III. "On the Thermo-electricity of Ferruginous Metals, and on the Thermal Effects of stretching Solid Bodies." By J. P. JOULE, F.R.S. Received January 29, 1857.

The experiments on the above subjects were made with a thermomultiplier placed in the vacuum of an air-pump. Its sensibility was such that with the junction antimony and bismuth, a thermometric effect not greater than of a degree Centigrade could be estimated. In determining the thermo-electric position of the metals, it was necessary to increase the resistance of the instrument a hundredfold, by placing in the circuit a coil of fine wire. In thermo-electric arrangement steel was found to be nearer copper than iron was. By hardening, steel was raised almost to the place of copper. Cast iron was found to surpass copper; so that the junction cast iron and copper is reverse to that of wrought iron and copper, and the arrangement cast iron and wrought iron is much more powerful than copper and wrought iron. A new test of the quality and purity of ferruginous metals is thus indicated, which will probably be found of value to the arts.

The experiments on the stretching of solids showed, in the case of the metals, a decrease of temperature when the stretching weight was applied, and a heating effect when the weight was removed. An iron wire of an inch in diameter was cooled of a degree Centigrade when stretched by a weight of 775 lbs. Similar results were obtained with cast iron, hard steel, copper, and lead. The thermal effects were in all these cases found to be almost identical with those deduced from Professor Thomson's theoretical investigation, the particular formula applicable to the case in question being II=— × Pe, where H is the heat absorbed in a wire one foot long, t the absolute temperature, J the mechanical equivalent of the thermal unit, P the weight applied, and e the coefficient of expansion per 1°. With gutta-percha also a cooling effect on extension was observed; but a reverse action was discovered in the case of vulcanized indiarubber, which became heated when the weight is laid on, and cooled when the weight was removed. On learning this curious result, Professor Thomson, who had already intimated the probability of 2 E

VOL. VIII.

a reverse action being observed under certain circumstances with india-rubber, suggested to the author experiments to ascertain whether vulcanized india-rubber stretched by a weight is shortened by increase of temperature. Accordingly, on trial, it was found that this material, when stretched by a weight capable of doubling its length, has that length diminished by one-tenth when its temperature is raised 50° Centigrade. This shortening effect was found to increase rapidly with the stretching weight employed; and, exactly according with the heating effects observed with different stretching weights, entirely to confirm the theory of Professor Thomson.

February 5, 1857.

The LORD WROTTESLEY, President, in the Chair.

THE BAKERIAN LECTURE was delivered by MICHAEL FARADAY, Esq., F.R.S., "On the Relations of Gold and other Metals to Light."

The Lecturer gave an exposition, illustrated by experiments, of the substance of a paper presented by him to the Society under the above title. The following is an abstract :

:

The author of this paper hopes that the undulatory theory of light, when more fully and perfectly developed, may aid in comparing local actions with those which take place at a distance, and even help towards the comprehension of the physical means by which the latter are carried on; and with that view he endeavoured, experimentally, to subject a ray of light to the action of particles, so small in size as to have an immediate and near relation, not only to the undulations of light, but even to the far smaller motions of the parts of the ether, which are supposed to produce, by their joint and successive action, the light-wave. His hope was, that by choosing particles of a fitting substance, experimental results might be obtained which, in the hands of the mathematical philosopher, might aid in perfecting the theory; and for this purpose gold was selected, because of its high optical

qualities, shown in its comparative opacity, whilst possessing a real transparency; its high yellow reflexion and its true green transmission; its known action on light in very minute quantity; its capability of extreme division; its great gravitating force, which could be called upon for aid when the metal was in a state of extreme division; its elementary character; the integrity of its metallic state; the facilities of testing its presence and condition; and, finally, because known phenomena seemed already to indicate dif ferences of action on light consequent upon its division.

The first state of division or attenuation considered, was that conferred on gold by beating into leaves. These, with their dimensions and general characters, are well known. Being taken up on glass damped by breathing or moistening, and then water introduced between the glass and the gold as a cushion, the gold can be perfectly stretched, so that when dry it is fit for optical examination; or if a diluted solution of cyanide of potassium be in like manner introduced beneath the gold, it can be more or less attenuated by solution, and then washed and dried. If gold-leaf thus extended and attached, either to glass, or plates of rock-crystal, or mica, be heated, it gradually loses its reflective power and its green colour, and becomes translucent. This change takes place far below the fusing-point of gold, and at a temperature as low as the boiling-point of oil if continued for several hours. When the heat is considerable, the gold-leaf suffers retraction of its parts, and becomes perforated by many fine holes, often symmetric in their form and dimension; but when the heat applied. is the lowest competent to produce the change, it does not seem certain that the effect is due to such retraction; a good microscopic examination of this point is required. When pressure is applied to such decoloured gold by a convex piece of rock-crystal of short radius (as half an inch or less), the green colour of the transmitted ray reappears. This production of the green colour by pressure can often be referred to in different states of gold, as a proof, amongst others, that the metal is in the metallic condition. Silver-leaf undergoes a like change by heat, at even a lower temperature.

