In the third chapter, these principles are applied to the meaJuring of local humidity. Of all the hygroscopic fubftances which Mr. De Luc has tried for this purpose, that which answers the best is a flip of whalebone, cut tranfverfely to the direction of the fibres, and made extremely thin, for on this depends its fenfibility. A flip of 12 inches in length, and a line in breadth, he has made fo thin as to weigh only half a grain; it may be made ftill thinner, but is then of too great fenfibility, being affected even by the approach of the obferver. The flip is kept extended by a fmall fpring, and the variations in its length are measured by a vernier divifion, or by an index on a dial-plate the whole variation, from extreme drynefs to extreme moisture, is about 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 foaking them in water. He has now very ingeniously contrived to fix the other extreme, that of drynefs; which being producible only by means of ftrong fire, fuch as hygrometers cannot fupport, he ufes 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 whole 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 fenfible humidity: the hygrometers, inclofed with a large quantity of fresh-buint lime in lumps, acquire in three weeks the fame degree of drynefs with the lime, which cannot differ fenfibly from extreme dryness.

M. De Sauffure, in his ingenious Effais fur l'hygrometrie, makes choice of hairs, prepared by maceration in alcaline lye. Mr. De Luc fhews that hairs, and all the other animal or vegetable hygroscopic fubftances taken lengthwife, or in the direction of their fibres, undergo contrary changes from different variations of humidity; that when immerfed in water, they lengthen at first, and afterwards fhorten; that when they are near the greatest degree of humidity, if the moisture is increased, they fhorten themselves; if it is diminished, they lengthen themselves firft before they contract again. Thefe irregularities, which obviously render them incapable of being true measurers of humidity, he fhews to be the neceffary confequence of their organic reticular structure.

De Sauffure takes his point of extreme moisture from the vapours of water under a glafs bell, keeping the fides of the bell continually moiftened; and affirms, that the humidity is, there, conftantly the fame in all temperatures; the vapours even of boiling water having no more effect than thofe of cold. Mr. De Luc has fhewn, on the contrary, that the differences in

humidity

humidity under the bell are very great, though De Sauffure's hygrometer was incapable of difcovering them; and that the real, undecompofed, vapour of boiling water has the directly oppofite effect to that of cold, the effect of extreme dryness; and on this point he mentions an interefting 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 fo as to crack, as if exposed to the fire. Our Author mentions fome ftriking inftances, in which the imperfection of De Sauffure's hygrometer has led him into falfe conclufions refpecting phenomena, and into erroneous theories to account for them.

This inquiry into the conftitution and properties of watery vapour makes the firft Part of the work. In Part II. vapours are confidered as one class of expanfible fluids; and the other fubstances belonging to that clafs are examined feparately, on the principles which that inquiry has pointed out. The firft chapter afcertains the diftinctive character of vapours relatively to the aeriform fluids.

Of all the expanfible fluids immediately cognifable by us, LIGHT is probably the only one that is really elementary: all the others are compounds, and on their inceffant decompofition, and recompofition, moft of the phyfical phenomena depend. Light is the only one alfo that is not atmospheric, or has no fettlement in the atmosphere, its motion being fo rapid, that its gravitation to the earth bears no fenfible proportion to its velocity. When combined, by affinity, with other fubftances, its motion is not deftroyed, but only retarded, and changed from a rectilinear courfe to various curves. In this latent ftate, it is an ingredient in moft bodies: the atmospheric fluids owe to it, mediately or immediately, their expanfibility; immediately, when the light, as fuch, enters their compofition, in which cafe they cannot be decompofed without appearing luminous, that is, emitting the light in its own form;-mediately, when one of thefe first compounds enters as an ingredient in others, in which cafe decompofition may take place without any luminous appearance, by the feparation 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 fubftances: thus, in watery vapours, fire is the deferent fluid, and water the gravitating fubftance.

All the atmospheric fluids belong to the two claffes, vapours and aeriform; the diftinctive characters of which are these. Vapours are decompofed by preffure, as already explained; but the aeriform fluids bear the ftrongest compreffion without decompofition. Vapours are decompofed, in hermetically fealed veffels, by the fpontaneous

7*

fpontaneous escape of their deferent fluid, on an abatement of the external heat; but aeriform fluids can be decompofed only by means of fome fubftance, to which their gravitating matter has a greater affinity than to its own deferent Auid. In vapours, the proportions of the component parts are extremely variable, according to fubfifting circunftances; but aeriform fluids, when once formed, continue in the fame ftate; and they can only be changed by chemical caufes. All thefe properties of vapours depend upon one principle, the weak union of the gravitating matter with the deferent fluid: whence the former can feparate itself from the latter, by the mutual tendency of its own particles, when they are brought within a certain diftance of one another; the deferent fluid alfo can quit the gravitating matter, to establish certain equilibria refpecting itself; and the expanfive force is greater or lefs, according to the quantity of the deferent fluid.

