then placed this saturated solution under the exhausted receiver of an air-pump; in a short time the biniodide began to be deposited, and soon they increased and assumed the form of large regular cubes. The hydriodate of zinc that remained was capable of dissolving a fresh quantity of biniodide, or of redissolving that which was crystallized from it: the crystals contain no zinc; they are acted on by chemical agents and by heat just in the same manner as the precipitated biniodide, and are quite distinct from the hydrargo-biniodide of zinc to be mentioned hereafter. The biniodide of mercury is sufficiently soluble in an excess of the hydriodate of potassa, from which there results on slow evaporation a yellow salt, which I discovered many months ago, and called it the double hydriodate of the biniodide of mercury and potash, because I found that whenever it was brought in contact with water, instant decomposition took place, the water from its strong affinity for hydriodic acid removed it, and the red biniodide was precipitated. Bonsdorff, however, calls this compound the hydrargo-biniodide of potassium. * * * On mixing bichloride of mercury in powder to a saturated solution of hydriodate of potassa, and agitating together, the whole becomes nearly a solid red mass, and much heat is at the same time generated. *** This red iodide is formed by many other processes, as when a solution of bicyanuret of mercury is added to an alcoholic solution of iodine, in which case it instantly precipitates. When the yellow sesquiodide is kept for some time under water, and exposed to light, very good small cubic crystals of the red iodide are found covering the surface; but the method I described above is the best one for obtaining it in its crystalline form. I have reason to think that there is another iodide of mercury, of a blue colour; it is formed by freely exposing in an open vessel the red iodide with an excess of metallic mercury. In the course of three or four weeks the surface assumes a decidedly blue tint. I have not as yet further examined this compound. * * * * * * On examining the crystal of the iodide of lead with the aid of the microscope, it is found to be a flat six-sided crystal [prism?]. This, next to the tetrahedron, appears, from what I have observed to be the most common form of crystallization. ***. I found that when, instead of using just a neutralizing sufficiency of hydriodate of potassa and acetate of lead, the hydriodate was added in excess, there was thrown down a white soft powder and not the yellow iodide. And by ammonia the yellow iodide is converted into a similar white powder, which, perhaps, may be another iodide of lead. If metallic tin be boiled with the iodide of lead, no reaction takes place; but if the dry iodide be mixed with granulated tin, and exposed to heat, combination takes place, and a double iodine of tin and lead results, of a brown colour and differing from either iodide separately. By boiling this double iodide in water, very beautiful crystals of the yellow iodide of lead are obtained. [To be continued.] V. Observations on the Steam Engines of Cornwall; in Reply to John Taylor, Esq., F.R.S., Treas. G.S., &c. By W. J. HENWOOD, F.G.S. Lond. & Paris, Hon. M.Y.P.S., Curator of the Royal Geological Society of Cornwall. To the Editors of the Philosophical Magazine and Journal. GENTLEMEN, MR. R. TAYLOR's communication in your present month's Number* appears to imply that a rotatory single engine working expansively is something of a novelty. Now, Mr. Watt's first expansion engine was erected in 1778+; the patent for his rotatory engine was taken out in 1781, October 25th ‡; and that for his double engine in 1782, March 12th §. Thus the rotatory single engine working expansively preceded the invention of the double engine; and some of the former construction were erected by Messrs. Boulton and Watt on the Cornish mines. At Binner Downs mine in this county Messrs. Gregor and Thomas have erected five rotatory single engines working expansively; the first of them in 1828, the last in 1833. Captain Gregor also set up a similar one, for driving a common grist mill, for Messrs. Harvey and Co. of Hayle Foundry. All these have performed their work extremely well; are quite as manageable as double engines; and, where they have taken their place, have worked with much less coal. The duty of those at Binner Downs, which are used as winding (whim) engines, Captain Gregor estimates at about 15 millions of pounds raised one foot high by the use of each bushel of coal consumed ||. Mr. Taylor speaks of "the method of working high pressure steam expansively, which we owe to Mr. Woolf¶." • Vol. vii. p. 369. t Ibid., p. 346. + Farey on the Steam-Engine, p. 341. Captain Lean reports the duty of Mr. Sims's engine at Charlestown 44, and not 60, millions, as stated by Mr. Taylor. ¶ Lond. and Edinb. Phil. Mag., vol. vii. p. 369. Now, in former numbers of this Journal*, I have shown that in 1811-1812, Captain Trevithick erected a single engine at Huel Prosper mine, in which steam of above 40 pounds pressure on the square inch was worked expansively. Mr. Farey observest that Mr. Woolf came to reside in Cornwall about the year 1813, and the "first engines for pumping water from the mines were set up by him in 1814;" but these, he adds, had two cylinders. I therefore repeat, that we do not owe this practice to