Saturnians and Mercurials. And as a great latitude of conjecture may be allowed with regard to thefe ftrange perfonages, who live at such a diftance from us, we have supposed that the materials of which their bodies are conftructed, and the fenfibility of their nerves-if they have any nerves-are different from thofe of the inhabitants of the earth: fo that, to a Mercurial, a bason of our melted lead might prove a cooling and refreshing beverage; while a piece of the coldeft ice of Siberia might burn the chops of a Saturnian. But M. de Mairan's hypothefis prevents our indulging any longer in fuch violent conjectures. Had Mercury no other heat than that fmall portion which, according to this fyftem, he receives from the fun, his furface would be covered with eternal frofts: nor does Saturn suffer in the leaft, by his great diftance from that luminary. The central emanations fet every thing to rights; as the external cruft of these, and of all the other planets, is more or less permeable to the native, central fire which they contain, exactly in proportion to their greater or less distance from the fun fo that the heat in all the planets, from Mercury to Saturn, is nearly equal; and we must now confider the fun, as reduced and degraded, by this fyftem, from the honourable function of prime and sole dispenser of heat, to the more humble office of a regulator.

MEMOIR II. On the duration of the fenfations excited in us, through the organ of fight. By the Chevalier D'Arcy.

The impreffions made by fenfible objects on our organs, continue to affect the mind, fome time after the objects which produced them have ceased to act upon them. This circumstance has been more particularly attended to in the cafe of visible objects though we could offer inftances of this protracted sensation, produced by the objects of the other fenfes. When a firebrand, or other luminous body, is turned round with a sufficient velocity, it is well known that we perceive a compleat luminous circle; not because the eye involuntarily and imperceptibly follows, or can poffibly follow the luminous body in its circular courfe; but because the impreffion made by the body in any one point of its revolution continues to be felt till it has returned to the fame point again; or, in other words, becaufe the fenfation furvives, during a certain fpace of time, the caufe which produced it. To difcover the quantity of this interval of time is the intention of the Author of this memoir.

For this purpose M. D'Arcy invented a machine, which we fhall not here ftop to defcribe particularly. It may be sufficient

A continued light will be feen in any fingle part of the courfe of the revolving luminous body, even when the eye is fixed, or confined the fight of that part only, by viewing it through a fmall aperture.

only

only to observe that it was fo conftructed, as to measure, with due accuracy, the duration of the revolution of a red-hot coal, or other luminous body, fixed to one of the extremities of it. By repeated experiments made with this machine he found that, when the luminous body made a compleat revolution in 8 thirds of time, it prefented to the eye the appearance of a compleat circle of fire; and from thence concludes, that the fenfations excited by light continue to affect the mind during the 7th part of a fecond, after it has ceafed to act upon the organ for when the body took 9 thirds of time in performing its revolution, an evident difcontinuity or interruption was obfervable in different parts of the luminous circle.

From thefe and other experiments made with this machine, this fingular conclufion may be drawn; that if an opaque body were to pass before the eye, with fuch a velocity as to move through a space equal to its own diameter, in lefs time than the duration here affigned to the fenfation of fight, that is, in lefs than 8 thirds of time, the body would not be perceived by the eye. The Author afterwards relates fome experiments made with coloured bodies; the refult of which may, à priori, be naturally deduced from the preceding. A circular plane, half of which was painted blue, and the other half yellow, being turned round with great velocity by his machine, produced the appearance of an uniform green. The impreffions excited on the fenfe by the two colours, though acting on the organ in fucceffion, were, on account of their duration, prefent to the mind at the fame time; and accordingly excited the fame idea which would have been produced by the actual mixture of the two colours. In like manner, another disk, on which he painted the feven colours of the folar Spectrum, excited by its revolution the idea of a furface of an uniform, but fomewhat imperfect white.

