Изображения страниц

none could now be procured that diminished so much as the pare cel originally made use of. After various researches and experiments, dictated by a thorough knowledge of the subject, and which have discovered many curious particulars respecting the properties of this class of earths, he has recourse to an admixture of the earth of alum, the ingredient on which the diminution by fire and all the argillaceous properties depend; and by a due proportion of which the common porcelain clays of Cornwall are made to correspond in all degrees of heat with the original clay, receiving from it at the same time some other important advantages. Coincidence with the original,' he observes, was not indeed essential; but as many degrees of heat were already before the public, measured by thermometer-pieces made of the first clay, and as the correspondence of the first with Fahren: heit's scale, had likewise been in some measure ascertained *, it was desirable that the same degrees of heat should continue to be expressed by the same numbers.'

Of the embarrassing properties of some of the natural ciays, we shall mention one, on account of what appears to us a very important consequence resulting from it. "Though they continued diminishing with tolerable regularity, up to a certain period of heat, about that in which cast iron melts, yet many of the pieces, urged with a heat known to be greater ihan that, were found not to be diminished so much as those which had suffered only that lower heat. Further experiments shewed, that after diminiling to a certain point, they begin, upon an increase of the heat beyond that point, to swell again ; and as this effect is constant in certain clays, and begins earliest in those which are most vitriscible, and as clays are found to swell upon the approach of vitrification, I look upon this enlargement of bulk, however inconsiderable, as a fure indication of the clay having gone beyond the true porcelain state, and of a difpofition taking place towards vitrification.-The degree of heat, there. fore, at which this enlargement begins, may be considered as a criterion of the degree of vitriscibility of the composition ; which points out a new use of this thermometer, enabling us to ascertain the degree of vitriscibility of bodies that cannot ačtually be vitrified by any fires which our furnaces are capable of producing.' Observations on the Afinities of Substances in Spirit of Wine. By

John Elliot, M. D. This paper, in the form of a letter to Mr. Kirwan, contains some experiments in confirmation of a position which the Author had advanced in a former publication, that certain decompofitions will take place in spirit of wine, which will not at

* See Rev. vol. Ixxii. p. 250.

O 3


follows, evaporation. He spirit, and for

all in water, nor in the dry way.' The particular decomposition here created of, is that of diachylum plaster (a compound of litharge and oil) with sea-salt, by boiling them in spirit of wine: the acid of the sea.sale unites with the litharge, and its alcali with the oil, the two latter forming together a true soap, which diffolves in the spirit, and may be obtained in its proper form by evaporation. As no separation takes place in water, it follows, that the apparent affinities depend, not solely upon the attractions of the ingredients to one another, but in part upon their attractions to the liquid employed as an intermedium. In Spirit of turpentine, the affinities were still further diversified ; the diachylum dissolved, and the common salt remained at the bottom. New Experiments upon Heat. By Colonel Sir Benjamin

Thompson, Knt. F.R.S. From the striking analogy between the electric Auid and heat, in respect to their conductors and non-conductors (bodies which are good conductors of the one being generally so of the other also), it was natural to imagine that the Torricellian vacuum, which affords so ready a passage to the ele&ric fluid *, would do the same to heat. But a series of curious, and admirably wellcontrived experiments, detailed in this paper, prove, without leaving a lhadow of doubt, that the vacuum conducts heat far more fluggishly than air ;-that, nevertheless, air of different densities differs little in this respect ;- but that the condu&ting power of air is very greatly increased by humidity. On this remarkable effect of moisture in the air, the Author remarks, with what infinite wisdom and goodness Providence appears to have guarded us against the ill-consequences of excessive heat and cold in the atmosphere : for were it poflible for the air to be as damp during the severe colds of the winter months, as it sometimes is in summer, its power of conducting the heat from our bodies, and of course its apparent coldness to us, would be intolerable ; but happily its power of holding water in solution, and therewith its power of robbing us of our animal heat, dimia nilhes in proportion as its coldness is increased. If colds or c2tarihs are occasioned by our bodies being robbed of heat, the reason is plain why those disorders prevail moft during the cold autumnal rains, and on the breaking up of the froft in spring. li is hence also plain, why damp houses, and damp beds, are

• Though it has lately been discovered, that the perfeet vacuum does not conduct electricity, this does not affect the Author's comparisons ; for the vacuum, in his experiments, was such as may be presu ied to have been a conductor; and in either case, the difference, in that respect, between the electric fluid and heat is equally


fo dangerous; and why the evening air is so pernicious in fummer and autumn, and not so in the hard frosts of winter. Physicians have been puzzled to account for the generation of the extraordinary quantity of heat supposed to be carried off from animal bodies by the cold air in winter, above what they communicate to the warmer atmosphere in summer ; but it is more tban probable, that the difference in the quantities of heat so loft or communicated, is infinitely less than they have imagined.

The Author examines also 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 ro much hotter or colder to the touch than water does, though really of the same temperature ; for the intensity of those seniations 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 thort period, or the intensity of the communication.

Sir Benjamin proposes continuing his experiments on heat; and experiments so accurately and judiciously conducted, on so interesting and obscure a subject, cannot fail to afford important results. An Account of Experiments made by Mr. John MNab, at Henley

House, Hudson's Bay, relating to freezing Mixtures. By Henry Cavendith, Esq; F. R. S. and A. S.

