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In the third chapter, these principles are applied to the mea. suring of local humidity. Of all the hygroscopic substances which Mr. De Luc has tried for this purpose, that which answers the best is a flip of whalebone, cut transversely to the direction of the fibres, and made extremely thin, for on this depends its sensibility. A lip of 12 inches in length, and a line in breadth, he has made so thin as to weigh only half a grain ; it may be made ftill thinner, but is then of too great sensibility, being affected even by the approach of the observer. The flip is kept extended by a small spring, and the variations in its length are measured by a vernier division, or by an index on a dial-plate: the whole variation, from extreme dryness to extreme moisture, is about į of its length.

Mr. De Luc, in his former publications, had graduated his hygrometers (then made of ivory) from one fixed point only, that of extreme moisture, which is obtained by soaking them in water. He has now very ingeniously contrived io fix the other extreme, that of dryness; which being producible only by means of Atrong fire, such as hygrometers cannot support, he uses an intermediate body, quicklime; which, after having been deprived, by force of fire, of all its own humidity, has the property of flowly imbibing humidity again from the bodies in its neighbourhood; and whose capacity is fuch, that all the vapour which can be contained in a quantity of air equal to its own bulk, can give it no sensible humidity: the hygrometers, inclosed with a large quantity of fresh-buint lime in lumps, acquire in three weeks the fame degree of dryness with the lime, which cannot differ fenfibly from extreme dryness.

M. De Sauffure, in his ingenious Efais sur l'hygrometrie, makes choice of haiis, prepared by maceration in alcaline lye. Mr. De Luc Thews that hairs, and all the other animal or ve. getable hygroscopic substances taken lengthwise, or in the die rection of their fibres, undergo contrary changes from different variations of humidity; that when immersed in water, they lengthen at first, and afterwards shorten ; that when they are near the greatest degree of humidity, if the moisture is increased, they Thorien themselves; if it is diminished, they lengthen themselves first before they contract again. These irregularities, which obviously render them incapable of being true measurers of humidity, he news to be the necessary consequence of their organic reticular structure.

De Saussure takes his point of extreme moisture from the vapours of water under a glass bell, keeping the sides of the bell continually moiftened ; and affirms, that the humidity is, there, conftantly the same in all temperatures; the vapours even of boiling water having no more effe&t than those of cold. Mr. De Luc has thewn, on the contrary, that the differences in

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humidity under the bell are very great, though De Saussure's hygrometer was incapable of discovering them; and that the real, undecomposed, vapour of boiling water has the dire&ly opposite effect to that of cold, che effet of extreme dryness; and on this point he mentions an interesting fact, communicated to him by Mr. Watt, viz. that wood cannot be employed, in the fteam-engine, for any of those parts where the vapour of the boiling water is confined, because it dries so as to crack, as if exposed to the fire. Our Author mentions some striking in. ftances, in which the imperfection of De Saussure's hygrometer has led him into false conclusions respecting phenomena, and into erroneous theories to account for them.

This inquiry into the constitution and properties of watery vapour makes the first part of the work. In Part II. vapours are considered as one class of expansible fuids; and the other subItances belonging to that class are examined separately, on the principles which that inquiry has pointed out. The first chapter ascertains the distinctive character of vapours relatively to the aeriform Auids.

Of all the expansible Auids immediately cognisable by us, LIGHT is probably the only one that is really elementary: all the others are compounds, and on their inceffant decomposition, and recomposition, most of the physical phenomena depend. Light is the only one also that is not atmospheric, or has no settlement in the atmosphere, its motion being so rapid, that its gravitation to the earth bears no sensible proportion to its velocity. When combined, by affinity, with other substances, its motion is not destroyed, but only retarded, and changed from a rectilinear course to various curves. In this latent state, it is an ingredient in moft bodies: the atmospheric Auids owe to it, mediately or immediately, their expanfibility ; immediately, when the light, as such, enters their compofition, in which cale they cannot be decomposed without appearing luminous, that is, emitting the light in its own form ; -mediately, when one of these first compounds enters as an ingredient in others, in which case decomposition may take place without any luminous appearance, by the separation of the first compound entire.

All the atmospheric fluids being compounds, and their expanfibility owing to one ingredient, the Author calls that ingredient the deferent fluid, and the others mere gravitating lubftances : thus, in watery vapours, fire is the deferent Auid, and water the gravitating substance.

All the atmospheric Auids belong to the two classes, vapours and acriform; the distinctive characters of which are these. Vapours are decomposed by preffure, as already explained; but the aeriform fluids bear the strongest compression without decomposition. Vapours are decomposed, in hermetically sealed vessels, by the

spontaneous fpontaneous escape of their deferent Auid, on an abatement of the external heat; but aeriform fluids can be decomposed only by means of some substance, to which their gravitating matter has a greater affinity than to its own deferent fluid. In vapours, the proportions of the component parts are extremely variable, according to fubfifting circunstances; but aeriform fluids, when once formed, continue in the same ftale; and they can only be changed by chemical causes. All these properties of vapours depend upon one principle, the weak union of the gravitating matter with the deferent Auid: whence the former can separate itself from the latter, by the mutual tendency of its own particles, when they are brought within a certain distance of one another; the deferent fluid also can quit the gravitating matter, to eftablih certain 'equilibria respecting itself; and the expansive force is greater or less, according to the quantity of the deferent Auid.

