points like thofe of needles. If this neutral falt be diftilled, the cauftic volatile alkali rifes, and the acid remains in the retort, in dry powder, coloured with yellow. Combined with white magnefia the acid in queftion forms an intermediate falt, which it is not eafy to diffolve in water. It does not change the folutions of alum and lime; but it decompofes the earth, that is known under the denomination of terra ponderofa acetata. The precipitate is not foluble in water. It precipitates in a blue colour the flannum falitum, and in a white, the following metallic folutions, viz. the ferrum, zincum, cuprum, vitriolatum; argentum, mercur: plumbum, nitratum; and plumbum falitum; but it does not produce any change in the corrofive fublimate, or in the folution of gold. To afcertain ftill farther the peculiar nature of the acid of tung flen, our Author obferves, that when it is calcined in a crucible, it is no longer foluble in water; that it has the property of attracting the phlogifton, as appears from the blue colour which it receives from vitrifying fluxes; that it precipitates, in green, the folution of liver of fulphur, and, in white, the folution of phlogifticated alkali; that it affumes a beautiful blue colour when polished iron, zinc, or tin are placed in a folution of it by water, and when fome drops of the spirit of falt are mixed with this folution. Other properties of this acid are here enumerated; but we are obliged to abridge. As the acid of molybdena, or black lead, derives a blue colour from the metals now mentioned, fome may be led by this circumftance to identify this acid with the acid of tungen. But our Academician proves, by feveral facts and experiments, that, notwithftanding this fingle point of refemblance, thefe two acids have many different properties, which diftinguish them palpably from each other. This memoir is followed by an Appendix added to it by the late Sir TORBERN BERGMAN, which contains feveral obfervations on the tungsten. Mem. VI. Experiments on the Elafticity and Diftribution of Heat, confidered with reftect to the Afcent and Refrigeration of Vapours in rarefied Air. By M. J. C. WILCKE. The experiments of Otto Gueric, on the afcent and fubfequent defcent of vapours in the air-pump, is well known. It was thence concluded that air, when rarefied, is no longer capable of fupporting, and therefore lets fall (as fpecifically heavier) all thofe heterogeneous fubftances which before were fufpended, and, as it were, diffolved in it. But, while this principle was employed to explain the defcent of the mercury in the barometer, the falling of rain, and other phenomena of our atmosphere, the most remarkable circumftances of the experiment of Otto Gueric were by no means explained, fays our Academician, in a fatisfactory manner. manner. Thefe circumstances are-that the vapours which de fcend from the air, when rarefied, afcended previously, and were spread abroad through the atmosphere; that the rarefaction of the air occafoned their afcent; and that the cause of thefe phenomena is the expan fion and diftribution of heat. Our Academician, having repeated and diverfified this experiment, found that the refult was the fame which preceding philofophers had derived from it, when be introduced a humid body into the receiver: but when he employed a receiver that was clean, dry, and a little warmed, and a leather done over with wax or tallow-when, at the fame time, the plate of the machine and the air of the chamber were thoroughly dry, no vapours ever appeared at the first motions of the pifton; and when, by exhaufting and then letting in the air at different times, or by any other operation, fome humid particles were introduced into the receiver, then, vapours were ob ferved afcending in the receiver, and that in proportion to the moisture or drynefs, the coldness or warmth of the air. From thefe experiments our Academician drew the following conclufions; that it is necessary to introduce fome humid fubftance into the recipient, in order to render the vapours which it contains, visible; that these vapours rife, in effect, from humid furfaces placed under the recipient; that they are expanded in the rarefied air before they redefcend in the form of clouds and drizzling rain; and that thus the afcent of thefe vapours, and their subsequent descent, must be confidered as two diftinct effects, intimately connected with the rarefaction of the air. After having difcuffed and refuted the explications that have been given of this phenomenon by feveral learned men, M. WILCKE relates the experiments he made in order to discover the cause of this ascent and descent of vapours in rarefied air. It is evident, fays he, that heat and cold have fome particular and immediate affinity