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fo dangerous; and why the evening air is fo pernicious in fummer and autumn, and not fo in the hard frofts of winter. Phyficians have been puzzled to account for the generation of the extraordinary quantity of heat fuppofed to be carried off from animal bodies by the cold air in winter, above what they communicate to the warmer atmosphere in fummer; but it is more than probable, that the difference in the quantities of heat fo loft or communicated, is infinitely less than they have imagined.
The Author examines alfo the conducting power of mercury, and finds it to be greater than that of water, in the proportion of about 100 to 31. Hence it is plain why mercury appears fo much hotter or colder to the touch than water does, though really of the fame temperature; for the intenfity of thofe fenfations does not depend entirely upon the degree of heat in the body exciting them, but on the quantity of heat which it can communicate to or from us in a given fhort period, or the intensity of the communication.
Sir Benjamin propofes continuing his experiments on heat; and experiments fo accurately and judiciously conducted, on fo interefting and obfcure a fubject, cannot fail to afford important results.
An Account of Experiments made by Mr. John McNab, at HenleyHoufe, Hudson's Bay, relating to freezing Mixtures. By Henry Cavendish, Efq; F. R. S. and A. S.
These experiments were made at Mr. Cavendish's defire, with materials provided and adjusted by him, in order to afcertain fome particulars in his remarks fubjoined to Mr. Hutchins's Paper in the 73d volume of the Tranfactions. As heat is generated in the congelation of fluids, and cold in the liquefaction of folids; and as the cold produced by mixing fnow with fpirit of nitre is fuppofed to be owing merely to the liquefaction of the fnow, it fhould follow, that there may be a degree of cold, in which the nitrous fpirit, fo far from diffolving fnow, will fuffer part of its own water to freeze; and in that cafe no additional cold fhould be produced by the mixture of fnow with it. As ftrong fpirit of nitre generates heat with water, it does the fame with the water that is formed on the first addition of fnow to it, and no cold is produced till the fnow amounts to about one fourth of the weight of the acid: it fhould follow, that if the acid be diluted at first with one fourth of water, no heat will be ge
* Mr. Cavendish adopts Sir Ifaac Newton's idea, that heat is not a diftinct body, but a quality, produced by the inteftine action of the parts of bodies. So far as relates to the explication of thefe experiments, the theory affects only the mode of expreffion; inftead of saying that heat is generated, it is now more cuftomary to fay that it is dijengaged, or let loose.
nerated, and the cold will of courfe be greater. The experi ments are perfectly fatisfactory, and ascertain fome other interefting particulars, in a region of science which philofophers have but few opportunities of exploring.
It appears from thefe experiments, that the nitrous acid is not only fufceptible of an aqueous congelation, or freezing of the watery part, but of a spirituous, or freezing of the acid itfelf:That when cooled to the point of aqueous congelation, it has no tendency to diffolve fnow, and thereby produce cold, but on the contrary, is difpofed to part with its own water :-That the tendency to diffolve fnow, and produce cold, is by no means deftroyed by its being cooled to the point of fpirituous congelation, or even actually congealed:-That both the ftrong and diluted acids bear, like water, to be cooled very much below their freezing point before the congelation begins, and rife up to their freezing points as foon as it does begin :-That, contrary to water, they fhrink in freezing, very much, the furface becoming deprefled, and full of cracks, and the ice finking freely in the unfrozen fluid :--And that their freezing point varies according to a very unexpected law, the acid of a certain degree of ftrength freezing much easier than that which is either ftronger or weaker. The lowest heat in which any nitrous fpirit was found to freeze, was 1 below o of Fahrenheit: the ftrength of this fpirit was 411, that is, 1000 parts of it would diffolve 411 of marble; and when any ftronger or weaker spirit was frozen in part, the frozen part approached nearer to that strength than the unfrozen.
The general refults of the experiments on this head are expreffed in the annexed table. The first column fhews the strengths of the acids; and the fecond the degree below o at which they refpectively froze.