In the second chapter, the Author thews, that FIRE, the deferent Auid in watery vapour, is itself a compound, poffeffiog the same distinctive properties, and therefore belonging to the same class. it is composed of light, which ferves for its deferent Auid, and of a substance merely gravitating: this lait may be disengaged from the light by compreson; the light separates from it to restore certain equilibria; and the expansive force is greater as the quantity of light is more abundant,

The substance which, united with light, composes fire, is · called by the Author the matter of fire. It is not indeed known

in its separate state; but that is the case with so mnany other substances admitted by philosophers, that no objection can thence be made to its existence. The Author thews, in some detail, that almost all the substances immediately perceptible by us are only mixtures, whose component parts are known but by the modifications they produce in other bodies.

Fire is one of the most simple compounds of light; and it is in this compound state that light enters the composition of most other bodies : its luminous quality is supprefied by the gravitating matter in the fire; but a new one is produced, -that of heat.

Fire, like watery vapour, has a maximum of density, which it cannot exceed without partial decomposition. This maximum is incandescence; in which state, we may presume, that the particles of fire are brought so near to one another, that those of the gravitating matter unite together, and quit the light. It is by this maximum of density that the heat of our furnaces is limited, as the mechanic action of watery vapours is in every given temperature: when the decomposition extends to all the classes of the particles of light, so as to produce perfect whiteness, the fire is at its highest force.

By heat, in our furnaces, is meant the simple a&tion of fire; of which the principal effects are, expansion,-ihe transformation

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of solids into fluids, or fimple fusion,-and vaporisation. Simple fusion, produced by fire only, must be diftinguilhed from that which is aided by affi nities: in the former, the body resumes its original appearance on loling its excess of fire ; whereas, in the latter, in vitrifications for example, a new form is produced. The iame distinction obtains also in vaporisation.

That large burning-glafles should produce greater effects than our furnaces, is not owing to a greater density of fire produced by them, but to affinities: the substances fused by them are evidences of this. There are grounds to believe, that the sun's rays are not calorific of themselves, but fimply luminous; and that they produce heat, by increasing the expansive force of the fire already fubfisting-by forming new fire when they meet with the other ingredient-and, in some cases, by disengaging the fire that is combined in the composition of other bodies. The other ingredient of fire is probably diffeminated through the atmosphere, differently in different places and seasons, and over different foils, and chiefly through the lower Itrata. A decomposition of air ico self, or of some particular species of air, and of the body exposed to the burning-glass, in virtue of the affinities of light, may supply to that element the material which it wants for forming fire.

After some curious details on these objects, of which the Author promises a further investigation in bis future work, he proceeds (lect. 3. of this chapter) to the phenomena of heat, confi. dered, not as a cause, but as the effect of free fire in other bodies, or the actual degree of its expansive force ; for it is on this force, not on its density, that the mechanical effe&ts of fire depend; the same quantity, in different bodies, having different degrees of expansive force, and producing different degrees of heat. In this consists the phenomenon, lately discovered, of the different capacities of bodies for fire, which the Author had predicted, from his theory, in his former work, and expressed in the same terms as he does now after the event; and this, he says, has been the case in several other instances. The expansive force of fire depends upon the velocity of its motion, as well as its quantity; and the more free space a body contains for the particles of fire to move in, the greater will the velocity be.

The Author supposes, with M. Le Sage, that the particles of fire have a rotatory and progressive motion on different axes, so as to describe (pirals. These little spirals are extremely close ; and bence the flowness of the propagation of fire, even in the air, though its prodigious power of dilating bodies evinces a great expanfive force. On the same hypothefis, be accounts mechanically for the rectilinear motion of light being changed into this spiral one by the union of the matter of fire with it, and for the different affinities which the (wo Auids are observed to poflefs.

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The particles of light, for instance, though infinitely more subtile than those of fire, do not permeate all bodies : in opaque ones, all that enters is retained by affinity ; bodies to which light has no particular affinity, it traverses with the rapidity proper to it. Fire, on the contrary, is propagated slowly, but permeates almost all: the only ones which it does not pass through are, ice ready to melt, and solid bodies on the point of fimple fufion. Light pafles through ice in all its states, but fire only when its temperature is below freezing: as soon as it is disposed to melt, it becomes for fire what black bodies are for light; all the fire introduced remains in it, employed in converting it into water, while the light, emitted from the decomposition of a part of that fire, pafles through.

The next fection treats of the phenomena of heat which accompany combustion. Fire is an essential ingredient in all combustible bodies, and to its disengagement from them is owing great part of the heat produced in that operation. It enters the composition also of moft solid and liquid bodies, and expanfible Auids: one of these Auids, called dephlogisticated or pure air, is always concerned in combustion ; and when that air is deftroyed or decompounded in the process, a great additional heat is produced : thus, in the combustion of phosphorus and charcoal, in the experiments of De la Place and Lavoisier, the heats produced were in the proportion of 7 to 3; the pure air having been decompounded by the former, and furnished the greatest part of the fire, while all the fire, in the latter case, proceeded from the charcoal alone.

In the destruction of pure air, the combuftible body produces first inflammable air: when the pure air is not destroyed, its of fice is only to receive the gravitating substance of inflammable air ; by which means the fire belonging to this last air is extricated, and the pure air, by its union with that gravitating subftance (phlogiston), becomes fixed air.

When inÃammable air is formed, and comes in contact with the pure air of the atmosphere, a certain degree of heat is necessary for producing combustion. This heat appears to be about 6500 of Fahrenheit's thermometer : olive oil, heated to that degree, boiled violently, and its surface became covered with Hame: inflammable air was emitted in a much lower heat, but no fame was produced ; and cold oil, poured into the flaming oil, while it sunk the thermometer, quenched the fame wherever it touched. The Author points out some further experiments on this subject, particularly respecting Spontaneous accenfions ; of which he mentions one that, we believe, bas not been much noticed by philosophers, over the tall furnaces in which metals are run down from their ores: the inflammable air, perhaps mixed with fixed air, but transparent, chars wood in the top of the

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