stinctly formed black rings from the two positive structures, between which it lies. This state of the polarizing structure is shown in fig. 6, which is nearly the same as in the lens of the Cow. The structure No. 1, beginning at the centre, was pretty bright, but No. 3 was much more so, and No. 4 very faint, though perfectly distinct. On the second day the blue ring No. 2 was much enlarged, and had encroached greatly on the brightest structure No. 3, having reduced it both in breadth and intensity. No. 4 has also extended itself at the expense of No. 3. On the third day the new structure No. 2 had become the brightest of all. No. 4 had increased also, whilst No. 1 had become smaller and fainter, and No. 3 was wholly obliterated. In another pair of lenses one of them burst at this stage of the development of the polarizing structures, while in the other the effect was singularly fine. No. 3 was wholly, and No. 1 nearly obliterated; while the two new structures, which had no existence at first, were the only ones that remained. The new negative structure No. 2 consisted of four beautiful blue sectors of polarized light; but in consequence of the great absorption of distilled water, and the consequent distension of the lens, it soon burst. I have already remarked that only one case has occurred in the course of my experiments in which the central structure of the lenses of quadrupeds was negative, as in fishes. In this case, however, the centre of the lens had its structure affected by some change in the condition of the fibres at their union in the three septa, which were not only distinctly seen, but had the polarizing structure clearly related to them. The polarized light filled up each of the three angles of 120° which lay between the three septa, and the intensity of the light was a maximum close to the three septa. Hence it is evident that the central negative structure was the result of an induration of the lens related to the septa, and had obliterated the positive structure which would otherwise have existed there. In examining the lenses of the horse, I have observed the progressive development of its three structures as the animal advanced in age, and the extinction of all of them but one when the age of the animal was great. In both the lenses of a young horse three years old I found only one positive structure. In both the lenses of a horse whose age was unknown, I observed three structures beautifully developed. The central ones, which were extremely distinct and more beautiful in form and more intensely luminous than in any other quadruped which I had examined, were positive, the next structure negative, and the external one positive. In the lens of another horse, whose age was also unknown to me, the remains of three structures were visible; but the two positive ones, namely, the central and external structures, had just disappeared, but were not encroached upon by the intermediate negative one. They were therefore black when seen by polarized light, as shown in fig. 7, while the remaining negative one was of the most brilliant yellow colour. In the lenses of a third horse, probably of an intermediate age, I found a structure intermediate between that of the two preceding ones. The following were the dimensions of its lenses : : Longest diameter First lens. Second lens. 0.820 0.793 0.500 The first lens having been carefully prepared and immersed in distilled water, exhibited the beautiful optical figure which is but imperfectly represented in fig. 8. The central sectors were positive, but faintly illuminated. The wide and brilliant yellow and white structure was negative, and the external structure, which had just begun to appear, was positive. On the second day the black mass round the central sectors had enlarged itself, and become very black, having the form of a square lozenge. The yellow ring has risen in its tint to a brilliant pink yellow at the edges, the white ring within it having increased in width, and the white ring without it having diminished. On the third day the diameter of the lens had increased to 0.86 in all directions, and its thickness from 0·50 to 0·717 of an inch. The coloured ring has not changed greatly. On the fourth day the bright pink of the negative structure has risen to a bright blue, the pink and yellow being seen at its margin; and the external positive structure seems to be now conjoined with the blue negative structure, in consequence, no doubt, of the extension of the latter to the margin of the lens. The thickness of the lens was now upwards of 0.86, and the capsule came off, in consequence of which two of the blue sectors have become of a pale pink colour. The instant the capsule came off, the lens shrunk in all its dimensions nearly the tenth of an inch. The second lens on the third day gave exactly the optical figure shown in fig. 8, having been newly placed in distilled water; but the external ring seems to be slightly negative, like the yellow one. Its appearance was grayish and indistinct. On the fourth day the yellow ring had risen to a pale pink of the first order, and the outer ring was negative, as on the preceding day. On the fifth day the pink ring had increased in intensity, and the other structures remained the same as before. On the sixth day the pink had risen to a very bright blue. The diameter of the lens was now 0.867 of an inch, and its thickness 0.733, being an increase of 0.233 of an inch in thickness. On the seventh day the capsule burst, and upon removing it and the soft pulp which formed about one-tenth of an inch of the outer margin of the lens, the pink ring, with the white band both within and without it, and the black mass at the centre of the rectangular cross, were as distinct as ever. Hence it is manifest that the rise of the tint from yellow was not the effect of any expansive pressure produced by the swelling of the lens and the reaction of the capsule. The descent of the tint from bright blue to pink was no doubt owing to the polarizing action of the extended capsule being withdrawn. In order to prove this, I took the capsule, which is a tough and elastic membrane, and having stretched it, I found that it polarized, just before it tore, a white of the first order. Now the value of this tint is nearly equal to the difference between the values of the pink and blue of the second order of colours. The preceding results throw much light on the physiology