Mr. Woolf, but to Captain Trevithick. But it has been already shown that this is only an extension of Mr. Watt's practice of 1778. The advance in the duty of steam engines which has taken place in this county within the seven years last past, is principally, if not entirely, due to Captain Grose; and is obtained mainly by the application of substances which transmit heat very slowly, to the surfaces of the portions of the apparatus containing dense steam. I remain, Gentlemen, 1, Morrab Place, Penzance, November 28, 1835. Yours, &c. W. J. HENWOOD. VI. On an Error in Dr. Apjohn's Formula for inferring the Specific Heats of dry Gases. By H. HUDSON, M.D., I M.R.I.A. To the Editors of the Philosophical Magazine and Journal. GENTLEMEN, BEG to point out a serious error in the formula given (in p. 385 of your 7th volume) by Dr. Apjohn for ascertaining the specific heats of gases by their action on the "evaporation thermometer." I have shown (in your Journal for October last, same vol., p. 257,) that (taking the density of air at 212° under 30 pressure as unity) the density of the vapour of saturaf tion at t° is = × 13.75, consequently (the weight of 448 +t a cubic inch of air at 212° under 30 being ⚫327958 +) the weight of a cubic inch of the vapour of saturation at t° is = f 448+t Grs. x 3.27958; also the latent heat of vapour at t° be ⚫ Phil. Mag. and Annals, vol. vii. p. 323, March 1830, and vol. x. p. 98, August 1831. ↑ Ibid., vol. viii. p. 308. 327958 is the weight at 32° (Prout), and 1.375 the expansion between 32o and 212° (Gay-Lussac). ing = 1168-t (capacity of water being 1168-t 448+t Grs. ׃ × 3.27958 of water in falling 1° will give out the quantity of heat necessary to produce this cubic inch of vapour. But in the experiments with dry gases an equal volume (i. e. one cubic inch) of the gas falling Vo produces this same effect; consequently (S being the weight of a cubic inch of the gas at t°) VxS gives the weight of the gas which would produce this effect in falling 1°; from whence it is obvious that 1 (the capacity of water): C (the capacity of the gas) :: Sx V: f VxS x 3.27958; or (since S', the weight of a cubic inch of Gr. Now, if V (the depression of wet thermometer) in hydrogen gas be 20° (t, the temperature of wet ball being 48°), and if V in atmospheric air be = 25° (t being = 43°), and taking weight of cubic inch of hydrogen at 60° = 0·02153, and weight of cubic inch of air at 60° = 0·3099*; consequently, Grs. Dr. Apjohn appears to have used (for every gas) the weight of a cubic inch of air, instead of S, the weight of the particular gas. Accordingly, the experiments (except with hydrogen) rather favour my view that the capacities of gases are equal in equal volumes. The same I believe to be true with hydrogen, and that V will be found the same in every gas (with an improved apparatus for trying the experiments) under similar circumstances of temperature and pressure, if the current of gas be sufficiently powerful. * I have supposed P (the pressure) = 30. The requisite alteration in 1168 -t × f the value of S' gives C = * .006456 × 30. VXS'XP Hoping for an immediate insertion of this, I shall reserve any remarks on the probable errors in the experiments, and their causes, for a future occasion. I am, Gentlemen, yours, &c. 24, Stephen's Green, Dublin, Nov. 11, 1835. H. HUDSON. VII. Remarks on a Paper on the Transmission of Calorific Rays, &c. by M. MELLONI, in the Phil. Mag. and Journal of Science, No. 42. By the Rev. B. PowELL, M.A., F.R.S., Savilian Professor of Geometry, Oxford,* IN N the last Number of this Journal (vol. vii. p. 475) a short communication appears from M. Melloni, in which that distinguished experimenter has honoured me with a reference to the experiment which I tried in 1825, which forms the basis of a peculiar view of the nature of the heat originating from luminous hot bodies, and which M. Melloni has since successfully verified with his extremely delicate apparatus, so as entirely to remove all doubt, which (I presume from the silence of physical writers) must previously have been felt on the subject. But while he speaks with approbation of that experiment, M. Melloni refers to my views connected with it in terms which imply a most singular misconception of them, and on which I therefore feel it necessary to offer a very few remarks. M. Melloni describes me as "endeavouring to explain his results by hypotheses" which are untenable. Now, I am not aware of having attempted to explain M. Melloni's results at all. All that I have contended for is, that if the distinction between two kinds of heat, "luminous and obscure" (as the author terms them), be admitted, (and he himself, I believe, admits it,) it will follow, that all results which have hitherto been commonly stated as referring to "radiant heat," will require now to be more precisely worded, and we must say which sort of radiant heat we mean, in all cases where there may be both present. That particular result which M. Melloni obtained in the repetition of my experiment with the thermomultiplier, and which so strongly confirms it, I have, indeed, adverted to as, to my apprehension, ill explained by the gratuitous hypothesis, that the heat acquires a new property with regard to its relations to surfaces by merely passing through glass; which seems to me at once needless and contrary to all analogy; * Communicated by the Author. |