These experiments were made at Mr. Cavendith's desire, with materials provided and adjusted by him, in order to ascertain some particulars in his remarks fubjoined to Mr. Huichins's Paper in the 73d volume of the Transactions. As heat is gene-. rated * in the congelation of Auids, and cold in the liquefaction of solids ; and as the cold produced by mixing snow with spirit, of nitre is supposed to be owing merely lo the liquefaction of the snow, it Thould follow, that there may be a degree of cold, in which the nitrous spirit, fo far from dissolving snow, will suffer part of its own water to freeze ; and in that case no additional cold should be produced by the mixture of snow with it. As Itrong spirit of nitre generates heat with water, it does the same with the water that is formed on the fift addition of snow to it, and no cold is produced till the snow amounts to about one fourth of the weight of the acid ; it should follow, that if the acid be diluted at first with one fourth of water, no heat will be ge

* Mr. Cavendish adopts Sir Isaac Newton's idea, that heat is not a diftinct body, but a quality, produced by the intestine action of the parts of bodies. So far as relates to the explication of these experiments, the theory affects only the mode of expression; instead of saying that heat is generated, it is now more customary to say that it is disengaged, or let looje.


[ocr errors]

nerated, and the cold will of course be greater. The experi. ments are perfectly satisfactory, and ascertain some other interest. ing particulars, in a region of science which philosophers have buc few opportunities of exploring.

It appears from these experiments, that the nitrous acid is not only susceptible of an aqueous congelation, or freezing of the watery part, but of a spirituous, or freezing of the acid itself:That when cooled to the point of aqueous congelation, it has no tendency to diffolve snow, and thereby produce cold, but on the contrary, is disposed to part with its own water :- That the tendency to diffolve snow, and produce cold, is by no means destroyed by its being cooled to the point of spirituous congelation, or even actually congealed :—That both the strong and diluted acids bear, like water, to be cooled very much below their freezing point before the congelation begins, and rise up to their freezing points as soon as it does begin :- That, contrary to water, they shrink in freezing, very much, the surface bem coming depressed, and full of cracks, and the ice finking freely in the unfrozen Auid :--And that their freezing point varies according to a very unexpected law, the acid of a certain degree of strength freezing much easier than that which is either stronger or weaker. The lowest heat in which any nitrous spirit was found to freeze, was i į below o of Fahrenheit: the strength of this spirit was 411, that is, 1000 parts of it would diffolve 411 of marble ; and when any stronger or weaker spirit was frozen in part, the frozen part approached nearer to that strength than the unfrozen.

The general results of the experiments on this head are ex, pressed in the annexed table. The first column shews the strengths of the acids; and the second the degree below o at which they re!pectively froze.

540 | 313

irituous congelation.
• 380

243 441

210 | 17 Š aqueous congelation, The vitriolic acid contracts in freezing, as the nitrous does, its ice finking to the botrom of the unfrozen Auid; but it does not appear to have any point of easiest congelation, seeming ra. ther to freeze easier and easier, the more concentrated it is, without limitation. It seems also, that certain parts are more congealable than the rest, and that the difference does not de. pend altogether on strength, but on some less obvious quality, probably on that, whatever it is, which forms the difference between glacial and common oil of vitriol,



Abiračt of a Register of the Barometer, Thermometer, and Rain, at Lyndon in Rutland, in 1785. By Tho. Barker, Esq. &c.

The depth of rain at Lyndon was about 201 inches; at South Lambeth it was only 191, at Fyfield 247, and at Selbourn 317. From some observations lubjoined to this register it appears, that the annual quantity of rain is very variable in the same place at different periods. At Lyndon, from 1740 to 1743 inclusive, the mean depth was only 16 inches in a year, and yet no com, plaint was made of dry summers in any of those years; the fummers were fhowery, but the winters dry. From 1741 to 1750, the mean depth was 181 inches ; from 1751 to 1760 223 ;~1761 to 1770, 231 ;–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 39. It is plain from these ob fervations, how little dependence can be had upon average quantiries taken on periods of small extent. Magnetical Experiments and Observations. By Mr. Tiberius Ca.

vallo, F.R.S. (The Lecture founded by the late Henry Baker, Esq. F.R.S.)

These experiments relate chiefly to the magnetic property which has lately been observed in some kinds of brass. The general result of them is, That most brass becomes magnetic by hammering: That the magnetism is destroyed by annealing, or softening in the fire, restored again by hammering, and thus alternately, as often as the hammering and annealing have been repeated : That the magnetism is not owing to any particles of iron communicated by the tools, for it is producible by beating the brass between pieces of fint or copper ; and that the destruction of the magnetism is not owing to the calcination of any iron particles, for it takes place though the brass be surrounded with charcoal powder in a close crucible, Mr. Cavallo is therefore of opinion, that the magnetism acquired by brass is not owing to any iron in it, but to some particular configuration of its component parts, occasioned by the ham, mering. There are indeed pieces of brass which have visible particles of iron in them, but there are magnetic in their soft as well as hard state ; and there are others, which cannot, by any known means, be rendered magnetic at all.

A proposition so 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 Poftscript to the Lecture, starts an objedion, and gives some experiments to obviate it. The brass may owe its magnetism to an irony matter intermixed, and this iron may be magnetic, or not magnetic, according as is


« ПредыдущаяПродолжить »