tic aerial ftate? This fuppofition becomes ftill more probable, if we obferve that when the folution of the fpar in an acid is precipitated by a mild alkaly, C. 1. 2. fome water enters into the compofition of the precipitate, for it weighs the fame as before it was diffolved, and yet contains only twenty ounce meafures of fixed air, whilft the native fpar contained twenty-five ounce measures; fo that there is an addition of weight equal to that of five ounce meafures of fixable air, or three three-fourth grains to be accounted for, which can only arife from the water; and this precipitate, thus united to water, readily lofes its aerial acid in the fire, E. 1.'^

In the obfervations on his analyfis, there are many other valuable chemical remarks; but we can only mention, that he finds the cauftic terra ponderofa has lefs affinity to the ni trous and marine acids, than the pure alkalies. In the dry way, it has lefs affinity even to the vitriolic acid than the vegetable fixed alkali.

Our author next examines the terra ponderofa vitriolata, found in Alfton-moor mines. It is the marmor metallicum of Cronstedt, and a hundred parts contain 32.8 of pure acid, and 67.2 of pure earth. There is reafon to think, from Dr. Withering's experiments, that this fubftance may prove an useful flux.

The third fection describes a variety of the fame fubftance, found in the Derbyshire mines, and called the Derbyshire cauk. Of this, a hundred parts contain 99.5 of marmor me tallicum, and 0.5 of calciform iron. Another variety, from Shropshire, in the fame quantity, contains 2.3 of the iron in a milar state. Dr. Withering feems to think, that the terra ponderofa claims a place between the earths and metals, as, in fome properties, it refembles both; but that the specific gravity of its compounds arifes from their containing no wa

ter.

Art. XXIV. Obfervations du Paffage de Mercure fur la Dif que du Soleil le 12 Novembre, 1782, fates à l'Obfervatoire Royale de Paris, avec des Réflexions fur un Effet qui fe faît fentir dans fes mêmes Obfervations femblable à celui d'une Réfraction dans l'Atmofphère de Mercure. Par Johann Wilhelm Wallot, Membre de l'Académie Electorale de Sciences & Belles Lettres du Manheim, &c.-Obfervations on the Pale fage of Mercury over the Sun's Difk, the 12th of November, 1782, made at the Obfervatory at Paris; with Reflections on an Effect, perceived in thefe Obfervations, fimilar to what would be produced by a Refraction in the Atmosphere of Mercury. By Joann Wilhelm Wallot.Mercury first touched the fun's limb at 2' 56', and the center of the planet was on it in two minutes more. The center was on the fun's limb in its

exit, at 4 zo, and the planet difappeared in little more than z". We cannot follow our author's aftronomical calculations in the fubfequent part; but he obferves, if the middle of the paffage, feparately determined from a comparison of the internal and external contacts, of the entrance and exit differ, the difference must probably depend on an atmosphere. This quantity was found to amoant to 26.4', and confequently 0.276" is allowed for the horizontal refraction of the atmosphere. The fame variations were found, in the tranfit of Venus, to amount only to 8 or 9"; confequently o.205" were allowed for her horizontal refraction. A remarkable difference in denfity, when the heat to which they are expofed is confidered; but, at the fame time, apparently well calculated for the existence of inhabitants! We ought however to add, that these obfervations must be frequently repeated, before their refults will be established as probable: they are at prefent little more than fufpicions.

Art. XXV. Thoughts on the conflituent Parts of Water and of Dephlogisticated Air. By Mr. James Watt,-Art. XXVI. Sequel to the Thoughts on the constituent Parts of Water and Dephlogifticated Air. By Mr. James Watt.-Mr. Watt concludes, or at leaft queftions, whether he is not authorised to conclude, from foine of Dr. Prieftley's experiments,

That water is compofed of dephlogisticated air and phlogifton, deprived of part of their latent or elementary heat; that dephlogisticated or pure air is compofed of water deprived of its. phlogitton, and united to elementary heat and light; and that the latter are contained in it in a latent ftate, fo as not to be fenfible to the thermometer or to the eye; and if light be only a modification of heat, or a circumftance attending it, or a component part of the inflammable air, then pure or dephlogifticated air is compofed of water deprived of its phlogifton and united to elementary heat ?'.

