« ÎnapoiContinuați »
up to a
rounded by a small vessel of water, the ether gined by some, that the artificial elastic fluids boils violently, and is dissipated in vapor, while have not the same mechanical property with the water freezes, and is cooled to a great degree. common air, viz. that of occupying a space inThe dissipation of this vapor shows that it has versely proportional to the weights with which an elastic force; and the absorption of the they are pressed : but this is found to be a misheat from the water shows, that this element take. All of them likewise have been found to not only produces the elasticity, but actually be non-conductors of electricity, though probaenters into the substance of the vapor itself; so bly not all in the same degree. See Air and that we have not the least reason to conclude ELECTRICITY. that there is any other repulsive power by which ELASTICITY. The cause or principle of elasthe particles are kept at a distance from one ano- ticity, or springiness, is variously assigned. The ther, than the substance of the heat itself. In Cartesians account for it from the materia subwhat manner it acts, we cannot pretend exactly tilis making an effort to pass through pores that to explain, without making hypotheses con
narrow for it.
Other philosophers, cerning the form of the minute particles of mat- in lieu of the subtile matter, substitute ether, ter, which must always be very uncertain. All or a fine etherial medium that pervades all known phenomena, however, concur in rendering bodies. Others, setting aside the precarious the theory now laid down extremely probable. notion of a materia subtilis, account for elasThe elasticity of the steam of water is exactly ticity from the great law of attraction, or the proportioned to the degree of heat which flows cause of the cohesion of the parts of solid into it from without; and, if this be kept and firm bodies. Thus, say they, when a hard
ient degree, there is no mechanical body is struck or bent, so that the component pressure which can reduce it into the state of parts are moved a little from each other, but not water. This, however, may very easily be done quite disjointed or broken off, or separated so far by abstracting a certain portion of the latent heat as to be out of the power of that attracting force it contains : when the elastic vapor will become a whereby they cohere; they must certainly, on the dense and heavy fluid. The same thing may be cessation of the external violence, spring back to done in various ways with the permanently elastic their former natural state. Elasticity has also been fluids. Thus the purest dephlogisticated air, resolved into the pressure of the atmosphere : for when made to part with its latent heat, by a violent tension, or compression, though not so burning with iron, is converted into a gravi- great as to separate the constituent particles of tating substance of an unknown nature, which bodies far enough to let in any foreign matter, adheres strongly to the metal. If the decompn- must yet occasion many little vacuola between sition is performed by inflammable air, both to the separated surfaces; so that upon the removal gether unite into a heavy, aqueous, or acid Aluid; of the force they will close again by the pressure if by mixture with nitrous air, still the heat is of the aerial Auid upon the external parts. discernible, though less violent than in the two See ATMOSPHERE. Lastly, others attribute the former cases. The decomposition indeed is slower, elasticity of all hard bodies to the power of resibut equally complete, and the dephlogisti- lition in the air included within them: and so cated air becomes part of the nitrous acid, from make the elastic force of the air the principle of which it may be again expelled by proper means: elasticity in all other bodies. These are clearly but of these means heat must always be one ; for the mere conjectures of philosophy. thus only the elasticity can be restored, and the M. Libes, the author of the Nouveau Dictionair be recovered in its proper state. The same naire de Physique, has in that work given a new thing takes place in fixed air, and all other per- explication of the phenomena of elasticity, which manently elastic fluids capable of being absorbed depends upon the following principles :by others. The conclusion therefore, which we
i. The signs of elasticity suppose a comprescan only draw from what data we have, concern- sion effected, that is, an alteration in the figure, ing the composition of elastic vapors, is, that all of todies produced by the mutual approach of of them are formed of a terrestrial substance, the moleculæ : whence it results, that bodies, united with the element of heat in such a man- whose moleculæ yield with a very great facility per, that part of the latter may be squeezed out to the slightest pressure, so as to roll one over from among the terrestrial particles; but in such another without changing their figure, cannot give a manner, that, as soon as the pressure is taken sensible signs of elasticity. Such in general are off, the surrounding fluid rushes in, and expands liquids. 2. When an elastic body is compressed, them to their original bulk: and this expansion some of its integrant moleculæ are brought nearer or tendency to it will be increased in proportion to one another, while others undergo a farther to the degree of heat, just as the expansion of a separation nearly equal to the approach of the sponge would be exceedingly augmented, if we former. 3. At the haoitual degree of heat and could contrive to convey a stream of water into pressure which we experience, all bodies bave a the heart of it, and make the liquid flow out with volume determined by the ratio of equality, which violence through every pore in the circumference. exists between the aitractive force of their partiIn this case, it is evident that the water would act cles, and the repulsive force communicated by as a power of repulsion among the particles of the caloric combined with those particles. the sponge, as well as the fire does among the These being granted, the re-establishment of particles of the water, charcoal, or whatever other solid bodies, after the compression, appears to substance is employed. Thus far we may reason be the result of the combined action of the caloric from analogy, but in all probability the inter- and of attraction. For in the moleculæ brought nal and essential texture of these vapors will nearer by the compression, the repulsive force for ever remain unknown. It has been ima- augments, and the attractive force likewise augments; but the augmentation of the former force fore surprising, that bodies in which we cannot exceeds that of the latter. For, at the epoch of produce compression should not give any sign of the formation of the body, such as it existed pre- elasticity. 3dly. Although caloric be the princivious to the compression, the repulsive force com- ple of elasticity, it does not follow that all bodies municated to its moleculæ by the caloric, was
