leat. purpose as well as any others: but ftill we ought to remember, that thefe are inaccurate; and when we begin to argue from them as if they fully and exactly determined the mode in which the fluid acts, or rather is acted upon (for both these words fuppofe heat to be paffive, and not active), we must certainly err. As to the phrafes capacity for containing heat, abfolute heat, &c. they are ftill more inaccurate than the words abforption and attraction, and cannot convey any diftinét idea; whence the fyftems founded upon the explanations of these terms, affumed gratis dictum without the leaft proof, have never been able to fupport themfelves, but are liable to endlefs and infuperable objections.

It is by no means indeed eafy, nay we may boldly fay that it is abfolutely impoffible, for human genius to inveftigate all the phenomena of this fubtile and invifible element. All that can be done is, to discover a few general rules according to which the fluid acts in certain cafes. From thefe we can only reafon ana-. logically to cafes where its action is lefs obvious. But we are not to expect that by reasoning in this manner we can folve every phenomenon: nor can it be any recommendation to an hypothefis, merely that it folves Some phenomena, unlefs we were able by its means to folve them all; but this no wife man will pretend to do, nay, not even to know them all. It appears exceedingly erroneous therefore to invent folutions of certain phenomena, and then to argue for the truth of the hypothefis from the facility with which the phenomena are explained by it. The true and proper method of proceeding in this cafe is to lay down certain principles eftablished from the obvious phenomena of nature, and to reason from them fairly as far as we can; but where this ends, our knowledge must stop, and we cannot by any means proceed farther upon a fure foundation.

The only general principles as yet certainly eftablished from obvious phenomena upon this fubject are the following: 1. Heat and cold are found to expel one another Hence we ought to conclude, that heat and cold are both pofitives; for a negative can neither be expelled nor accumulated. 2. Heat is vifibly occafioned by the rays of the fun concentrated, and likewife by the fluid of electricity concentra ted. If fire, therefore, properly fo called, be the caufe of heat, than which nothing can be more evident to our fenfes, we are certainly intitled to conclude, that both the light of the fun and the electric fluid are clementary fire. Hence alfo we conclude their identity; for two different fubftances cannot by any means produce conftantly the fame effect when put in the fame circumftances, which both light and electricity do in this cafe, merely by concentration, or dif charging a great quantity of the fluid upon a fmall portion of any terreftrial body. 3. Heat expands bodies in every direction: whence we conclude, that the fluid, when producing heat, acts from a centre towards a circumference; and by analogy, that when it produces cold it acts from a circumference towards a centre. 4. It appears from undeniable experiments, that heat, fomehow or other, is the caufe of fluidity. As the action of the fluid has already been shown, when it produces heat, to be from a centre to a circumference, it follows, that when the expansive action of the fluid is confined within the furface of any body,