with the afcent of vapours in the airpump; because the greatest part of these phenomena depend more especially on the degree of abfolute and relative heat, which exifts in the air, the water, the glass, and even the machine, at the time of the experiment. They appear always to greater advantage when the air and the bodies are warm, than during a keen and fharp cold. It is also eafy to perceive how differently they are affected by a warm or by a cold receiver. The former prevents the free expanfion of the vapours; the latter promotes it. The former remains clear and pure; the latter, as foon as the air is introduced, is obfcured on all fides with vapour and moisture. Our Academician thinks that hence, without any farther researches, it is natural to conclude, that the paffage and diftribution of heat among bodies placed under the recipient and in rarefied air, must be confidered as the true caufe of the afcent, the mo dification, and the defcent of vapours. He does not, however, reft his proofs here; for, fays he, in order to complete my own conviction and that of others, and to unfold more particularly the mechanism of thefe effects, I made the following experiments: 1. Two thermometers were nicely conftructed, exactly correfponding with each other. The one was fufpended under a dry receiver the other was placed near it, but on the outside of the receiver. After having left them in this pofition for as long a space of time as was fufficient to make them contract the temperature of the medium, in which they were placed, I pumped out the air, and found, that after the vacuum had been produced, the infide thermometer had fallen two degrees, but that It rofe again when the air was introduced anew. This effect ceased when the tube of the thermometer was opened; which proves that it was owing to the expanfion of the ball of the thermometer, and the preffure of the external air. The temperature of the chamber was afterwards changed, and it was then observed that the two thermometers rofe and fell exactly together; and this evinces the equal and correfpondent diftribution of heat in the external dense air, and in the internal rarefied air. This takes place as long as the ball of the interior thermometer continues dry; but as foon as it contracts the fmalleft degree of moisture, the correfpondence is interrupted, and the variations are remarkable. 2. If the ball of the interior thermometer be immersed in a veffel filled with water, and the air be pumped out, it remains at the fame point during the whole of this operation; but it falls feveral degrees the moment that it is taken out of the water, and does not rife again to its former height till the ball becomes dry, and all the moisture has evaporated. 3. To preferve the moisture the longer, and in a greater abundance about the ball, it was furrounded with a piece of fine linen, thoroughly wet; and, as foon as the piston began to work, the liquor fell five or fix degrees, and fometimes fourteen, when the vacuum was completed; the temperature of the chamber and that of the water being about ten degrees. The thermometer rofe again when the moisture bad evaporated, but did not return to its former height until the ball was entirely dry. M. WILCKE remarks, that in the preceding experiments the vapours, which arife from the veffel full of water, form a palpable obftacle to the fall of the thermometer, which always defcends fome degrees lower under a dry receiver, where there are no other vapours but those which arife from the ball; and the. greatest descent takes place when the ball is moistened before the thermometer is placed under the receiver. After having proved, by thefe experiments made with water, that the rarefaction of the air is favourable to evaporation, and, at the fame time, to the refrigeration and defcent of the thermometer, our APP. Rev. Vol. LXXV. Academician Nn Academician employed liquors of a more volatile nature, and more fufceptible of evaporation.-4. The ball of the thermometer, being furrounded with a piece of fine linen, was moiftened with spirit of wine, highly rectified, and the confequence was, that the thermometer fell from the 17th degree above the freezing point to the 8th, and even to the 12th below it: with vitriolic ether it defcended from the 18th degree above o to 18 degrees below it. By this method of proceeding, water, in a warm chamber, was converted into ice with great facility, by placing it in a glass veffel fufpended under the receiver. 