therefore, take place; the balance of force which determines its particular direction in the instances pointed out, forms an interesting subject of investigation, which, together with the cleavage and dissection of crystals, and the manner in which they are affected by light and heat, may ultimately contribute to the explanation of the laws of molecular attraction. I shall conclude this paper with the result of some experiments upon the mutual action of mercury and platinum. EXPERIMENT XIV. There is no apparent action whatever between mercury and a bar of platinum, at the common temperature of the atmosphere; but when exposed together for a short time to the boiling point of the former, the latter becomes superficially coated with the fluid. The combination is so slight, that the mercury may easily be wiped off mechanically, as water from wet glass. Platinum, which has been kept constantly wetted with mercury for six years, has not become disintegrated, or in any way changed its properties. EXPERIMENT XV. A few grains of spongy platinum, formed from the ammoniomuriate, were violently agitated with mercury and a few drops of water in a test-tube: a kind of thick scum, or semifluid amalgam, speedily collected upon the surface, from which the still fluid metal could easily be poured off. EXPERIMENT XVI. The foregoing experiment was repeated; but the water was acidified with acetic acid. The test-tube was five inches long, and about half an inch diameter. The mercury occupied about an inch, and the weak solution of the acid about half an inch of its depth. The platinum was thrown in, and the whole shaken together for a short time; when the tube became filled with an amalgam, of the consistence of soft butter. When the tube was upset, a very few drops of fluid mercury ran out of it; and when the amalgam was shaken out into a saucer, it retained its consistence for many weeks. It possessed a dullish metallic hue, like that of lead which has become tarnished; and very much resembled the amalgam formed by the electrization of mercury in contact with ammonia. The experiment was frequently repeated, sometimes with the substitution of some neutral salt for the acid, and always with similar results. When the amalgam was laid upon filtering paper, the moisture was gradually absorbed and evaporated, and the mercury returned to the fluid state. EXPERIMENT XVII. The experiment was varied by filling a tube, which was some inches longer, with the weak acid solution; and after the formation of the amalgam by agitation, inverting it in a cup of mercury. Minute bubbles of gas were immediately seen rising from the amalgam through the fluid, and collecting in the upper part of the tube. Upon close examination, particles of the spongy platinum could be discovered between the sides of the glass and the mercurial paste, round which bubbles of gas gradually accumulated, which gave the whole a honeycombed appearance. These, as they increased in size, slowly crept up the sides of the tube, till, reaching the fluid, they rapidly ascended to the top. In twelve hours' time, nearly the whole of the liquid had been expelled from the tube, and when a light was applied to the gas it exploded. Some of the acetic solution, which had been frequently employed in repetitions of the experiment, was slowly evaporated, and afforded crystals of prot-acetate of mercury. EXPERIMENT XVIII. I endeavoured, in vain, to produce analogous results, by agitating amalgam of gold and other amalgams with diluted acetic acid and solutions of neutral salts. No action was apparent, and in no instance was anything like the frothy amalgam produced. Hence it appears that, when minutely divided platinum is agitated with mercury, and moisture is present, an electrical action takes place, which, when heightened by the addition of a diluted acid, or the solution of a neutral salt, is sufficiently energetic to decompose water and evolve hydrogen: the oxygen at the same time combines with the mercury, and a solution is effected by the acetic acid, which its unassisted affinity could not have produced. This action appears to be of the same nature as that described by Mr. Faraday *, in his account of the Alloys of Steel; during his experiments upon which, he found that steel, alloyed with an hundredth part of platinum, was acted upon by dilute sulphuric acid, with infinitely greater rapidity than the unalloyed steel, and that an acid, which scarcely touched the pure steel, dissolved the alloy with energetic effervescence. It also appears that this electrical action communicates an adhesive attraction to the particles of the metal, by which the particles of liquid and aëriform bodies are entangled and retained, a kind of frothy compound formed, and the fluidity of the mercury destroyed. The appearance of this amalgam is so very like that of the ammoniacal amalgam formed by exposing a solution of ammonia in contact with mercury to the influence of the Voltaic pile, or when an amalgam of potassium and mercury is placed upon moistened muriate of ammonia, that it is impossible not to be struck with the resemblance. I am inclined, indeed, to believe, that the production of the latter may be explained upon the same principles as that of the former. When the effect