which contain caloric must exhibit this property. sufficient to give the degree of separation that 1. Too much or too little caloric may equally was peculiar to the body: it was, therefore, su- weaken the elastic force. The different forms perior to the attractive force until the moment which distinguish the integrant moleculæ of difwhen the moleculæ had become so far separated ferent bodies; the different arrangement assumed as was consistent with the natural state of the by those moleculæ, according to circumstances, body. Whence it results, that if the particles are may be sometimes more or less favorable, at brought nearer together by compression, that is, others more or less prejudicial to elasticity. 2. if they are contracted with the caloric into a Soft bodies, such as butter, humid clay, &c., exsmaller space, the ratio of equality which sub- perience in their soft state a commencement of sisted between the attraction and the repulsion solution by water, which must alter the repulsive before the compression, must be destroyed in force of their moleculæ, and consequently hurt favor of the repulsion; and consequently, on the the elasticity. This is so true, that these bodies, cessation of the compression, this surplus of re- freed from their aqueous parts, without changing pulsive force will act so as to separate again the their temperature, yield sensible signs of elasticity. moleculæ brought nearer by the compression, un
M. Libes, instead of attempting to explain, as til the equilibrium is re-established between the some would expect, why caloric is elastic, says it attraction and the repulsion : and this equilibrium is not necessary to suppose it so. For it may can only be re-established, when the molecules communicate this property to other bodies withhave recovered the degree of separation which out being so itself
. This position he illustrates they had previous to the compression. For as follows: when dry bread is immersed in water, similar reasons the attraction will predominate that bread becomes swoln, its particles being over the repulsion in those particles which have farther separated from one another: water, theresuffered a farther separation than is due to the fore, by penetrating the pores of the bread, comnatural state: it must, therefore, act to draw the municates to its particles a repulsive force; yet particles nearer, and re-establish the equilibrium it would be ridiculous to conclude from hence, of those forces : and this equilibrium cannot be that the moleculæ of water mutually repel one re-established until the molecules have recovered another. In like manner, when we subject a the relative distance which they had before the body to the action of heat, its integrant molecules compression.
are separated from one another, and acquire a This theory M. Libes applies to an elastic repulsive force by their combination with caloric: sphere, as an ivory ball when falling upon a but this phenomenon, like the preceding, depends plane, to a plate of steel, whose extremities are probably upon the concourse of several attracbrought towards each other by bending, and to tive forces, such as that of the moleculæ of the the known effects of tempered metals, &c. In caloric, that of the particles of the body for one explaining the elasticity of aëriform fluids, M. another, and, lastly, the reciprocal attraction of Libes calls in to his aid a new force. For, in this the particles of the caloric and those of the body kind of substances, the repulsion having prevailed penetrated by that fluid: whence it results that over the attraction, their particles are retained in the elasticity of bodies by no means presupposes their mutual position by the pressure of the at- that of the caloric which has given rise to it. mosphere. But this force, it may be observed, Indeed, M. Libes does not regard as completely being constant, makes no change in the results demonstrated the existence of the fluid called "ust stated; except that, if the pressure be taken caloric; he assumes the hypothesis as a matter away, the particles of the gas will be separated of convenience, which enables us to abridge the from one another, until their relative distance at- language of philosophical discussions, and to tains a point determined by the equality between found upon our reasonings an analytical calculus. the attraction of the earth and the repulsion of He next, therefore, proceeds to state his theory those particles. Now, since all bodies contain analytically; and deduces from his formula the caloric, it may be asked how it happens that all following results among others. 1. That when bodies are not elastic, if caloric be the principle we compress elastic fluids, the repulsive force of elasticity ?
becomes more powerful than the attractive, and To this M. Libes replies by the following re- consequently when the compression ceases, the marks :-1st. Since there is not in nature any moleculæ ought to return towards their first pobody, either perfectly hard, or perfectly soft, sition. 2. That liquids and aeriform fluids have there is none but what possesses some degree of the exclusive privilege of assuming a larger elasticity. 2dly. Perceptible signs of elasticity volume when the pressure of the atmosphere is suppose the compression effected : it is not there- taken away. Dict. de Phys. Retrospect, No. 8.
END OF VOL. VII.