5. An equal quantity of ether was put into two tea-cups, one of which was placed under the receiver, and the other without. The ether evaporated much more fpeedily in vacuo, just as warm water cools fooner in vacuo than in the open air. As in the preceding experiments a great quantity of vifible vapours are emitted from the ball of the thermometer, which is fufpended in rarefied air, it is evident that these vapours arise from humid furfaces, and carry with them the heat of the bodies of the furfaces from which they proceed; and that thus the affumption and the paffage of the heat from the mass of bodies in rarefied air, muft be confidered as the immediate and true cause of the eruption and afcent of the vapours. The following experiments will enable us to form a ftill clearer idea of the heat itfelf, and of the mechanifm of its effects in the air-pump. 6. A round plate of polished copper of the fame diameter with the receiver, and fupported by a glafs foot in a horizontal fituation, was placed under a receiver at about the middle of its height. On working the pump, the receiver was filled with vapours, and when a wax candle and the eye were placed in the level of the plate, it was eafy to perceive diftinctly, when the plate was heated, that the vapours kept themselves conftantly at a certain diftance from the metal, which was furrounded with a clear diaphanous fpace. Over this the vapours were fufpended, fell and vanifhed without arriving at the surface of the plate, which was afterwards found to be as dry and unfullied as it had been before the experiment. On the contrary, when the plate was colder than the receiver, no trace of fuch an atmofphere was to be feen; and the vapours moving in all directions on the cold furface of the metal, covered it with a kind of dew. After the relation of the experiments now mentioned, and of others of a fimilar kind made for the fame purpose, our Academician offers fome judicious obfervations on the analogy between these phenomena and thofe of electricity. His conclufion is, that they muft both be explained by the fame theory; and he deduces from them the following propofitions: The fcale of the Swedish thermometer makes the freezing point o, and boiling water 100. Heat Heat is a moft fubtile and expanfible matter, whofe parts repel each other reciprocally. This matter is also most powerfully attracted by that of other bodies: hence the reafon why it not only penetrates and fills their pores, furrounds their furfaces, and dilates them by its telafticity and abundance, but also separates, and, under the name of evaporation, carries off with it the fmalleft parts of bodies. Of these it forms folutions or elaftic vapors, whole kind is determined by the nature of the matter of which thefe bodies are compofed, and whofe degree of elafticity depends upon the quantity of the repulfive heat. Different kinds of matter attract the heat with different degrees of force, according to the nature of each matter. Thus, in the preceding experiments, the heat is attracted the most powerfully by the air, lefs by water, ftill less by glass, and leaft of all by the mercury of the thermometer. The fame kind of body or matter receives and retains, according to its different flates and modifications, a different quantity of heat. This twe fee clearly when a fufficient quantity of heat transforms the bodies into folutions or elaftic vapours, or when they are under the preffure of an exterior force: in the first of these two cafes, their parts are furrounded with the heat that is neceffary to feparate them from each other, and furmount their mutual attraction; in the fecond, they can neither receive nor retain all the heat, which, in a state of full liberty, would get the better of their attraction. Thus a warm air, when strongly compreffed, difcharges, like a fpunge, the heat which it contains, but refumes it when it becomes more free, and can dilate itself. In the fame manner the elastic heat expands itself in that direction where it finds the leaft refiftance. Hence it follows, that as foon as the quantity and preffure of the air within the receiver are diminished by the effect of the pump, the equilibrium of the heat is disturbed, the particles of air which remain in the receiver have more room to acquire and retain, as a kind of atmosphere, a greater quantity of heat than before. This heat is furnished by the furrounding bodies, and comes principally from thofe which have it in excefs, or which attract and retain it with the leaft force: by its peculiar elafticity it directs its courfe towards that quarter where the equilibrium ceafes, and the refiftance is diminished at the fame time; and, when the nature of the body admits of this, it carries off from it the more fubtile exterior parts; and, in this feparation, thefe parts attract, and are furrounded by, a greater portion of heat, which is difengaged from the body whence they have been separated, and which is confiderably cooled by * Boerhaave Elem. Chem. P. II. P. 480. Na 2 this |