5

this may be called its latent heat; because it extends not beyond the surface, and therefore cannot affect the thermometer, or be known to us as heat by the fenfe of feeling. But when this expanfive action is transferred from the internal parts of the fubftance to the furface, it then affects the thermometer, and the body is faid to become botter at the fame time that it congeals or is faid to be frozen. This is what fome philofophers call the converfion of the latent into fenfible heat; others, the alteration of the capacity but whatever term we give to the effect, the caufe muft remain the fame, viz. the oppofite actions of the fame fluid; the expanfive power in fome cafes counteracting or overcoming the condenfing one, and vice versa. 5. Though fometimes the expanfive action is fufficiently ftrong to produce fluidity naturally, and in moft cafes may be made fo ftrong artificially as to make bodies fluid, yet in all cafes it is not fo. A certain degree of expanfive power exifts in all bodies whatever, and this by philofophers is called the jpecific heat of the body. 6. Whatever is called the cooling of any body is only the diminution of the expanfive action upon its furface, or, if we may ufe the expreffion, on the furface of its particles. This is accomplished by an oppofite power or modification of the fluid taking place on the outfide; but when this becomes fufficiently ftrong to penetrate the whole fabftance, it then expels part of the fluid acting in the oppofite direction, and then fome change takes place in the texture of the body. It is, however, impoffible to speak very perfpicuoufly upon this fubject, as the fubtility and indivifibility of the fluid render all reafonings upon it very precarious. 7. It is altogether impoffible to calculate the quantity of abfolute heat contained in any subftance, because this depends on the proportion betwixt the quantity of fluid acting expanfively and that acting. in the oppofite direction in the fame. Thefe two muft fome way or other counterbalance each other throughout the whole fyftem of nature; and we may fay with certainty, that any fubftance in which the one exifts without the other, is none of thofe fubject to the investigation of our fenfes, and all fpeculations concerning it must be vain. 8. When the fluid contained in any fubflance is vehemently agitated, this naturally produces an expanfion in it; and therefore bodies become hot by violent friction, percuffion, &c. In these cafes, however, we have no right to say that the fluid is expelled, but only that its mode of action is altered; for this is conftantly fufficient to produce heat, and in this indeed the very effence of heat confiits. 9. When the expanfive action of elementary fire within any fubftance becomes greater than is confiftent with the cohefion of that fubitance, it is diffipated or refolved into vapour. This, however, may be done in fuch a manner that the heat ftill acts upon the feparated parts of the body without fpending any of its force upon external fubftances. Hence vapour continues to exift in a temperature much below that in which it was originally produced; nay, will fometimes be exceffively cold to the touch, when it really contains as much heat, though in a latent ftate, as before. 10. When this latent heat is transferred to external bodies, the vapour then ceafes to be vapour, or is condenfed, and in fome cafes returns to its original fate; in others, it is productive of light and vehement fenfible heat:

whence

Making trial of the heat of a lamp, he found that Ha it also had a confiderable effect. The ball of one being blackened, and both fet at two inches distance from the flame of a lamp, they both rofe from 58 to 65% deg. and the thermometer which was blackened to 67+ Another time the uncoloured thermometer rofe to 671, and the coloured one to 68. From a number of trials it at laft appeared, that the difference at this diftance from the lamp amounted generally to about a degree. When the thermometers were removed farther than two inches from the lamp, the difference decreased; and at the distance of about 14 or 15 inches it vanished entirely.

whence all the phenomena of DISTILLATION, EVAPORATION, FLAME, IGNITION, COMBUSTION, &c. Thefe are the principal facts which can be looked upon as established with regard to heat confidered in a philofophical view. In common difcourfe it is always spoken of as a certain fubftance diftinct from all others, and may properly enough be reckoned fo with regard to all the purpofes of life. In this fenfe, heat is accumulated by certain bodies in a much greater proportion than others. Dr Franklin made the experiment with pieces of cloth of various colours laid upon fnow and expofed to the funshine, and in all cafes found that the pieces dyed with the darkeft colours funk deepelt in the fnow. Mr Cavallo examined the matter more accurately; first by obferving the height to which a thermometer with a blackened bulb rofe in comparison with one of clear glass, and then by comparing the heights of different thermometers whofe bulbs were painted of various colours. Having therefore conftructed two thermometers whofe fcales exactly correfponded with each other, he fixed them both upon the fame frame, about an inch asunder, having the balls quite detached from the frame; and in this manner expofed them to the light of the fun or of a lamp. When these were expofed to the fun or kept in the fhade, with the glafs of both bulbs clear, they fhowed precifely the fame degree; and the difference between the degree fhown by the thermometers when expofed to the fun and when kept in the shade, at about the fame time of the day, was very trifling.

The ball of one of the thermometers being painted black, and that of the other left clean, they showed different degrees of temperature on being expofed to the fun; the difference fometimes amounting to 10°: but was never conftant; varying according to the clearnefs of the fun's light as well as of the air, and likewife according to the different degrees of temperature in the atmosphere.

On keeping the thermometer with the painted ball on the infide of a window, Mr Cavallo obferved that ftrong day-light had an effect in raifing the mercury as well as the fun's light. To afcertain this, he cleaned the bulb of the painted thermometer, and blackened that of the other; but the effect was conftant, viz. the quickfilver in the tube of the thermometer, whose ball was painted black, was conftantly higher than the other whenever they were expofed to the strong daylight. The difference was commonly about one-third of a degree, but fometimes it amounted to three-fourths, or even to a whole degree; and the experiment anfwered even when the fun was hid by clouds, which feems to indicate that every degree of light is accompanied with a correfponding one of heat.

