Imágenes de páginas
[merged small][ocr errors]

points like those of needles. If this neutral falt be distilled, the caustic volaule alkali sises, and the acid remains in the retort, in dry powder, coloured with yellow. Combined wiih white magnesia the acid in question forms an intermediate salt, which it is not easy to d:fi lve in waier. It does not change the rolu. tions of alum and lime; but it decomposes the earth, chat is knowo under the denomination of terra ponderosa acetata. The precipitare is not soluble in water. It precipitates in a blue colour the Plannum falitum, and in a while, the following metallic solutions, viz. the ferrum, zincum, cuprum, vitriolatum; argentum, mercur: plumbum, nitratum ; and plumbum falitum ; but it does not produce any change in the corrosive sublimate, or in the solucioa of gold.

To ascertain still farther the peculiar nature of the acid of tung flen, our Author observes, that when it is calcined in a çrucible, it is no longer soluble in water; that it has the property of attracting the phlogiston, as appears from the blue colour which is receives from vitrifying fluxes; that it precipitates, in green, the solution of liver of sulphur, and, in white, the solution of phlogisticated alkali ; that it assumes a beautiful blue colour when polished iron, zinc, or tio are placed in a solution of it by water, and when some drops of the spirit of salt are mixed with this solution. Ocher 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, some may be led by this circunstance to identify this acid with the acid of tung fien. But our Academician proves, by several facts and experiments, that, notwithftanding this single point of resemblance, these two acids have many different properties, which distinguish them palpably from each other.

This memoir is followed by an Appendix added to it by the Jate Sir TorBERN BERGMAN, which contains several observa. tions on the tungsten.

Mem. VI. Experiments on the Elasticity and Distribution of 'Heat, considered with refoect to the Ajcent and Refrigeration of Vác pours in rarefied Air. By M. J. C. WILCKE.' The experie ments of Ouo Gueric, on the ascent and subsequent descent of vapours in the air-pump, is well known. It was therce concluded that air, when rarefied, is no longer capable of supporting, and therefore lets tall (as specifcally heavier) all those heterogeneous substances which before were suspended, and, as it were, diffolved in it. But, while this principle was employed to explain the descent of the mercury in the barometer, the falling of rain, and other phenomena of our atmosphere, the moft remarkable circumstances of ine experiment of Otro Gueric were by no means explained, says our Academician, in a satisfactory manner. These 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 occafioned their ascent; and that the cause of these phenomena is the expan"Fron and distribution of heat. Our Academician, having repeated and diversified this experiment, found that the result was the fame which preceding philosophers 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 same 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 piston ; and when, by exhausting and then letting in the air at different times, or by any other operation, some bumid particles were introduced into the receiver, then, vapours were observed ascending in the receiver, and that in proportion to the moisture or dryness, the coldness or warmth of the air. From these experiments our Academician drew the following conclu. fions ; that it is necessary to introduce some humid subfiance into the recipient, in order to render the vapours which it contains, visible; that these vapours rise, in effect, from humid surfaces placed under the recipient; that they are expanded in the rarefied air before they redescend in the form of clouds and drizzling rain ; and that thus the ascent of these vapours, and their subsequent descent, must be considered as two distinct effects, intimately connected with the rarefaction of the air.


After having discussed and refuted the explications that have been given of this phenomenon by several 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, says he, that heat and cold have some particular and immediate affinity with the ascent of vapours in the air. pump; because the greatest part of these phenomena depend more especially on the degree of absolute and relative heat, which exists 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 Marp cold. It is also easy to perceive how differently they are affected by a warm or by a cold receiver. The former prevents the free expansion of the vapours; the latter promotes it. The former remains clear and pure; the latter, as soon as the air is introduced, is obscured on all sides with vapour and moisture. Our Academician thinks that hence, without any farcher researches, it is natural to conclude, that the passage and distribution of heat among bodies placed under the recipient and in rarefied air, must be considered as the true cause of the ascent, the modl:fication, and the descent of vapours. He does not, however, reft


his proofs here ; for, says he, in order to complete my own conviction and that of others, and to unsold more particularly the mechanism of these effects, I made the following experiments: 1

1. Two thermometers were nicely constructed, exa&tly cora responding with each other. The one was suspended 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 sufficient 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 inside thermometer had fallen iwo degrees, but that it rose again when the air was introduced anew. This effect cealed when the cube of the thermometer was opened; which proves that it was owing to the expansion of the ball of the there mometer,' and the pressure of the external air. The temperature of the chamber was afterwards changed, and it was then observed that the two thermometers rose and fell exactly together; and this evinces the equal and correspondent distribution of heat in the external dense air, and in the internal rarefied air. This takes place as long as the ball of the interior therinometer continues dry; but as soon as it contracts the smallest degree of moisture, the correspondence is interrupted, and the variations are remarkable.

2. If the ball of the interior thermometer be immersed in a vessel filled with water, and the air be pumped out, it remains at the same point during the whole of this operation ; but it falls several degrees the moment that it is taken out of the water, and does not rise again to its former height till the ball becomes dry, and all the moisture has evaporated.