of the crystalline lens; and I shall have occasion, in a separate paper, to point out the conclusions to which they lead respecting the cause and cure of cataract. Allerly by Melrose, May 6, 1837. XXXI. On the supposed Identity of the Agent concerned in the Phænomena of ordinary Electricity, Voltaic Electricity, Electromagnetism, Magneto-electricity, and Thermo-electricity. By M. DONOVAN, Esq., M.R.I.A. FULLY [Continued from p. 127.] aware that a conviction of the identity of the agent in all the phænomena called electric is firmly established in the minds of the scientific, and that experiments of apparently so convincing a nature have been brought to bear upon the subject that doubts seem to be no longer entertained, I scarcely know how to declare, in terms that shall protect me from the imputation of presumption, that I have never been able to view the matter in the same light. It is my duty to assign reasons for thus venturing to dissent from universally accredited opinions: I shall therefore enter into a critical examination of the chief arguments which have been made use of to establish the alleged identity. In doing so, I shall take occasion to refer to the explanations of these phænomena suggested by the views developed in the preceding pages of this essay, relative to the presumed compound nature of the electric fluid which with this object I stated in the commencement. Professor Faraday informs us, that at the period when he undertook the investigation of the agent concerned in these phanomena, the question of identity was as yet undecided. He says, "Notwithstanding, therefore, the general impression of the identity of electricities, it is evident that the proofs have not been sufficiently clear and distinct to obtain assent of all those who were competent to consider the subject:" hence his investigations rendered it necessary for him to "ascertain the identity or difference of common and voltaic electricity."-(266.) Being assured by so high an authority that, up to the period indicated, the question was doubtful, and having long felt a strong impression that the identity, far from being established, was rendered more problematical by every newly-discovered fact when viewed without prepossession or prejudice, I was anxious to put myself in a condition to appreciate Professor Faraday's reasonings. To these I have therefore addressed myself, aware that if they do not establish their object, the question which so long occupied the attention of inquirers would return to that original state of doubt in which this eminent philosopher found it, when, as he declares, he deemed a resumption of the investigation necessary. The vast difference of properties observable in electric and voltaic phænomena has been conceived to be explicable on the supposition, that in the former the quantity of electricity is small and the intensity great; while in the latter, the quantity is great and the intensity low. Professor Faraday thus expresses this universally accredited proposition: "Hence arises still further confirmation, if any were required, of the identity of common and voltaic electricity, and that the differences of intensity and quantity are quite sufficient to account for what were supposed to be their distinctive qualities*." Again, he says, "The great distinction of the electricities (common and voltaic) is the very high tension to which the small quantity, obtained by the aid of the machine, may be raised; and the enormous quantity in which that of comparatively low tension, supplied by the voltaic battery, may be procured+." In another place he says, "The general conclusion which must, I think, be drawn from this collection of * Researches, par. 378. See also 360. + Ibid. 451. facts is, that electricity, whatever may be its source, is identical in its nature. The phænomena in the five kinds or species quoted differ, not in their character, but only in degree; and in that respect vary in proportion to the variable circumstances of quantity and intensity*.” In this expression of opinion I believe most, if not all, philosophers of the present day agree, whether they view the electric fluid as homogeneous or not. The same view is given by Davy, Becquerel, Roget, and other eminent authorities, all following in the footsteps of Cavendish; I shall therefore devote my chief attention to the consideration of it. It is, in the first place, necessary to understand precisely what is meant by these terms quantity and intensity; they have been always used in explaining the difference between frictional and voltaic electricity; but, as it appears to me, without giving any satisfactory account of how these conditions of the electric fluid act. Dr. Faraday thus expresses himself:-"The term quantity in electricity is perhaps sufficiently definite as to sense; the term intensity is more difficult to define strictly. I am using the terms in their ordinary and accepted meaning." To understand his views, it is therefore necessary to inquire what the "ordinary and accepted meaning" of the word intensity is. Professor Hare gives a definition which I believe is a concise enunciation of the opinions of most philosophers on this subject. He asks, "What does intensity mean as applied to a fluid? Is it not expressed by the ratio of quantity to space? If there be twice as much electricity within one cubic inch as within another, is there not twice the intensity+?" Elsewhere he says, he is unable to form any other idea of intensity than "that of the ratio of quantity to space." Sir H. Davy gives his opinion to the same effect. Dr. Roget, in his excellent essay on galvanism in the Encyclopædia Metropolitana, p. 208, says, "that when all the circumstances relative to the conductors and the surrounding bodies are the same, the intensity will increase in a certain ratio with the absolute quantity of electricity that has been given to the conductor by the machine." Mr. Snow Harris's opinion may be resolved into the same meaning. Pouillet says that intensity is "precisely in the inverse ratio to the length of the circuit §." The same opinion is conveyed by Gmelin in the following sentence:-"The difference between the effects of the voltaic pile and those of the electric machine consists of the two following points: 1, by means of the latter a large quantity of *Researches, par. 360. † Philosophical Magazine, 1821, p. 292. Elements of Chemical Philosophy, p. 137. |