In the fecond chapter, the Author fhews, that FIRE, the deferent fluid in watery vapour, is itfelf a compound, poffeffing the fame diftinctive properties, and therefore belonging to the fame class. It is compofed of light, which ferves for its deferent fluid, and of a fubftance merely gravitating: this laft may be difengaged from the light by compreffion; the light feparates from it to reftore certain equilibria; and the expanfive force is greater as the quantity of light is more abundant.

The fubftance which, united with light, compofes fire, is called by the Author the matter of fire. It is not indeed known in its feparate ftate; but that is the cafe with fo many other fubftances admitted by philofophers, that no objection can thence be made to its exiftence. The Author fhews, in fome detail, that almost all the fubftances immediately perceptible by us are only mixtures, whofe component parts are known but by the modifications they produce in other bodies.

Fire is one of the moft fimple compounds of light; and it is in this compound ftate that light enters the compofition of most other bodies: its luminous quality is fupprefled by the gravitating matter in the fire; but a new one is produced,—that of heat.

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

By heat, in our furnaces, is meant the fimple action of fire.; of which the principal effects are, expansion,-the transformation

of

of folids into fluids, or fimple fufion,-and vaporisation. Simple fufion, produced by fire only, must be diftinguished from that which is aided by affinities: in the former, the body refumes its original appearance on lofing its excefs of fire; whereas, in the latter, in vitrifications for example, a new form is produced. The fame diftinction obtains alfo in vaporifation.

That large burning-glaffes fhould produce greater effects than our furnaces, is not owing to a greater denfity of fire produced by them, but to affinities: the fubftances fufed by them are evidences of this. There are grounds to believe, that the fun's rays are not calorific of themfelves, but fimply luminous; and that they produce heat, by increafing the expanfive force of the fire already fubfifting-by forming new fire when they meet with the other ingredient-and, in fome cafes, by difengaging the fire that is combined in the compofition 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 ftrata. A decompofition of air itfelf, or of fome particular (pecies of air, and of the body expofed to the burning-glafs, in virtue of the affinities of light, may fupply to that element the material which it wants for forming fire.

After fome curious details on thefe objects, of which the Author promises a further investigation in his future work, he proceeds (fect. 3. of this chapter) to the phenomena of heat, confidered, not as a caufe, but as the effect of free fire in other bodies, or the actual degree of its expanfive force; for it is on this force, not on its denfity, that the mechanical effects of fire depend; the fame quantity, in different bodies, having different degrees of expanfive force, and producing different degrees of heat. In this confifts the phenomenon, lately difcovered, of the different capacities of bodies for fire, which the Author had predicted, from his theory, in his former work, and expreffed in the fame terms as he does now after the event; and this, he fays, has been the case in feveral other inftances. The expanfive 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 fuppofes, with M. Le Sage, that the particles of fire have a rotatory and progreffive motion on different axes, fo as to defcribe fpirals. These little fpirals are extremely close; and hence the flownels of the propagation of fire, even in the air, though its prodigious power of dilating bodies evinces a great expanfive force. On the fame hypothefis, he accounts mechanically for the rectilinear motion of light being changed into this fpiral one by the union of the matter of fire with it, and for the different affinities which the two fluids are obferved to poffefs.

The

The particles of light, for inftance, though infinitely more fubtile than thofe 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 flowly, but permeates almost all: the only ones which it does not pass through are, ice ready to melt, and folid bodies on the point of fimple fufion. Light paffes through ice in all its ftates, but fire only when its temperature is below freezing: as foon 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 decompofition of a part of that fire, paffes through.

The next fection treats of the phenomena of heat which accompany combuftion. Fire is an effential ingredient in all combustible bodies, and to its difengagement from them is owing great part of the heat produced in that operation. It enters the compofition also of most solid and liquid bodies, and expanfible fluids one of these fluids, called dephlogifticated or pure air, is always concerned in combuftion; and when that air is deftroyed or decompounded in the procefs, a great additional heat is produced: thus, in the combuftion of phofphorus and charcoal, in the experiments of De la Place and Lavoifier, 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 cafe, proceeded from the charcoal alone.

In the deftruction of pure air, the combuftible body produces firft inflammable air: when the pure air is not deftroyed, its of fice is only to receive the gravitating fubftance of inflammable air; by which means the fire belonging to this laft air is extricated, and the pure air, by its union with that gravitating subftance (phlogifton), becomes fixed air.

When inflammable air is formed, and comes in contact with the pure air of the atmosphere, a certain degree of heat is neceffary for producing combuftion. This heat appears to be about 650° of Fahrenheit's thermometer: olive oil, heated to that degree, boiled violently, and its furface became covered with Aame: inflammable air was emitted in a much lower heat, but no flame was produced; and cold oil, poured into the flaming oil, while it funk the thermometer, quenched the flame wherever it touched. The Author points out fome further experiments on this fubject, particularly refpecting Spontaneous accenfions; of which he mentions one that, we believe, has not been much noticed by philofophers, over the tall furnaces in which metals are run down from their ores: the inflammable air, perhaps mixed with fixed air, but tranfparent, chars wood in the top of the

furnace