burns slowly along the tube, with a very small sharp tongue of pale flame, a jet of flame is obtained at the mouth of the tube, by the burning of the gas evolved by the decomposition of the gun-cotton. Sometimes, and especially when wider tubes are employed, the slow combustion will proceed only for a short distance, and then, in consequence of the ignition of a mixture of the combustible gases and air within the tube, the gun-cotton will explode with great violence, the tube being completely pulverized, and portions of unburnt cotton scattered by the explosion. If still wider tubes are employed, the cotton will flash into flame almost instantaneously throughout the tube directly the flame reaches the opening in these cases the explosion is not violent; sometimes the tube escapes fracture, and at others is broken in a few places, or torn open longitudinally, a slit being produced in the tube directly over the gun-cotton. By using narrow tubes and gradually shortening the tube through which the gun-cotton was passed, pieces of the twist being allowed to project at both ends, it was found, upon inflaming the material which projected on one side, that the slow form of combustion, induced in it as soon as it burned into the tube, was maintained by that portion which burned in the open air on the other side, when the combustion had proceeded through the tube. Eventually, by the employment of a screen of wood or card-board containing a perforation of the same diameter as that of the gun-cotton-twist, through which the latter was partially drawn, the alteration of the combustion of the material from the ordinary to the slow kind was found to be invariably effected. On the one side of the screen, the gun-cotton burned with the ordinary flame and rapidity, until the combustion extended to the perforation, when the flame was cut off and the material on the opposite side of the screen burned only slowly, emitting the small-pointed tongue of pale yellow flame. These results indicate that if, even for the briefest space of time, the gases resulting from the first action of heat on gun-cotton upon its ignition in open air are impeded from completely enveloping the burning extremity of the gun-cotton-twist, their ignition is prevented; and as it is the comparatively high temperature produced by their combustion which effects the rapid and more complete combustion of the gun-cotton, the momentary extinction of the gases, and the continuous abstraction of heat by them as they escape from the point of combustion, render it impossible for the gun-cotton to continue to burn otherwise than in the slow and imperfect manner, undergoing a transformation similar in character to destructive distillation. These facts appear to be fully established by the following additional experimental results: 1. If, instead of employing in the above experiments a moderately compact gun-cotton-twist, one of more open structure is used, it becomes difficult or even impossible to effect the described change in the nature of the combustion, by the means described, because the gases do not simply burn cat, or escape from, the extremity of the twisted cotton, but pass readily between the separated fibres of the material, rendering it difficult or impossible to divert them all into one direction; and hence they at the same time transmit the combustion from particle to particle, and maintain the heat necessary for their own combustion. 2. If a piece of the compactly twisted gun-cotton, laid upon the table, be inflamed in the ordinary manner, and a jet of air be thrown against the flame, in a line with the piece of cotton, but in a direction opposite to that in which the flame is travelling, the combustion may readily be changed to the slow form, because the flame is prevented from enveloping the burning cotton, and thus becomes extinguished, as in the above experiment. 3. Conversely, if a gentle current of air be so directed against the guncotton, when undergoing the slow combustion, that it throws back upon the burning cotton the gases which are escaping, it will very speedily burst into the ordinary kind of combustion. Or, if a piece of the gun-cottontwist, placed along a board, be made to burn in the imperfect manner, and the end of the board be then gradually raised, as soon as the material is brought into a nearly vertical position, the burning extremity being the lowest, it will burst into flame. By applying to the extremity of a piece of the compact twist a heated body (the temperature of which may range from 135° C. even up to a red heat), provided the source of heat be not very large in proportion to the surface presented by the extremity of the gun-cotton, the latter may be ignited with certainty in such a manner that the slow form of combustion at once ensues, the heat applied being insufficient to inflame the gases produced by the decomposition of the gun-cotton. By allowing the guncotton thus ignited to burn in a moderately wide tube, closed at one end, the inflammable gases produced may be burned at the mouth of the tube, while the gun-cotton is burning in the interior; or they may be ignited and the gun-cotton consequently inflamed, by approaching a flame, or a body heated to full redness, to the latter, in the direction in which they are escaping. It need hardly be stated that these results are regulated by the degree of compactness of the gun-cotton, the size of the twist, and the dimensions of the heated body. Thus a small platinum wire heated to full redness, or the extremity of a piece of smouldering string, will induce the slow combustion in a thin and moderately compact twist; but a larger body, such as a thick rod of iron, heated only to dull redness, will effect the ignition both of the gun-cotton and of the gases evolved by the combustion of the first particles, so that the material will be inflamed in the ordinary manner. Similarly the red-hot platinum wire, or a stout rod heated to redness barely visible in the dark, if they are maintained in close proximity to the slowly burning surface of gun-cotton, will eventually cause the gases evolved to burst into flame. The more compact the twist of the gun-cotton, the more superficial is the slow form of combustion induced in it, and a condition of things is readily attainable, under which the guncotton-twist will simply smoulder in open air, leaving a carbonaceous residue; and the heat resulting from this most imperfect combustion will be abstracted by the gases evolved more rapidly than it is generated, so that in a brief space of time the gun-cotton will cease to burn at all in open air *. The remarkable facility with which the nature of combustion of guncotton in air or other gases may be modified, constitutes a most characteristic peculiarity of this substance as an explosive, which is not shared by gunpowder or explosive bodies of that class, and which renders it easily conceivable that this material is susceptible of application to the production of a comparatively great variety of mechanical effects, the nature of which is determined by slight modifications in its physical condition, or by what might at first sight appear very trifling variations of the conditions attending its