We have already obferved the inconfiftencies of even the beft chemists, on the fubject of phlogiston, and the difference in opinion of others; but the position of Mr. Watt feems to be on a more fecure foundation than many late theories. It was an early opinion of Dr. Black, that fixed air was a compofition of common air and phlogiston, at a time when this was the only new fpecies of air difcovered. After we became acquainted with phlogisticated and inflammable airs, the opinion kelas to have been abandoned, though now again revived. We have already mentioned the experiments of Dr. Priestley, which feem to fhow that fixed air is compounded of dephlogifticated and inflammable airs; and Mr. Watt thinks it probable that fixed differs from phlogisticated air, in containing

a greater

If this be true, and we think it

a greater fhare of phlogiston. may be fupported both by reafon and facts, inflammable air must be in the firft rank, and phlogiflicated air in the laft: the one containing the greatest, and the other the least quantity of phlogiston. Dephlogifticated air, on the other hand, he thinks, is produced from water, by any fubftance which has a ftronger attraction for phlogifton than the principle of water, properly enough ftyled humor, to diftinguith the fence from the form. This our author has elucidated by many experiments, which pretty clearly establish this opinion; and it is nearly that of Mr. Cavendish, fee page 251. We indeed always concluded, from the facts already in our hands, that the air obtained from the different calces, by means of nitrous acid, was not produced, but only feparated in a purer form. This opinion we have had frequent occafion to repeat; and are of courfe pleafed to fee it more firmly established.

The first fact eftablished by Mr. Watt is, that when the pure air is obtained, the acid, however pure when first added, became phlogifticated, and this phlogiftication constantly occurred, on repeating the affufion of pure acid on the fame minium. But, on repeating the experiments with accuracy, large quantities of pure air were produced, with a very small lofs of weak acid. Thirty-fix ounce meafares of air were procured by means of four hundred and eighty grains of weak dephlogifticated nitrous acid, and the whole acid was recovered ex cept five grains.

The paper concludes with fome obfervations on the fyftems of Mr. Scheele and others, with fome general reafoning on the fubject. As the water produced, on firing pure and inflammable air, occafioned the other experiments, we are informed that the heat produced is much greater than is fufficient to convert the water into feam. As no direct experiments have been made to afcertain the fact exactly, the author affumes thofe of Lavoifier and De la Place, in the last volume of the Memoirs of the Academy of Sciences. In these it appears that the heat, extricated during the combuftion of inflammable and dephlogisticated airs, was very great. One Paris ounce, ffomewhat larger than an English) of dephlogisticated air, decomposed by burning phofphorus, gave 9265°.5 of Fahrenheit. It does not feem of great confequence, or to vary the experiment confiderably, from what fource the phlogifton is procured. The fecond paper only relates to fome circumstances neceffary to be attended to by those who wish to repeat Mr. Watt's experiments.

XXVII. An Attempt to compare and connect the Thermometer for Atrong Fire, defcribed in Vol. LXXII. of the Philo

fophical

fophical Tranfactions, with the common Mercurial ones. By Jofiah Wedgwood, F. R. S.-It is of confequence to connect the fcale of Mr. Wedgwood's thermometer with that in common ufe. This is now performed in a dextrous and accu ate It appears, from this manner by the intervention of another. comparison, that one of Wedgwood's degrees is equal to 130° of Fahrenheit's fcale. Mercury boils at about 600 of Fahrenheit, or rather below that point. Red heat, vifible in the day light, is found to be at the 1077th degree, nearly at the point where it was fixed by Dr. Black and fir Ifaac Newton. But the extremity of the fcale, or Wedgwood's 240th degree, is at 32277° of Fahrenheit. The leaft welding heat of iron, ufually reckoned about 5000°, is now found to be 12777°. What an immenfe fcale! From freezing water to vital heat is fcarcely the FIVE HUNDREDTH part of it; ten times that quantity may be taken away, in the higher degrees, without occafioning a fenfible difference. In the nicer chemical manufactures, particularly in enamelling, this gage of heat must be extremely valuable.