3. To preserve the moisture the longet, and in a greater abundance about the ball, it was surrounded with a piece of fine Jinen, thoroughly wet; and, as soon as the piston began to work, the liquor fell five or six 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 rose again when the moisture had 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 arise from the vessel full of water, form a pale pable obstacle to the fall of the thermometer, which always de. scends some degrees lower under a dry receiver, where there are no other vapours but those which arise 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 these experiments made with water, that the rarefaction of the air is favourable to evaporation, and, at the same time, to the refrigeration and descent of the thermometer, our APP. Rev. Vol. LXXV. Νη


[ocr errors]

b fpirit bermometer and evene 78th degring, watlicy, by

[ocr errors]

Academician employed liquors of a more volatile nature, and more susceptible of evaporation.-4. The ball of the thermometer, being surrounded with a piece of fine linen, was moistened with spirit of wine, highly rectified, and the consequence was, that the thermometer tell from the 17th degree above the freezing point to the 8ch, and even to the 12th below it: with vitriolic ether it descended 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 vessel suspended 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 ir vacuo, juft as warm water cools sooner in vacuo than in the open air.

As in the preceding experiments a great quantity of visible vapours are emitted from the ball of the thermometer, which is fuspended in rarefied air, it is evident that these vapours arise from humid surfaces, and carry with them the heat of the bodies of the surfaces from which they proceed; and that thus the affumption and the passage of the heat from the mass of bodies in rarefied air, muft be considered as the immediate and true cause of the eruption and ascent of the vapours. The following experiments will enable us to form a still clearer idea of the beat itself, and of the mechanism of its effects in the air-pump.

6. A round plate of polished copper of the same diameter with the receiver, and supported by a glass foot in a horizon. tal situation, 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 easy to perceive diftinaly, when the plate was heated, that the vapours kept themselves con. fancly at a certain diftance from the metal, which was surrounded with a clear diaphanous space. Over this the vapours were fuspended, fell and vanilhed without arriving at the surface of the plate, which was afterwards found to be as dry and unsullied as it had been before the experiment. On the contrary, when she plate was colder than the receiver, no trace of such an atmosphere was to be seen ; and the vapours moving in all directions on the cold surface of the metal, covered it with a kind of dew.

After the relation of the experiments now mentioned, and of others of a funilar kind made for the same purpose, our Academician offers some judicious observations on the acalogy be. tween these phenomena and those of electricity. His conclusion is, that they muh both be explained by the same theory; and he deduces from them the following propofitions :

• The scale of the Swedish thermometer makes the freezing point @, and boiling water 19o.


241 · Heat is a most subtile and expansible matter, whtse parts repel the term each other reciprocally. a robot This matter is also most powerfully attracted by that of other nuke bodies : hence the reason why it not only penetrates and fills -pet their pores, surrounds their surfaces, and dilates them by its

elasticity and abundance, but also separates, and, under the name are out of evaporation, carries off with it the smallett parts of bodies,

Of these it forms solutions or elastic vapoars, whose kind is deFacial termined by the nature of the matter of which these bodies are E' composed, and whose degree of elasticity depends upon the 1.0 quantity of the repulsive heat.

Different kinds of matter attract the heat with different degrees of But force, according to the nature of each matter. Thus, in the ;" preceding experiments, the heat is attracted the most powerfully of uit by the air, less by water, still less by glass, and least of all by

Full the mercury of the thermometer. Dout & The same kind of body or matter receives and retains, according to its of the different states and modifications, a different quantity of heat. This bere we see clearly when a sufficient quantity of heat transforms the boof the dies into solutions or elastic vapours, or when they are under do the pressure of an exterior force : in the first of these two cases,

o their parts are surrounded with the heat that is necessary to reof thrones parate them from each other, and surmount their mutual attrace

tion; in the second, they can neither receive nor retain all the are not heat, which, in a state of full liberty, would get the better of

their attraction. Thus a warm air, when strongly compressed, discharges, like a spunge, the heat which it contains *, but refumes it when it becomes more free, and can dilate itself. In the same manner the elastic heat expands itself in that direction where it finds the least resistance.

Hence it follows, that as soon as the quantity and prerCosure of the air within the receiver are diminished by the effect of

the pump, the equilibrium of the heat is disturbed, the particles clei 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 surrounding bodies,

and comes principally from those which have it in excels, or non which attract and retain it with the least force : by its peculiar

elafticity it directs iis course towards that quarter where the equilibrium ceases, and the resistance is diminished at the fame

time; and, when the nature of the body admits of this, it carceries off from it the more subtile exterior parts ; and, in this se

paration, these parts attract, and are surrounded b;', a greater
portion of heat, which is disengaged from the body whence
they have been separated, and which is conliderably cooled by
* Boerhaave Elem. Chem. P. II. p. 480.
• Nn 2



[ocr errors]
[ocr errors]
« AnteriorContinuar »