We pass over several obfervations relating to fome varieties occurring in these experiments, as well as the Author's fuggeftions with regard to fome new objects of enquiry; particularly, whether the various intenfities of light, and the supposed different velocities which have by fome been attributed to the differently coloured rays, would affect the duration of the fenfations; and what varieties may be produced by the different fenAbility of different obfervers. He feems to think the determi nation of some of these questions, notwithstanding the fmallness of the quantities concerned, to be of importance with regard to the accuracy of aftronomical obfervations; and inftances, as a proof of the juftice of this opinion, the celebrated discovery of the nutation of the earth's axis, which results from obfervations of a quantity fo fmall as 18 feconds in 9 years: but the cafes, we apprehend, are by no means parallel.

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MEMOIR

MEMOIR III. Obfervations made at Solfatara, near Naples. By M. Fougeroux de Bondarcy.

This remarkable place, formerly known by the names of Forum Vulcani, Phlegrai Campi, &c. has attracted the attention of many philofophers. Several obfervations have been made on this fpot by Monf. Geoffroy, the Abbé Nollet, M. de la Condamine, and the Abbé Mazeas, part of which are repeated in this memoir: but they are here illuftrated by figures, to which are joined feveral new obfervations peculiar to the Author, principally relative to the formation and effects of Vulcanoes, and to the chemical procefles which have, for many ages paft, been carried on, both by nature and art, at this place.

Solfatara is a natural bason about 1200 feet long, and 800 feet broad, formed by mountains which nearly furround it; and has all the appearance of a ruined vulcano, in the last ftage of its existence, but not yet extinguished. Towards the middle of it, the earth founds hollow under the feet, and on letting a large ftone fall upon it, a noife is produced refembling the report of a cannon. In feveral parts of this bafon are little openings or fillures, through which imali ftones are thrown up, with fome degree of force, and from whence proceeds a smoke, attended with a heat fufficient to burn the hand; but not confiderable enough to fet fire to fulphur, with which the earth is in fome places impregnated to a fourth, and even a half of its whole mafs. The manufacture of this commodity was carried on in this place lo long ago as the time of Pliny. [See Hiftor. Natur. lib. 35. cap. 15.] The Author minutely describes its prefent ftate, as well as the manufacture of alum, which is extracted from the ftones that abound in this place, and on which nature has performed the preliminary procefs, of calcination. But the most remarkable production of this place is a falt, which fublimes through the fiflures abovementioned, into pots placed over them, inverted; where it is detained and collected, and which the natives affirm to be fal ammoniac.

Difficult as it may hitherto have appeared, to conceive from whence the volatile alcali, which is one of the two conftituent principles of that falt, draws its origin, in a place where there is no animal or vegetable matters to furnish it; the Author's experiments and obfervations afford juft grounds to believe that the falt of Salfatara is of the fame nature with the true fal ammoniac, which we receive from Egypt, and which is a factitious fubftance, procured by fublimation from the foot of the dung of camels and other animals. Nevertheless, authors of the greatest credit, either mifled by the love of fyftem, or not having had opportunities to analyfe the fubftance naturally fublimed at Solfatara, have affirmed it to be nothing more than a mere foffile fea-falt, elevated by fubterranean heat, and affording cubic

crystals

cryftals after diffolution and evaporation. This was the opinion of Geoffroy, which is maintained likewife by the intelligent and more modern authors of the Encyclopedie: and Cartheufer denies even the existence of any native ammoniacal salt whatever. Nunquam,' says he, fal ammoniacum nativum, vulgari fimile, in ullo terrarum angulo repertum fuit, &c.' We fhall give fome of the leading characters by which the Author eftablishes its existence at Solfatara; leaving to the philofophical chemifts to discover the particular origin of its volatile alcaline principle. We fhall only ftop to hint, that we have obferved some modern chemists naming certain foffile fubftances as capable of furnishing it, by the affiftance of fire, without the admixture of any animal or vegetable matter; particularly foffile coal; cretaceous, calcareous, and other earths mixed with pyrites, or the marine acid; and even iron: any or all of which may be fupposed to exift in this place. An inquiry into the manner in which fal ammoniac is produced by nature, is not a matter of mere curiofity, but of importance fufficient to deferve the attention of the practical chemift, as well as of the philofopher: as will readily be acknowledged by thofe who are acquainted with the many ineffectual attempts, which have been made in this kingdom, to produce the true fal ammoniac, at a lefs expence than that at which it is procured from Egypt.-But to return to the Author's proof of the identity of the native and factitious falt.

