be propagated in circular vortices. To this objection Dr. Young endeavours, in vain, to reply. In short, in the words of Macquer, quoted by an ingenious author on this subject, a body, whose motion we can perceive, whose velocity we can calculate, whose direction we can change, which we can accumulate and disperse, whose constituent parts we can separate and re-unite, which we can combine with and separate from other bodies, must be a substance peculiar and distinct.' III. An Analysis of a mineral Substance from NorthAmerica, containing a Metal hitherto unknown. By Charles Hatchett, Esq. F.R.S.' We have already alluded to this mineral, and regretted that it should have been first announced in a French publication. We called it, however, an earth; but it appears to be a metallic substance. We shall transcribe our author's description of this ore, which was found among the specimens in the British Museum, and was sent by Mr. Winthrop to sir Hans Sloane, probably from some mine in the Massachusets. DESCRIPTION OF THE ORE. • The external colour is dark brownish gray. • The internal colour is the same, inclining to iron gray. • The longitudinal fracture is imperfectly lamellated; and the cross fracture shews a fine grain. The lustre is vitreous, slightly inclining in some parts to metallic lustre. It is moderately hard, and is very brittle. The colour of the streak or powder is dark chocolate brown. P. 50. It consists of an oxyd of iron, with a white substance which appears to be metallic; but it is not very heavy, has no perceptible flavour, and is not soluble in water: when moistened, it communicates an evident redness to paper. The preceding experiments shew, that the ore which has been analysed, consists of iron combined with an unknown substance, and that the latter constitutes more than three-fourths of the whole. This substance is proved to be of a metallic nature, by the coloured precipitates which it forms with prussiate of potash, and with tincture of galls; by the effects which zinc produces, when immersed in the acid solutions; and by the colour which it communicates to phosphate of ammonia, or rather to concrete phosphoric acid, when melted with it. Moreover, from the experiments made with the blow-pipe, it seems to be one of those metallic substances which retain oxygen with great obstinacy, and are therefore of difficult reduction. It is an acidifiable metal; for the oxide reddens litmus paper, expels carbonic acid, and forms combinations with the fixed alkalis. But it is very different from the acidifiable metals which have of late been discovered; for, 1. It remains white when digested with nitric acid. 2. It is soluble in the sulphuric and muriatic acids, and forms colourless solutions, from which it may be precipitated, in the state of a white flocculent oxide, by zinc, by the fixed alkalis, and by ainmonia. Water also precipitates it from the sulphuric solution, in the state of a sulphate. The following results of some experiments which I have purposely made, will shew how much the specific gravity of this ore is different from that of Wolfram, and Siberian chromate of iron. Pure Wolfram, free from extraneous substances, at temp. 65° 6955. 3728. 4355. The Siberian chromate of iron, like all other mineral substances which are not crystallised, and which consequently are not always homegen cous, must evidently be liable to considerable variations in specific gravity.' 3. Prussiate of potash produces a cop us and beautiful olivegreen precipitate. 4. Tincture of galls forms orange or deep yellow precipitates. 5. Unlike the other metallic acids, it refuses to unite with ammonia. 6. When mixed and distilled with sulphur, it does not combine with it so as to form a metallic sulphuret. 6 7. It does not tinge any of the fluxes, except phosphoric acid, with which, even in the humid way, it appears to have a very great affivity. 8. When combined with potash and dissolved in water, it forms precipitates, upon being added to solutions of tungstate of potash, molybdate of potash, cobaltate of ammonia, and the alkaline solu tion of iron. These properties completely distinguish it from the other acidifiable metals, viz. arsenic, tungsten, molybdena, and chromium; as to the other metals lately discovered, such as uranium, titanium, and tellurium, they are still farther removed from it. The colours of the precipitates produced by prussiate of potash and tincture of galls approach the nearest to those afforded by titanium. But the prussiate of the latter is much browner; and the gallate is not of an orange colour, but of a brownish red, inclining to the colour of blood. Besides, even if these precipitates were more like each other, still the obstinacy with which titanium refuses to unite with the fixed alkalis, and the insolubility of it in acids when heated, sufficiently denote the different nature of these two sub, stances.' P.61. The olive-green prussiat, and the orange-gallate, are said to be fine colours, not affected by light, and promise to be useful as pigments. In the conclusion, it is justly remarked that apparently new bodies may be combinations of others well known; but when they possess new properties, they should, for a time at least, be considered as distinct substances. This argument we have had occasion to urge, as well as to explain the advan tages of such a theory. The specimen was a small one; but further trials must be made when the mine is again discovered. IV. A Description of the Anatomy of the Ornithorhynchus paradoxus. By Everard Home, Esq. F. R.S.' This singular animal is an inhabitant of fresh-water lakes, and resembles the amphibia (reptilia) in its structure and mode of living. Indeed, it appears to connect the aquatic birds and reptiles, and to be one of those varying shades which set systems at defiance. The heart contains two auricles and two ventricles; but the mode of increase resembles that of the lizard. The animal is, in fact, oviparous, nearly in the same way as the lizard. The particulars of the anatomy it is impossible to detail in this place without transcribing almost the whole article. V. On the Independence of the analytical and geometrical Methods of Investigation; and on the Advantages to be derived from their Separation. By Robert Woodhouse, A. M. Fellow of Caius College, Cambridge. Communicated by Joseph Planta, Esq. Sec. R.S.' This most able and comprehensive paper is, in some respects, a continuation of an inquiry communicated in the last volume of the Transactions. Mr. Woodhouse contends for the strict independence of the analytical and geometrical methods of investigation, showing, at the same time, their connexion, and the mutual assistance they afford. Des Cartes, Newton, and D'Alembert, enlarged the kingdom of algebra, by extending its powers in proportion to their wants, and the necessary questions to be investigated. The respective advantages of each analysis are next examined. The geometrical method is decidedly more perspicuous when its subjects are simple and easily comprehended; the analytical calculus more commodious; and it has been carried to a greater extent, while, at the same time, it is distinguished by superior success. The question, then, concerning the respective advantages of the ancient geometry and modern analysis, may be comprised within a short compass. If mental discipline and recreation are sought for, they may be found in both methods; neither is essentially inaccurate; and, although in simple inquiries the geometrical has greater evidence, in abstruse and intricate investigation the analytical is most luminous; but, if the expeditious deduction of truth is the object, then I conceive the analytical calculus ought to be preferred. To arrive at a certain end, we should surely use the simplest means; and there is, I think, little to praise or emulate, in the labours of those who resolutely seek truth through the most difficult paths, who love what is arduous because it is arduous, and in subjects naturally difficult toil with instruments the most incommodious.' P. 122. The author then endeavours to show the general superiority of the analysis, and notices the great difficulties felt by those, who, with professor Stewart, apply geometry to the explanation of natural phænomena. A slight censure is also extended to the mathematicians who explain the doctrine of logarithms by the introduction of the property of curves, which we have always thought one of the happiest and most successful applications of geometry. The author's great object is to show that algebra wants no aid from geometry. We do not think he has succeeded. The geometer sees the whole subject with a luminous precision; the algebraïst gropes in the dark; and, when he has attained his end, cannot always perceive the means, nor confirm his conclusions. VI. Observations and Experiments upon oxygenized and hyperoxygenized muriatic Acid; and upon some Combinations of the muriatic Acid in its three States. By Richard Chenevix, Esq. F.R.S. and M.R.I.A.' |