The appendix to this article contains fome curious experiments, by Mr. Wedgwood, on thawing ice. He repeated Lavoifier's experiments; but found, on dropping the heated thermometer piece into pounded ice, that very little of the water, neceffarily produced, appear in that form. It seemed to have been abforbed by the furrounding ice as by a spunge; but it became alfo ice, and connected the feparate molecules with confiderable force. Theafon of the abforption is obvious; and the author accounts for the freezing, from vapour requiring a lefs degree of heat, to fix it in a folid form, than water. But this is only a part of the caufe. Melting ice is remarkably greedy of heat; and indeed every body in a capacity to abforb heat, and to convert it from a fenfible to a latent ftate, attracts it fo powerfully as to feparate it from any other body. We want no additional evidence of this quality, than the umple fact, that all our freezing mixtures, or rather the mixtures which produce cold in the furrounding fluids, effect this change, while they themselves are in a thawing flate. Fuming fpirit of nitre, poured on fnow, always melts it, while it congeals the water round which it is placed. A piece of ice, in the fire, will extinguish it, at a distance to which the water produced cannot reach. Inftances of this kind are numerous. Though fomewhat must be allowed to the facility with which fteam freezes, yet the chief effect seems to be owing to the abforption of the water firft produced, and the abftraction of the heat from the continued operation of the hot clay. In the cellar, when the tea-cup froze to the upper part of the ice, while

the

the reft was in a thawing ftate, the general caufe only operat ed, viz. the fuperior attraction of the melting ice, for the heat contained in the veffel.

The neceffary length of our obfervations obliges us to defer the future confideration of this part of the annual volume till another opportunity.

[To be continued.]

A Differtation on Milk. By Samuel Ferris, M. D.

THIS

HIS is one of thofe effays which obtained the prize-medal of the Harveian Society at Edinburgh, in 1782. The experiments to afcertain the nature of milk are well directed'; but the fubject has been fo frequently examined that it will not eafily admit of much new light. Our author indeed found that, in making butter, no air was really generated; what appeared to be fuch was only air rarefied by the heat raised in the procefs. This fact acts in a very extenfive circle; for the fermentation, raised in making butter, was one of the arguments from which the antagonists to thofe who deduced animal heat from the friction of the fluid against the fides of the vesfels, endeavoured to combat that theory: they attributed this heat to fermentation. But, nce the refult is fcarcely a tertium quid, but rather feparated than produced, and one of the conftant attendants on fermentation, is wanting, that oppofition must be materially weakened. We cannot, at this time, attribute animal heat to motion only; but we fee no reafon why it may not have fome share.

Another part of this Differtation is, An Enquiry into the Similarity between Blood and Milk. The fpontaneous feparation of each into parts, flightly refembling each other, has influenced fome fuperficial phyfiologifts to conclude, that milk was only blood deprived of its colouring parts; and they thence deduced confequences which materially influenced the practice of phyfic, and did much harm. It was not one of the leaft, that the ftomachs of hectic and cachectic patients were loaded with too great a quantity of this ready-made blood. Dr. Ferris examines the relation of thefe fluids experimentally; and, in the most important circumftances, finds little refemblance. We wish that the nature of the milk of carnivorous animals had been more accurately afcertained. Mr. J. Hunter, we have been informed, found the milk of a bitch, which had been fome time fed with even putrid fiefh, acefcent. Dr. Young and Dr. Ferris found it alkalefcent; but the latter tells us very candidly, that he determined the point only by the VOL. LIX. June, 1785. Ee change