The falt of Solfatara fublimes into the pots, in the form of needles. It affects the tongue with a brifk, acrid tafte, exactly fimilar to that of the common fal ammoniac. In the act of diffolution in water, it produces a degree of cold even fuperior, as the academicians at Naples affirm, to that caused by the laftmentioned falt. The folution, after evaporation, fhoots into fimilar vegetations. Like it, this falt being laid on a red-hot coal, is entirely diffipated, without any previous fufion; and the vapour strikes the nose with the peculiar pungency of the volatile alcali. Finally, the Author having added fome foffil fixed alcali to a folution of this falt, the fame volatile alcaline vapour arofe; and having evaporated the liquor, he obtained cubical chryftals, as well as others of a different figure; but which were both true fea-falt, formed by the union of the marine acid with the fixed alcali which had taken the place of the volatile alcali. This native falt therefore contains, according to the Author, a volatile alcali combined with the marine acid, and therefore differs not from the true fal ammoniac. He might, however, have placed the truth of this conclufion out of all doubt, had he actually collected the volatile alcali, by the well known process by which it is procured from the true fal ammonia; and by other experiments might have discovered whether

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any

any other fubftances are combined with these two principles, befides fome which he mentions, and which have poffibly contributed fo long to difguife its true nature.

MEMOIR IV. Obfervations on a coal-mine, which has continued burning for a long time paft. By the fame.

Our own country furnishes many inftances of this kind. The mine, which is the fubject of this memoir, is fituated at St. Genis, near the city of St. Etienne in France, and has been on fire for the pace of 100 years paft. The ground is burnt to a confiderable extent, and in different parts of it are fpiracles, or openings, through which a hot vifible vapour ascends in the day, and a flame is faid to be fometimes perceived rifing in the night; the heat at all times being fufficient to be employed in fome articles of the cookery of the neighbouring peasants. The Author relates fome ineffectual and ridiculous attempts which have been made to extinguifh this fire, by cutting trenches round it; and a proposal of extinguishing it by water. He apprehends danger to the neighbouring cities of Chambon and St. Etienne, from the progrefs of this undermining fire, in a country where the vein of coal is very rich and extenfive; and proposes the ftopping up all the communications with the external air, as one of the most probable means to extinguish it.

MEMOIR V. Botanical and meteorological obfervations made at Denainvilliers, in the Year 1764. By M. Du Hamel.

We find nothing obfervable in this meteorological journal; nor, for the fame reafon, fhall we make any extract from fome fhort phyfical obfervations, with which this clafs is terminated.

ANATOM Y.

MEMOIR I. and II. On the circulation of the blood in the liver of the fetus. Second and third memoirs. By M. Bertin.

In the memoirs for the year 1753, M. Bertin firft proposed his new theory on this fubject. He endeavoured to prove, in oppofition to the opinion of the most celebrated anatomists, that the liver of the foetus, in utero, receives the greatest part of its blood from the umbilical yein; that this vein forms two branches, one of which enters into the vena cava, and the other into the vena porta; and that the blood moves in this vein through the liver from the left to the right hand, though, according to the common opinion, it is fupposed to follow a contrary direction: but that, at the inftant of birth, on the ligature of the umbilical chord, the umbilical vein lofing its former office, the blood in the vena porta turns back, in order to enter into the veflels of the umbilical vein, which it fills by a communication established between them, and in which the blood afterwards flows, till the death of the subject, in a direction |