is produced by the direct application of the electrical current, by means of the battery, it ceases the moment the connexion between the poles is broken; and when brought about by the agency of the amalgam of potassium, the electrical action is doubtless excited by the contact of the two dissimilar metals, and the frothy compound lasts no longer than the existence of the potassium in the metallic state. In the action which I have just described, between mercury and finelydivided platinum, the permanence of the metals produces a much more lasting effect, and the soft amalgam may be preserved for a great length of time without altering its appearance. At all events, these results cannot but increase the strong doubts which previously existed concerning the hypothesis of the metallization of ammonia, and the supposed compound of mercury and ammonium. *Philosophical Transactions, 1822. Part II., p. 262. ON THE MEANS OF GIVING A FINE EDGE TO RAZORS, LANCETS, AND OTHER CUTTING INSTRUMENTS. BY THOMAS ANDREW KNIGHT, Esq., F.R.S., IN the preparation of steel, and in the art of subsequently forming it into cutting instruments, the British manufacturers are, I believe, unrivalled; and they have probably approximated, if they have not attained, perfection: but in the art of giving the finest possible edge to their instruments, when formed, I think that they have generally still something to learn; for I hear surgeons often complaining, that they rarely find themselves in possession of a perfectly well set instrument; and I have never yet, in any instance, seen a razor come from a cutler so set that I could use it with any degree of comfort, though I have obtained razors from many of the most eminent manufacturers of the metropolis. The machinery which they employ has long appeared to me to be imperfect and uncertain in its mode of operating, and in many respects inferior to that which I have been some years in the habit of using, and which I shall proceed to describe. This consists of a cylindrical bar of cast steel, three inches long without its handle, and about one-third of an inch in diameter. It is rendered as smooth as it can readily be made with sand, or, more properly, glass-paper, applied longitudinally; and it is then made perfectly hard. Before it is used, it must be well cleaned, but not brightly polished, and its surface must be smeared over with a mixture of oil and the charcoal of wheat straw, which necessarily contains much siliceous earth in a very finely reduced state. I have sometimes used the charcoal of the leaves of the Elymus arenarius and other marsh grasses; and some of these may probably afford a more active and (for some purposes) a better material; but upon this point I do not feel myself prepared to speak with decision. In setting a razor, it is my practice to bring its edge (which must not have been previously rounded by the operation of a strop) into contact with the surface of the bar at a greater or less, but always at a very acute angle, by raising the back of the razor more or less, proportionate to the strength which I wish to give to the edge; and I move the razor in a succession of small circles from heel to point, and back again, without any more pressure than the weight of the blade gives, till my object is attained. If the razor have been properly ground and prepared, a very fine edge will be given in a few seconds; and it may be renewed again, during a very long period, wholly by the same means. I have had the same razor, by way of experiment, in constant use during more than two years and a half; and no visible portion of its metal has, within that period, been worn away, though the edge has remained as fine as I conceive possible; and I have never, at any one time, spent a quarter of a minute in setting it. The excessive smoothness of the edge of razors thus set led me to fear that it would be indolent, comparatively with the serrated edge given by the strop; but this has not in any degree occurred; and therefore I conceive it to be of a kind admirably adapted for surgical purposes, particularly as any requisite degree of strength may be given with great precision. Before using a razor after it has been set, I simply clean it on the palm of my hand, and warm it by dipping it into warm water; but I think the instrument recommended operates best when the temperature of the blade has been previously raised by the aid of warm water. A steel bar, of the cylindrical form above described, is, I think, much superior to that of a plane surface for giving a fine edge to a razor or penknife; but it is ill calculated to give a fine point to a lancet; and I therefore cause a plane surface to be made, a quarter of an inch wide, on one side of the bar, by cutting away a part of its substance; and I have found this form to be most extensively useful. The edge of some razors, whether formed of wootz, of mixed metals, or of pure steel, but particularly of mixed metals, has generally appeared to me to be more keen and active when used a few seconds after it had been applied to the bar, than on the following day; and I have often seen the utmost activity restored to the edge of such instruments, so instantaneously, and by so apparently inadequate means, that I have been |