540 | 31
14 380 45
The vitriolic acid contracts in freezing, as the nitrous does, its ice finking to the bottom of the unfrozen fluid; but it does not appear to have any point of cafieft congelation, feeming rather to freeze eafier and eafier, the more concentrated it is, without limitation. It feems alfo, that certain parts are more congealable than the reft, and that the difference does not depend altogether on ftrength, but on fome lefs obvious quality, probably on that, whatever it is, which forms the difference between glacial and common oil of vitriol.
Abstract of a Regifler of the Barometer, Thermometer, and Rain, at Lyndon in Rutland, in 1785. By Tho. Barker, Elq. &c. The depth of rain at Lyndon was about 20 inches; at South Lambeth it was only 19, at Fyfield 241, and at Selbourn 31. From fome obfervations fubjoined to this regifter it appears, that the annual quantity of rain is very variable in the fame place at different periods. At Lyndon, from 1740 to 1743 inclufive, the mean depth was only 16 inches in a year, and yet no complaint was made of dry fummers in any of those years; the fummers were showery, but the winters dry. From 1741 to 1750, the mean depth was 18 inches; from 1751 to 1760 221-1761 to 1770, 23;-1770 to 1780, 26. In three of the years of this last period, 1773, 4, and 5, the mean annual depth was 32 inches; and in one of them, from October 1773 to September 1774, the depth was 391. It is plain from these ob fervations, how little dependence can be had upon average quantities taken on periods of fmall extent.
Magnetical Experiments and Obfervations. By Mr. Tiberius Cavallo, F. R.S. (The Lecture founded by the late Henry Baker, Efq. F. R. S.)
Thefe experiments relate chiefly to the magnetic property which has lately been obferved in fome kinds of brafs. The general refult of them is, That moft brafs becomes magnetic by. hammering: That the magnetifm is deftroyed by annealing, or foftening in the fire, reftored again by hammering, and thus alternately, as often as the hammering and annealing have been repeated: That the magnetifm is not owing to any particles of iron communicated by the tools, for it is producible by beating the brafs between pieces of flint or copper; and that the deftruction of the magnetism is not owing to the calcination of any iron particles, for it takes place though the brafs be furrounded with charcoal powder in a close crucible. Mr. Cavallo is therefore of opinion, that the magnetifm acquired by brass is not owing to any iron in it, but to fome particular configuration of its component parts, occafioned by the ham mering. There are indeed pieces of brafs which have vifible particles of iron in them, but these are magnetic in their soft as well as hard ftate; and there are others, which cannot, by any known means, be rendered magnetic at all.
A propofition fo fingular and important, as the existence of magnetism, or the power of attraction to the magnet, independ ent of iron, is certainly not to be admitted without rigorous examination. The Author himself, in a Poftfcript to the Lecture, ftarts an objection, and gives fome experiments to obviate it. The brafs may owe its magnetifm to an irony matter intermixed, and this iron may be magnetic, or not magnetic, according as it
is in a metallic or calcined ftate: by hammering, fome phlogifton may be transferred from the brass to the iron; and in the fire, it may be absorbed again from the iron by the brass. To afcertain this point, he introduced fome calx of iron into minute cavities in pieces of naturally unmagnetic brafs, and found-that hammering did not produce magnetifm in this cafe, but that fire did; the reverfe of what happens to the magnetic brafs. He therefore thinks it demonftrated, as far as the fubject will admit of demonftration, that the magnetifm acquired by brass is not owing to iron contained in it.
It must be obferved, however, that the objection fuppofes the iron to be diffufed through the brafs, in a ftate of intimate, or chemical union; whereas, in the experiments, the iron was diftinct, having no degree of union with the brafs; circumstances obviously very different. This infufficiency of the above experiments feems to have been pointed out to the Author by Dr. Blagden; for the Poftfcript is followed by a letter to that gentleman, giving an account of an experiment made plainly in confequence of fuch a remark. A very fmall portion of iron was united by fufion with unmagnetic brafs, which thereby became very weakly magnetic, every part of its furface juft fenfibly attracting one end of the needle; and this weak degree of magnetifm was neither increafed by hammering, nor annihilated by foftening.
