This phenomenon has particularly attracted the attention of M. Arago, who is of opinion that an opaque atmosphere may be produced at will, and thereby lead to the solution of most important problems on the polarization of light, which have hitherto not been resolved for want of the means of observation. The discovery of the chlorous acid is a fact of high importance. It completes the series of the combination of the chlorine and oxygen, and establishes an analogy the more between these compounds and the corresponding compounds of Mining Jour. azote. Improved Modification of a New Electrotype Process. By M. NEWSAM. A discovery has been recently made of etching copper plates, by first gilding them by the electrotype, and then delineating the drawing through the coat of gold to the copper. I tried the gilding process several times, but could not succeed in producing a firm coat of metal; and I am acquainted with many scientific gentlemen who arrived at the same result-namely, a dirty black powder, easily rubbed off, instead of the "glittering metallic surface of incomparable beauty," as it has been designated by some. Indeed I am half inclined to doubt that such a thing has ever been accomplished. My trials with the "noble" metals proving in every case a decided failure, I thought of trying what could be done by means of the cupreous deposit, as I always found it perfectly pure and firm, provided, only, care were taken in conducting the process. My first experiment proved quite successful; the subject of it was an ordinary steel plate. I covered it with a thin film of copper, which I afterwards etched, and bit up with dilute sulphuric acid. Far finer and closer lines can be formed on this ground than on that in general use at present, which arises from the latter being required to be laid comparatively thicker on, which, together with its property of adhering to the point of the tool, during the process, sometimes sends a whole body of fine line-work into a broken unmeaning mass. What I consider the greatest advantage attending working on a metaliic "ground," is, that after the effect, &c., is bit in, the plate can again undergo the same process, if the resulting etching be not satisfactory-an object not to be attained by the employment of grounds of a waxy or resinous nature. Such grounds always require to be totally removed before the picture can be pronounced perfect-at least as regards the etching; and if the same be found, on inspection, to be not of the requisite degree of color, or depth of tone, the touching up is attended with labor and uncertainty. A plate furnished with a metallic ground, can, however, be bit deeper, even after a proof has been printed from it, the coat of metal, of course, being allowed to remain on the plate. Mech. Mag. Remedy for Spontaneous Combustion of Coals. Captain Carpenter, of Her Majesty's steam frigate Geyser, has suggested the following remedy for spontaneous combustion. It is proposed to have several cast iron tubes, with holes at the lower part, as shown in the following figure, passing into the bunker, or coal-hole, nearly to the bottom of the vessel, and only a few inches from the ship's side, properly secured. The upper parts of these tubes are to come up to the deck, and to be contrived so as to give ventilation without allowing wet to go down amongst the coals. At the same time, means are to be afforded of pouring water into the bunkers, so as to flood them at the bottom in case of ignition. The water, in that case, would have a twofold effect, as it would not only extinguish the fire at the place where danger is to be apprehended, but at the same time, the water poured into the tubes would destroy all ventilation, and have a tendency to smother the part ignited. The cause of spontaneous combustion is, evidently, first, the accumulation of gases from the moisture of the coals; and then either heat or friction gives rise to ignition. To obviate this evil, if you allow the atmospheric air to pass freely amongst the coals, of course the gases could not accumulate, and combustion would not take place. If, however, there should be parts where the air did not penetrate, then the remedy is effected by pouring water into the bottom of the coal-box, and extinguishing the fire. The smoke issuing from the tube on deck, would always give timely notice of danger; and the hose on deck, pointed into the upper part of the tube, would provide a sufficient supply of water always at hand. The tubes would be about six inches in diameter, and about a quarter of an inch thick; their length would be regulated by the depth of the coal-box. Hay-stacks are provided with the same remedy against taking fire, by introducing a large basket tube down the centre; and why should not the same result take place in the manner proposed? Ibid. VOL. IV, 3RD SERIES. No. 6.-DECEMBER, 1842. 336 |