By this confideration Mr Cavallo was induced to try whether, by directing the concentrated light of the moon upon the blackened bulb of a thermometer, it would be raifed higher than a clean one standing in the fame. The experiment was feveral times tried with a large lens, and afterwards with a burning mirror of 18 inches diameter; yet fometimes for want of proper means of obferving the height of the mercury in the tubes of the thermometers, fometimes for want of a continued clear light of the moon, or in fhort from fome unfavourable circumftance or other, he was never able to make a fair and decifive trial of this experiment.

N° 149.

On this occafion Mr Cavallo had an opportunity of making a curious obfervation concerning the decrease of heat at different diftances from the centre." It is mathematically true, that emanations which proceed from a centre, and expand in a fphere, muft become more and more rare in proportion to the fquares of the diftances from the centre. Thus it is faid, that the intenfity of light proceeding from a luminous body, at the double, treble, quadruple, &c. distance from that body, must be respectively four, nine, fixteen, times, &c. lefs denfe. The fame thing may be said of heat; but with refpect to the latter, it appeared, that its intenfity did not decrease exactly in the duplicate proportion of the diftances from the flame of the lamp, but fhowed a very odd irregularity. It seemed to decreafe fafter than the duplicate proportion of the diftances for the space of two inches and a half or three inches, after which it decreased much flower; but whether this proceeded from fome different ftate of the air's purity at different diftances from the flame of the lamp, or from the vapours coming from the flame, I cannot take upon me to determine."

Mr Cavallo next made fome experiments upon thermometers, the balls of which were painted of various colours. His view was to examine with precision the degrees of heat imbibed by differently coloured fubftances, in order to determine whether they kept any proportion to the spaces occupied by the prifmatic colours in the prifmatic fpectrum, or if they followed any other law. In these experiments he met with confiderable difficulties, chiefly arifing from the different nature of the colours with which the bulbs were painted. By reafon of this diverfity the bulbs could not be made equally smooth, which occafioned a confiderable difference in the effect; as he found by painting two bulbs of thermometers with the fame colour, only making the one smooth and the other rough.

To avoid this inconvenience, he attempted to make thermometers with tubes of differently coloured glafs; but when a ball was formed with any of these, the glafs of the ball was fo thin, that it differed very little from that which was entirely colourless. He then included the thermometers in boxes, where the rays entered through coloured glaffes; but here the rays were not only far from being homogeneous, but there was fuch a difference in the transparency of fome of the coloured glaffes, that this method proved alfo ineffectual. The leaft ambiguous method, therefore, was that of painting the balls of the thermometers with water-colours, taking care to lay them on as equally and smooth as poffible. In this manner the experiments were repeated, ufing sometimes a dozen of ther

mometers

when firft weighed and when it is afterwards heated Heat. to one degree; but by an eafy calculation it will be found, that this difference is fo exceedingly small that it cannot be perceived with our most exact inftruments either of weight or measure.

mometers at once, whose balls were painted with various colours, and were exposed to the fun: but from a vaft number of experiments, and fome weeks obfervation, it could only be deduced, that if the colours with which the balls of the thermometers were painted had any confiderable resemblance to thofe of the prifm, those which were nearest to the violet fhowed a greater degree of heat than the others; but they were all, even that painted with white lead, in fome intermediate degree between the blackened thermometer and that which was left quite clear. If the colours had not the proper denfity, the effects were different: thus, a thermometer painted with a light blue ftood lower than another painted with good carmine.