employment. There is little doubt that the products of decomposition of gun-cotton vary almost as greatly as the phenomena which attend its exposure to heat under the circumstances described in this paper. A few incidental observations indicative of this variation were made in the course of the experiments. Thus, in the instances of the most imperfect metamorphosis of gun-cotton, the products included a considerable proportion of a white vapour, slowly dissolved by water, as also small quantities of nitrous acid and a very large proportion of nitric oxide. The latter gas is invariably formed on the combustion of gun-cotton in air or other gases; but the quantity produced appears always to be much greater in instances of the imperfect or slow combustion of the material. The odour of the gases produced in combustions of that class is powerfully cyanic, and there is no diffi culty in detecting cyanogen among the products. I trust before long to institute a comparative analytical examination of the products resulting from the combustion of gun-cotton under various conditions; meanwhile I have already satisfied myself, by some qualitative experiments, of the very great difference existing between the results of the combustion of guncotton in open air, in partially confined spaces, and under conditions precisely similar to those which attend its employment for projectile or destructive purposes. I have, for example, confirmed the correctness of the statement made by Karolyi in his analytical account of the products of decomposition of gun-cotton, that no nitric oxide or higher oxide of nitrogen is eliminated upon the explosion of gun-cotton under considerable pressure, as in shells. Coupling this fact with the invariable production of nitric oxide when gun-cotton is exploded in open air or partially confined spaces, there appears to be very strong reason for the belief that, just as the reduc *By enclosing in suitable cases solid cords, made up of two or more strands, and more or less compactly twisted, I have succeeded readily in applying gun-cotton to the production of fuses and slow-matches, the time of burning of which may be accurately regulated. tion of pressure determines a proportionately imperfect and complicated transformation of the gun-cotton upon its exposure to heat, the results of which are more or less essentially of an intermediate character, so, conversely, the greater the pressure, beyond the normal limits, under which gun-cotton is exploded-that is to say, the greater the pressure exerted by it, or the resistance presented at the first instant of its ignition, the more simple are the products of decomposition, and the greater are the physical effects attending its explosion, because of the greater energy with which the chemical change is effected. III. "On Magnesium." By Dr. T. L. PHIPSON, F.C.S. Communicated by Prof. G. G. STOKES, Sec. R.S. Received March 9, 1864. (Extract.) Iodine and Sulphur.-I find that iodine can be distilled off magnesium without attacking the metal in the least. In the same manner I distilled several portions of sulphur off magnesium without the metal being at all attacked. Decomposition of Silicic Acid.-Heated for some time in a porcelain crucible with excess of anhydrous silica, the metal burns vividly if the air has access; and a certain quantity of amorphous silicium is immediately formed. Magnesium is therefore capable of reducing silicic acid at a high temperature. The reason why potassium and sodium cannot effect this is simply because these metals are highly volatile and fly off before the crucible has attained the proper temperature. Magnesium being much less volatile than the alkaline metals, takes oxygen from silica before volatilizing. If the silicic acid be in excess, a silicate of magnesia is formed at the same time; if the metal is in excess, much siliciuret of magnesium is produced. The presence of the latter is immediately detected by throwing a little of the product into water acidulated with sulphuric acid, when the characteristic phosphoric odour of siliciuretted hydrogen is at once perceived. Decomposition of Boracic Acid.-With boracic acid the phenomena are rather different; the acid melts and covers the metal, so that it does not inflame ever when the crucible is left uncovered. A certain quantity of boron is soon liberated, and the product forms a greenish-black mass, which oxidizes and becomes white in contact with water, and disengages no odoriferous gas in acidulated water. Decomposition of Carbonic Acid.-I thought it would be interesting to try a similar experiment with carbonic acid. Accordingly dry carbonate of soda was heated with a little magnesium in a glass tube over a common spirit-lamp; and before the temperature had arrived at a red heat I observed that carbon was liberated abundantly, and magnesia formed. Action of Alkalies.-A solution of caustic alkali or ammonia has little or no action upon magnesium in the cold. Precipitation of Metallic Solutions.-Magnesium precipitates nearly all the metals from their neutral solutions. When these are taken in the form of protosalts, even manganese, iron, and zinc are precipitated as black powders. Aluminium and uranium (and perhaps chrome) are only precipitated as oxides. Alloys of Magnesium.-I have examined only a few alloys of magnesium. Unlike zinc, magnesium will not unite with mercury at the ordinary temperature of the air. With tin 85 parts, and magnesium 15 parts, I formed a very curious alloy of a beautiful lavender-colour, very hard and brittle, easily pulverized, and decomposing water with considerable rapidity at ordinary temperatures. If the air has access during the formation of this alloy, the mixture takes fire; and if the crucible be then suddenly withdrawn from the lamp, the flame disappears, but a vivid phosphorescence ensues, and the unfused mass remains highly luminous for a considerable time. A white powdery mass, containing stannic acid and magnesia, is the result. [With platinum, according to Mr. Sonstadt, magnesium forms a fusible alloy; so that platinum crucibles can be easily perforated by heating magnesium in them.] Sodium and potassium unite with magnesium, and form very malleable alloys, which decompose water at the ordinary temperature. It is probable that an alloy of copper and magnesium, which I have not yet obtained, would differ from brass, not only in lightness, but by decomposing water at the ordinary temperature with more or less rapidity. Uses.-Magnesium will be found a useful metal whenever tenacity and lightness are required and tarnish is of no consequence. The light furnished by combustion of the wire has already been utilized in photography at night. In the laboratory it will be found useful to effect decompositions which sodium and potassium cannot effect on account of their greater volatility. April 28, 1864. Dr. W. A. MILLER, Treas. & V.P., in the Chair. The following communications were read :— I. "On the Magnetic Elements and their Secular Variations at All observations and results to be mentioned here relate to Longitude 13° 23′ 20′′ E. from Greenwich. 1. Horizontal Intensity. Denoting by (1800+t) the date of observation in tropical years of the |