This experiment appears conclufive; and yet the conclufion drawn from it feems to be rather weakened by fome obfervations which the Author has fubjoined: he found that naturally magnetic brafs loft its power of acquiring magnetifm by being melted partially, and recovered it again by being fairly melted; and that a long continuance of a heat juft below fufion generally dimi nishes, and fometimes quite deftroys that property.
The needle employed in thefe experiments deferves notice, on account both of the fimplicity of its conftru&ion, and its having been found much more fenfible than the best of thofe now in ufe. It is fufpended by a chain, confifting of five or fix circular links of horse hair, each link about three quarters of an inch in diameter, and the ends of each of the pieces of hair joined by a knot and secured with a little fealing wax. The upper link is hung on a pin, and a piece of fine filver wire, about 1 inch long, hooked to the lower. The lower end of the wire goes round a fmall cylindrical bit of cork, through which a common fewing needle, made magnetic, is thruft horizontally. links, on account of the fmoothness and lightness of the hair, move freely on each other, and allow the needle more than a whole revolution round its center, with fo fmall a degree of friction, that it may be confidered as next to nothing.
Particulars of the prefent State of Mount Vesuvius; with the Account of a fourney into the Province of Abruzzo, and a Voyage to the Iland of Ponza. By Sir William Hamilton, K. B. F. R. S. The many curious particulars contained in this paper will not admit of an abftract. The Author obferves, upon the whole, that the more opportunities he has of examining this volcanic country, the more he is convinced, that volcanoes should be confidered in a creative rather than a destructive light. Many new discoveries have been made of late, particularly in the South Seas, of iflands, which owe their birth to volcanic explosions ; and fome, where the volcanic fire ftill operates. It is probable, that on further examination, moft of the elevated iflands at a confiderable diftance from continents would be found to have a volcanic origin; as the low and flat islands appear, in general, to have been formed of the fpoils of marine productions. -Vefuvius, the Solfaterra, and the high volcanic ground on which great part of Naples is built, were once probably iflands, and the Author conceives the islands of Procita, Ifchia, Ventotiene, Palmarole, Ponza, and Zannone, to be the outline of a new portion of land, intended by nature to be added to the neighbouring continent: the Lipari iflands, all of which are volcanic, are looked upon in the fame light with respect to a future addition of territory to the island of Sicily.
This paper is accompanied with three plates; one, a plan of the island of Ponza; the other two, views of rocks of lava and bafaltes upon the island.
An Account of a Subfidence of the Ground near Folkftone, on the Coast of Kent. In a Letter from the Rev. John Lyon, M. A. to Edward King, Efq. With Remarks by Mr. King.
In No. 349 of the Phil. Tranfact. (for the year 1716) there is an account, by the Rev. Mr. Sackette, of an uncommon finking of the earth in the fame neighbourhood, which is there attributed to the hills or cliffs, which confift of great ragged fandftones, ftanding on a wet bottom of flippery clay, having flidden forwards, towards the fea, as a fhip is launched upon tallowed planks. The fubfidence defcribed in the prefent Paper happened in September 1785, and is accounted for fomewhat differently, viz. from the foundation being undermined by waterdrains, which abound in the loofe marley bottom: the ground now resembles an arch that has funk down, and which has left its two abutments, a hill and a cliff, ftanding; and the preffure of the hill, being more than a counter-balance to that of the finking ground, upon the ftratum of wet marle, has forced upwards fome rocks, at the distance of a few yards, and fqueezed the wet marle in many places up with them. The fituations of the parts in which the alteration has taken place, and of the adjacent country, are reprefented in two plates,