In the course of his thermometrical experiments, Mr Cavallo likewise discover a new method of determining the expanfion of mercury by weight, which feemed capable of being carried to a greater degree of exactnefs than any other hitherto propofed. Having firft blown a ball to a capillary tube, fuch as are commonly used for thermometers, he weighed it, and found the weight when empty to be 79.25 grains; and he obferves, that in this experiment it is a precaution abfolutely neceffary to have the glafs as accurately cleaned as poffible. Some mercury was then introduced into the ftem of the thermometer, taking care that none of it entered the ball; and by adapting a scale of inches to the tube, obferved that 4.3 inches of it were filled with the mercury. The thermometer was now weighed again; and from this the weight of the glafs being fubtracted, the remainder, viz. 0.24 gr. fhowed the weight of that quantity of quickfilver which filled the 4.3 inches of the tube. Now the ball of the thermometer, and alfo part of the tube, were entirely filled with quickfilver; and in order to find out the weight of the mercury contained in it, the thermometer was weighed for the laft time; and the weight of the glass being fubtracted from this, the remainder, viz. 3205 grains, fhowed the weight of the whole quantity of quickfilver contained in the ther

mometer.

By comparing this inftrument with a graduated thermometer of Fahrenheit, and by applying a fcale of inches, he found, that 20° on the new thermometer was equal to 1.37 inches. But 0.24 grains was the weight of as much mercury as filled 4.3 inches of the tube. Therefore, by the rule of proportion, it will be found, that the weight of as much quickfilver as fills 1.33 inches of the tube, viz. the length of 20°, is equal to 0.0742 of a grain nearly; and that the weight of as much quickfilver as fills a length of the tube equivalent to one degree, is equal to 0.00371 grains. Now it is clear, that the weight of the whole quanti ty of quickfilver contained in the thermometer is to the weight of as much as fills the length of one degree of the tube, as the bulk of the whole quantity of quickfilver in a given degree of heat to the increase of bulk that the fame whole quantity of quickfilver acquires when heated but one degree; viz. 32.05 grains is to 0.00371 grains as I to 0.0011+. By which experiment it appears, that one degree of Fahrenheit's thermometer increases the bulk of mercury not above eleven hundredth thousandth parts. A small deviation from mathematical exactnefs is indeed produced by the difference of weight between the quickfilver of the tube VOL. VIII. Part I.

On repeating this experiment with other thermometers, each procefs varied a little from the other; which irregularity, Mr Cavallo thinks, was certainly owing to the imperfection of his fcales: but by taking a mean of various experiments, it appears, that one degree of heat, according to Fahrenheit's thermometer, increases the bulk of a quantity of quicksilver in the temperature of 50° by about nine parts in 100,000; that is, if the bulk of any quantity of quickfilver in the temperature of 50° be 100,000, it will be 100,009 in the temperature of 51°.

In making experiments of this kind, it is necessary to have the bores of the tubes abfolutely cylindrical; and the fcales fhould be fo exact as to turn with the hundredth part of a grain when charged with half an ounce weight.

HEAT of Burning Bodies. See COMBUSTION. HEAT of Chemical Mixtures. This is a phenomenon neceffarily refulting from the change of form produced in the different fubftances which are mixed together; and the manner in which it happens may be cafily underftood from the example of oil of vitriol and water. If equal quantities of concentrated vitriolic acid and water are mixed together, a very great degree of heat immediately takes place; infomuch, that if the veffel which contains the mixture is made of glass it will probably break; and after it is cold, the mixture will be found to have fhrunk in its dimenfions, or will occupy lefs space "than the bulk of the water and acid taken feparately. In this cafe we know that the water, while in its fluid ftate, hath as much latent heat as it can contain; i. e. the elementary fire within it expands or feparates its parts from each other, as much as is confiftent with the conftitution of the body. If any more is added, it cannot be absorbed, or direct its force upon the particles of the water without raising them in vapour: of confequence, part of this additional expanfive power will be employed in the formation of vapour, and the reft will be discharged upon the neighbouring bodies, i. e. will be converted into fenfible heat. The vitriolic acid, in its concentrated ftate, contains a great quantity of latent heat, which is neceflary to preferve its fluidity. But when it is mixed with the fluid water, the latent heat contained in the latter is abundantly fufficient for both of confequence, the great expanfive power in the oil of vitriol itself becomes now totally useless, and therefore exerts its force upon the neighbouring bodies; and when the mixture returns to the original temperature of the oil of vitriol and water, it fhows a lofs of subftance by its diminution in bulk. This may serve to explain all cafes in chemistry where heat or cold is produced and it will generally be found, that where bodies, by being mixed together, produce heat, they fhrink in their dimenfions; but when they produce cold, they are enlarged.

: