Nov. 4 Published the First Day of every Month.-Price 2s. 6d. THE LONDON, EDINBURGH, AND DUBLIN PHILOSOPHICAL MAGAZINE, AND JOURNAL OF SCIENCE. Being a Continuation of Tilloch's Philosophical Magazine,' Nicholson's 'Journal,' and Thomson's Annals of Philosophy.' CONDUCTED BY SIR ROBERT KANE, M.D. F.R.S. M.R.I.A. AND WILLIAM FRANCIS, Ph.D. F.L.S. F.R.A.S. F.C.S. FOURTH SERIES. N° 243.-OCTOBER 1868. WITH A PLATE, Illustrative of Prof. R. BUNSEN's Paper on Rhodium. LONDON: PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET, Printers and Publishers to the Univer y of London. Sold by Longmans, Green, Reader and Dyer; Kent and Co.; Simpkin, Marshall and Co.; Whittaker and Co.; and H. Baillière, London:-and by A. and C. Black, and Thomas Clark, Edinburgh; Smith and Son, Glasgow:-Hodges and Smith, Dublin:-and Putnam, New York. ROYAL SCHOOL OF MINES. Director-Sir Roderick Impey Murchison, Bart., K.C.B., F.R.S., &c. During the Eighteenth Session 1868-69, which will commence on the 5th of October, the following COURSES of LECTURES and PRACTICAL DEMONSTRATIONS will be given : 1. Chemistry. By E. Frankland, Ph.D., F.R.S. 2. Metallurgy. By John Percy, M.D., F.R.S. 3. Natural History. By T. H. Huxley, LL.D., F.R.S. 4. Mineralogy. By Warington W. Smyth, M.A., F.R.S. 5. Mining. 6. Geology. By A. C. Ramsay, LL.D., F.R.S. 7. Applied Mechanics. By Robert Willis, M.A., F.R.S. 8. Physics. By Instruction in Mechanical Drawing, by the Rev. J. Haythorne Edgar, M.A. The Fee for Students desirous of becoming Associates is £30 in one sum, on entrance, or two annual payments of £20, exclusive of the Laboratories. Pupils are received in the Royal College of Chemistry (the Laboratory of the School), under the direction of Dr. Frankland, and in the Metallurgical Laboratory under the direction of Dr. Percy. Tickets to separate Courses of Lectures are issued at £3 and £4 each. Officers in the Queen's Service, Her Majesty's Consuls, Acting Mining Agents and Managers, may obtain Tickets at reduced prices. Certificated Schoolmasters, Pupil Teachers, and others engaged in education, are also admitted to the Lectures at reduced fees. His Royal Highness the Prince of Wales grants two Scholarships, and several others have also been established by Government. For a Prospectus and information apply to the Registrar, Royal School of Mines, Jermyn Street, London, S.W. TRENHAM REEKS, Registrar. THESAURUS SILURICUS. This day is published, demy 4to, half cloth, 18s., THE FLORA AND FAUNA OF THE SILURIAN PERIOD. By JOHN J. BIGSBY, M.D., F.G.S., Formerly British Secretary, Canadian Boundary Commission, &c. John Van Voorst, Paternoster Row. In 8vo, with 12 Charts and Diagrams, price 14s. cloth, A TREATISE ON THE ACTION OF VIS INERTIÆ IN THE OCEAN; With Remarks on the Abstract Nature of the Forces of Vis Inertia and Gravitation, and a New Theory of the Tides. By WILLIAM LEIGHTON JORDAN, F.R.G.S. Just published, in 4to, on fine paper, Parts VI. and VII. of Being Contributions to the Archæology and Palæontology of Périgord and the adjoining Provinces of Southern France. By EDOUARD LARTET and HENRY CHRISTY. Edited by T. RUPERT JONES, Professor of Geology, &c., Royal Military College, Sandhurst. This work will be completed in about 20 Parts (price 3s. 6d. each); to appear at short intervals. Each Part will contain Six Plates, besides Letterpress. H. Baillière, Publisher, London; J. B. Baillière & Fils, Paris; C. Bailly-Baillière, Madrid; Baillière Brothers, New York; F. F. Baillière, Melbourne. [ADVERTISEMENTS continued on 3rd page of Cover. XXXII. On Some Effects of a Chemically Clean Surface. THE HE supersaturated solutions of a number of salts contained in chemically clean vessels can be kept during a long time without crystallizing, and even be reduced to temperatures much below the freezing-point of water, provided they are protected from the motes and dust of the air, and other chemically unclean bodies, which act as nuclei. For this purpose it is sufficient to close the mouths of the tubes and flasks with cotton-wool, which filters the air as it enters the vessel during the period of a rising barometer. When, however, the barometer falls, a portion of the air escapes from the tube, and carries with it some of the aqueous molecules of the solution; and the effect of this action often repeated is to lower the surface of the liquid and to deposit all along the width of the depression a crystalline crust of the salt. This crust is formed with singular facility in supersaturated solutions of magnesic sulphate; it may often be noticed in similar solutions of sodic sulphate, ammonia phosphate, and some others. Many persons will think it a remarkable fact that this crystalline crust does not act as a nucleus to the solution itself when this is brought into contact with it. Nor does the solution dissolve it, even though the tube be kept inclined for days together so as to immerse a portion of the crust. It cannot do so, since its high state of saturation has more than satisfied its solvent powers. I say more than satisfied, because the water contains a much larger quantity of salt than it can take up at the temrature of the air. * Communicated by the Author. Phil. Mag. S. 4. Vol. 36. No. 243. Oct. 1868. R The reason why this crystalline crust does not act as a nucleus to the rest of the solution is, that, being chemically clean, there is perfect adhesion between it and the solution as a whole, and hence there can be no separation of the salt from the water of the solution. An objection to this theory has been put :-In ordinary crystallization, as in nursing a crystal of alum, are we not dealing with chemically clean surfaces? My answer is a decided negative. The evaporating-dish that contains the solution is not chemically clean; the solution itself, exposed as it is to the air, is not chemically clean; nor is the hair by which the crystal is suspended; nor is the crystal itself, for this is frequently taken out and exposed to the air and handled, and abnormal growths chipped off with the thumbnail. Under such conditions of chemical impurity the crystal acts as a powerful nucleus; and if the fresh crystalline deposits made upon it are chemically clean, they do not long remain so. A chemically clean octahedron of alum is transparent; a nursed crystal is opake, (as I believe) in consequence of the multitude of minute crystals formed upon the impurities of the surfaces. The general opinion is that the most powerful nucleus for the crystallization of a saline solution is a crystal of the salt itself. It is nevertheless possible, with strict attention to chemical purity, to introduce a crystal into a highly supersaturated solution of the same salt, and yet the crystal shall not act as a nucleus, the solution remaining perfectly liquid as before. For this purpose a solution of two parts by weight of magnesic sulphate and one of water was boiled and filtered into a flask made clean by means of strong sulphuric acid, and rinsing with water; the solution was again boiled in the second flask; and while steam was issuing from the neck, a short tube, suspended by a wire, full of crystals of magnesic sulphate (all previously dipped in sulphuric acid and rinsed with water), was held in the neck, and secured there by putting in the plug of cotton-wool at the same time that the spirit-lamp was removed. The flask was left to cool during about fifteen hours, and then, without disturbing the cotton-wool, the tube with the crystals was lowered into the solution: there was no crystallization; and during forty hours, though the flask was shaken several times, the solution remained unchanged. A similar solution was filtered into clean tubes, which were closed with cotton-wool, and when nearly cold were put into strong sulphuric acid, covered with a receiver, and the air exhausted. In the course of twenty minutes a crystalline crust formed on the surface; and this, during the shaking of the pump, fell to the bottom of the solution; but it did not induce crystal lization. After some hours the tubes were taken out, the cotton-wool removed, when the solution in one tube immediately became solid, and that in the other tube did the same the moment it was touched with a bit of wire. I sent in a note of these experiments to the Royal Society by way of addendum to a paper "On Supersaturated Saline Solulutions" which I had the honour of submitting to that body on the 28th of May last*. I also brought the substance of that note before the Chemical Section of the British Association at Norwich on the 20th ult. The subject excited some interest, and several gentlemen who spoke on the occasion called for a definition of a chemically clean, in contradistinction to a dirty surface, one gentleman quoting the adage that dirt is something in the wrong place. As a suggestion from any one of the distinguished observers who spoke on this occasion must be highly prized by me, and as all of them seemed to think I had not sufficiently defined the basis of my theory, I adopt the course recommended by no less a man than Descartes-that is, not to answer an objection at the time it is made, but to carry it home, consider it well, and then reply to it in writing. I therefore suspend for a short time the further prosecution of this subject in order to define more precisely what I mean by chemical purity, and to illustrate that definition by briefly recapitulating the work that has been done by means of chemically clean surfaces. The work that remains to be done seems to me to be as important as any that has yet been accomplished. There are several sets of phenomena at present wrapped up in vague molecular theories, which I am presumptious enough to think admit of simple explanation under the theory I am advocating. In the first place, what is a chemically clean surface? A chemically clean surface is one that has on it no film or coating of any substance whatsoever foreign to its own composition. As oxidation by the air, organic matter, and floating motes are the most usual forms of films, we might say loosely, that any substance which has been exposed for some time to the air is chemically unclean; but speaking strictly, a film of any foreign matter will render a surface unclean for some conditions or other in the experiments in hand. A chemically unclean surface, then, may be generally defined as anything that is exposed to the products of respiration or of combustion, or to the touch, or to the motes and dust of the air, and so becomes covered with a film more or less organic. So *Proceedings of the Royal Society, vol. xvi. p. 403. See also 'Chemical News' for July 3rd, 1868, and the Philosophical Magazine for September 1868. |