lent philofophical and critical obfervations. Some of these we fhould gladly communicate to our readers: but we cannot do juftice to them, without greatly exceeding the limits which we are obliged to prefcribe to this article.

We come next to the memoirs; which, as ufual, we shall diftribute according to the feveral branches of fcience to which they relate.

GENERAL PHYSICS.

Memoir concerning the double Refraction of Iceland Crystal. By the Abbé HAUY.

This phenomenon is explained by Sir Ifaac Newton, in his optics; and the prefent ingenious author's investigation, though conducted in a different manner, leads him to a theory not materially different from that of our great philofopher.

Method of fo conftructing an Arcometer, or Hydrometer, that the fpecific Weights, which it indicates, fhall be in the reciprocal Ratio of the Volume that it measures, and which therefore points out the specific Gravity of Fluids by Immerfion alone, without requiring any Calculation. By M. BRISSON.

After a very minute enumeration of the inaccuracies to which most of the areometers hitherto invented are liable, M. BRISSON proceeds to give an account of his inftrument; which is certainly founded on a very ingenious application of fimple and eafy principles. It is evident that an hydrometer, the weight of which remains unaltered, when immersed in fluids of different denfities, muft indicate volumes of thefe fluids which are in reciprocal proportion to their respective denfities or specific gravities; fo that the volume, which finks in any given fluid, exceeds that which finks in a more dense fluid, in proportion as the denfity of the latter exceeds that of the former. Hence, in order to conftruct an inftrument, which, by immerfion only, fhall indicate the fpecific gravity of a fluid, nothing more is requifite than to know the exact proportion between the volume of the part which finks in this fluid, and the volume of that which finks in diftilled water. It is alfo certain, that hydrometers of different weights, immerfed in the fame fluid, will fink deeper in proportion to their feveral weights, If, for inftance, we immerfe in water an hydrometer weighing nine drachms, and repeat the experi ment after adding a drachm to its weight, the volume which finks in the first trial will be to that which finks in the fecond, as nine is to ten-but if, after reducing the hydrometer to its primitive weight or nine drachms, we immerfe it in a fluid lefs denfe, in which it finks to the fame depth that it attained

in water when its weight was ten, we muft naturally conIclude that the volumes of this fluid and of the water, thus measured, are as ten to nine, and that their denfities are as nine to ten.

On thefe principles is founded M. BRISSON's method of graduating an hydrometer, that fhall accurately indicate the proportion between the fpecific gravities of diftilled water, and that of any fluid in which it is immerfed. In order to afcertain the feveral divifions of the fcale, he adds to, or fubtracts from, the primitive weight of the hydrometer, fuch quantities as bear a certain proportion to this weight determined by the degree fought, and then obferves the depth to which the inftrument finks in water. To find thefe quantities, let a reprefent the primitive weight of the hydrometer, which expreffes the gravity of water, and put b for the volume of water difplaced by it: fuppofe the inftrument then plunged into another fluid fpecifically lighter, the gravity of which is to that of water as n to a; and let b+x exprefs the volume displaced of this fluid; it is evident that, in this cafe, the abfolute weight of the volume difplaced of the latter fluid will be b+x Xn, which will be equal to the abfolute weight of the bulk of water, or to bXa: hence bn+nx=ba or x=b×a—”. Thus the quantity to be added to, or fubtracted from, the primitive weight of the hydrometer, is expreffed by a fraction of this weight; the denominator of which is the denfity or degree required, and the numerator, the difference between this density and that of

water.

a-n

n

a-n

In order to fhew how this principle is applied, let us fuppose it required to graduate an hydrometer intended for fluids lighter than water. Let the inftrument be fo loaded with mercury, that its bulb and a finall part of its ftem fhall fink in diftilled water, and the part level with the furface be marked as 1000 degrees: it must then be taken out, and its weight, which conftitutes the quantity a, be accurately afcertained. Then, to find that part of the ftem, which answers to the 990th degree, add as much mercury as fhall be equal to orth of the original weight; and mark the point to which it finks after this, taking out the added weight, add and the point, which is then level with the furface of the water, will be the place of the 980th degree. The apparatus for afcertaining the divifions on the ftem of the inftrument is fimple and ingenious: but, for particulars relative to this and other circumstances of the procefs, we muft refer to the me

th,

moir itself. Tables are added, in which M. BRISSON has taken the trouble of calculating the parts of the primitive weight for every degree of the scale, from 7c0 to 1750. This is a greater degree of accuracy than is neceflary; as he acknowleges that only every tenth degree needs be thus afcertained.

A fixth Memoir on Electricity. By M. COULOME.

This ingenious philofopher here purfues the fubject which has fo long employed his attention, and concerning which he has already published several memoirs, that were noticed, when the academical publications, in which they were inferted, came under our review. The doubts that we then entertained concerning the fundamental principles of this hypothefis, are by no means removed. A variety of experiments and calculations are here particularifed, with a view to afcertain the proportion in which the electric fluid is distributed among conducting bodies, placed in contact with each other, and its denfity on the feveral parts of their furfaces. M. COULOMB adopts the theory of two electric fluids, though he acknowleges that the hypothefis of Dr. Franklin will account for the phenomena obferved; and his objections to this hypothefis feem to refult from his fuppofition that chemical principles must be applied to explain the appearances of electricity, rather than from folid arguments founded on the facts themselves. He promifes, however, to examine more particularly, in a future memoir, the several theories which have been proposed by different philofophers.