Division by the Leyden Deflagration.-When a gold wire is deflagrated near the surface of glass plates by a strong electric discharge, it is dissipated in minute particles, which are deposited on the glass. These are seen by the microscope to be of different sizes;

but by far the greater part are so minute as not to be distinguishable separately. The general film is of different colours by transmitted light, being grey, violet, or green; and often on the central or nearest part of the discharge, where the heat has been active, is of a fine ruby colour. All these particles act with acid and chemical reagents as gold acts; and there is no reason to believe they are anything other than metallic gold. They appear with precisely the same colours and characters, whether the deflagrations are made in common air, in oxygen, or in hydrogen; and whether the deposits are formed on glass, rock-crystal, topaz, or mica. When heated by any ordinary means, the green and grey parts change to a ruby or ruby-amethystine colour, and that whether surrounded by air, or vapour of alcohol or ether. Even after heating, they adhere only as a dust to the plates, except when the temperature applied to those on glass has been very high. Agate pressure confers the green character on the heated deposits, and also, in frequent cases, upon that which has not been heated. All things considered, there can be no reason to doubt that the deposits thus made to vary in the colour of the transmitted light, consist of pure metallic gold.

Thin films of Gold.—If a very weak solution of chloride of gold, free from excess of acid, and containing about 1 grain of metal to 2 or 3 pints of water, be placed in a very clean glass or glazed vessel, in a quiet place, and then two or three small particles of phosphorus be laid floating on the surface, and the whole covered over and left for twelve or more hours, the gold will be reduced, covering the whole of the surface with a film, thicker near the phosphorus than at other parts. This film may be raised from the fluid by plates of glass, and washed and dried on the plates, and is then ready for examination. The thinner parts of such a film are scarcely visible, either by reflected or transmitted light; the transition to thicker parts is gradual, the thickest being opaque, and their reflexion that of dense gold. The colour by transmitted light varies, being grey, green, or dull violet. The films are porous, and act as pure gold, resisting all the agents which metallic gold resists. When heated, the transmitted colour changes towards amethyst and ruby; and then the effect of pressure in producing a green colour is in many cases very remarkable,—even a touch with a card or the finger being able to cause the change.

Gold fluids. Whilst the particles of phosphorus are producing a film on the surface, it frequently happens that streams of a red colour descend from them through the fluid; and if the phosphorus be submerged, and left for twenty-four or forty-eight hours, this red product is easily and abundantly obtained. If the gold solution be placed in a very clean bottle, and then a few drops of a solution of phosphorus in ether be added, and the whole agitated from time to time, the ruby fluid is obtained in a shorter period. This fluid is apt to change in colour, becoming amethystine, violet, purple, and finally blue; impurities of certain kinds in very small quantities cause this change. It is hardly possible to clean a vessel so well that the first portion put into it does not alter. Most saline bodies produce the change; a trace of common salt readily makes it manifest. That all these fluids are coloured by diffused particles is shown by the circumstance, that on being left for a shorter or longer time, the particles sink, forming a coloured stratum of deposit; many months, however, are required for even the partial separation of the finer ruby particles. When a light is looked at through the fluid, the latter appears transparent; but when the eye is on the illuminated side, then the fluid is seen opalescent. If a cone of sun-rays be thrown by a lens into the fluid, the illumination of the particles within the cone shows their presence as undissolved bodies. It is believed that all the particles being metallic gold, the ruby are in the finest state of division, the blue in a more aggregated condition. Though the ruby particles, whilst freely diffused, are easily changed in colour, and as it is supposed by aggregation, still they may in some degree be separated by a filter; for on passing the fluid several times through a paper filter, the latter associates much of the rubifying substance with itself, and becomes of a rose colour; it may then be well washed and dried, and contains the ruby particles located, as it is believed, and prevented by their attachment to the paper-fibres from undergoing mutual aggregation. In this state their character is not altered from ruby to blue by salt or acids; they resist those chemical agents which are resisted by gold, but are dissolved by chlorine, cyanides and the other substances capable of acting on gold. Heated either in oxygen, hydrogen, or air, no change of tint or quality is induced at such temperatures as the paper can bear; or, as far as can be judged, at any higher temperature.