Effay concerning the Uniformity of Measures both of Length and of Capacity. By M. BRISSON.

This academician propofes to make a pendulum, that swings feconds in the latitude of Paris, the ftandard of long meafure, accounting it as three feet, or half a toife, and to retain all the old divifions or denominations, regulating them by this ftandard. For the bafis of measures of capacity, he recommends a cubic foot of diftilled water of a given temperature, which he accounts as fixty-four pounds; and he would have all other measures either aliquot parts or multiples of this foot. It is eafy to fee that this scheme does not anfwer the end which ought to be held in view, and which alone can counterbalance the inconveniences attending every alteration of what long cuftom has eftablished: the divifions are not fuch as facilitate calculation, and the ftandard is too arbitrary to be generally received; as none, except the French, will fee any reason why it should be determined by the latitude of Paris rather than by that of any other place. The plan mentioned in the begin

ning of this article, is infinitely more liberal and philofophi

cal.

Inquiry into the best Kind of Steel for receiving Magnetic Virtue. By the fame.

From thefe experiments, we learn that English fteel is preferable to all other kinds for this purpofe; that the next beft is a German fteel, which the French call Etoffe de Pons; and that caft-fteel will receive but little magnetic power.

CHEMISTRY.

Analysis of the Prafius and Chryfoprafus, or Green Calcedony of Cofemitz in Silefia. By M. SAGE.

This agate, called Prafius, becaufe its green refembles that of the leck, is found to be coloured by nickel and cobalt : fometimes it contains iron ochre, by which it is marked with yellow fpots in this cafe it is diftinguished by the name of chryfoprafus.

Analyfis of the Aerated Ponderous Spar of Alton Moor. By

the fame.

This fpar Mr. Kirwan has defcribed by the name of aërated barytes. Its fpecific gravity is to that of vitriolated ponderous fpar as 42919 to 44400: when white and perfectly transparent, it contains neither calcareous earth nor any metallic calx: but in that which is opake, blend, pyrites, and iron ochre are frequently found, and fometimes calx of lead.

Memoir on the Combuftion of fome Subftances in Oxygenated Muriatic Acid Gas. By M. FOURCROY.

It was formerly fuppofed that all thofe elaftic fluids, which are unfit for refpiration, were equally improper for combuftion: pyrophorus, however, has been found to burn in nitrous gas. with greater violence than in atmospheric air; and it appears, from the experiments here related, that the oxygenated muriatic acid gas, or the dephlogifticated marine acid of Scheele, forms another exception to a notion once fo generally received. A wax taper immersed in this gas continued to burn; the flame, indeed, became longer and fmaller, and affumed a reddish hue like that of a torch feen though a mist: but it was observed that the wax burned faster, and that the wick was fooner confumed, than in common air: fimilar phenomena occurred on repeating the experiment with a lamp; the flame was red and gloomy, furrounded with a denfe vapour, and the carbonic fubftance of the oil feemed to be feparated with greater rapidity than usual, and to be whirled in a kind of torrent around the wick. The phofphorized hydrogen, or phofphoric gas of Gengembre, on coming into contact with the oxygenated › APP. REV. VOL. VIII.

N n

muriatic

muriatic acid gas, immediately took fire, and burned with a deflagration not lefs violent than in the atmosphere, but with a flame lefs bright than it yields in vital air. The fulphurated hydrogen, or hepatic gas, on the contrary, exhibited no inflammability on being thus mixed,

From thefe phenomena, M. FOURCROY concludes, that oxygen, in its union with the muriatic acid, is also combined with light and caloric: but that it contains lefs of the former, and that the latter is more compreffed, than in the state of vital air. He afcribes the vapour, which furrounds the flame, to the carbon raised in this form fafter than it can be confumed, and gradually depofited by the hydrogen, which, uniting with the oxygen, forms drops of water on the fides of the receiver.

This ingenious academician alfo difcovered, that some fubftances are inflammable in the oxygenated muriatic acid gas, which are not fo in atmospheric air. A piece of phosphorus took fire in it, and burned with great violence: this is the more remarkable, as phosphorus, when cold, is not inflammable in vital air, and, when lighted, does not burn in it with fo much brightnefs as in that of the common atmosphere. Another inftance of this kind occurred on bringing ammoniacal and oxygenated muriatic acid gas into contact with each other; these two elaftic fluids were immediately united and condensed; not only great heat, but even flame, was produced; a thick white vapour arose in the receiver, and drops of water were formed in great abundance on the glass.

These phenomena are alfo explained by the great compreffion of the caloric. It is obferved that fubftances do not eafily combine, unless their several densities be rendered nearly equal; hence phosphorus, in order to unite with, and be inflamed by, vital air, must be rendered lefs denfe by heat: but, in the oxygenated muriatic gas, as well as in the nitric acid, the oxygen, in confequence of containing lefs light, and having its caloric more compreffed, approaches nearer to the phosphorus in denfity, and therefore more rapidly combines with it. The author allo accounts for the inflammation of the ammoniacal gas on this principle: the flame produced by this decompofition fhews that the hydrogen of the ammoniac feparates from the oxygen of the muriatic acid gas a certain quantity of light existing in the burning principle, and proves that oxygen is combined with lefs light and heat in water than in the muriatic acid.

When M. FOURCROY poured the concentrated fulphuric acid on the oxygenated muriat of potafh, he observed a violent effervefcence, and the production of a white vapour, which, though it refembled in smell the oxygenated muriatic acid, had a character peculiar to itself: the falt and the acid both became

of