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tained when the balloon was surrounded with ice. The degrees are centigrade.

Excesses of temp.
or values of t.

Values of
V observed.

Values of V calculated.

80

cases.

Excess of the therm. Corresponding velocities

240
10.69

10.68 above the balloon.

of cooling.

220
8.81

8.89
200
7.40

7-34
240
10.69

180
6:10

6.03
220
8.81

160
4.89

4.87
200
7.40

140
3.88

3.89
180
6:10

120
3:02

3.05
160
4.89

100
2:30

2.33
140
3.88

80
1.74

1.72
120

3:02
100

2.30
1.74

The laws of cooling in vacuo being known, nothing is more simple than to separate from

the total cooling of a body surrounded with The first column contains the excesses of tem- air, or with any other gas, the portion of the perature above the walls of the balloon; that is effect due to the contact of the fluid. For this, to say, the temperatures themselves, since the it is obviously sufficient to subtract from the balloon was at 0°. The second column contains real velocities of cooling, those velocities which the corresponding velocities of cooling, calcu- would take place if the body, cæteris parilated and corrected. These velocities are the bus, were placed in vacuo. This subtracnumbers of degrees that the thermometer would tion may be easily accomplished now that sink in a minute. The first series shows clearly we have a formula, which represents this vethe inaccuracy of the geometrical law of Rich- locity with great precision, and for all possible mann; for, according to that law, the velocity of cooling at 200° should be double of that at 100°;

From numerous experimental comparisons the whereas we find it as 7:4 to 2-3, or more than following law was deduced : the velocity of triple; and in like manner, when we compare cooling a body, owing to the sole contact of a the loss of heat at 240° and at 80°, we find the gas, depends for the same excess of temperature first about six times greater that the last; while, on the density and temperature of the fluid; according to the law of Richmann, it ought to but this dependence is such, that the velocity of be merely triple. From the above, and some cooling remains the same, if the density and the analogous experiments, the following law has temperature of the gas change in such a way been deduced : when a body cools in vacuo, that the elasticity remains constant. surrounded by a medium whose temperature The influence of the nature of the surface of constant, the velocity of cooling for excess of bodies, in the distribution of heat, was first accutemperature, in arithmetical progression, in- rately examined by Mr. Leslie. "This branch of creases as the terms of a geometrical progres- the subject is usually called the radiation of casion, diminished by a certain quanty. Or, loric. "To measure the amount of this influence, expressed in algebraic language, the following with precision, he contrived a peculiar instruequation contains the law of cooling in vacuo: ment,* called a differential thermometer. It

consists of a glass tube, bent into the form of V=m. a (a − 1).

the letter U, terminated at each end with a bulb

The bore is, about the size of that of large therO is the temperature of the substance surrounding the vacuum ; and i that of the heated mometers, and the bulbs have a diameter of

one-third of an inch and upwards. Before body above the former. The ratio a of this hermetically closing the instrument a small progression is easily found for the thermometer, whose cooling is recorded above; for when portion of sulphuric acid, tinged with carmine,

is introduced. The adjustment of this liquid, so as 0 augments by 20o, t remaining the same, the

to make it stand at the top of one of the stems, velocity of cooling is then multiplied by 1:165, immediately below the bulb, requires dexterity which number is the mean of all the ratios in the operator. To this steni a scale divided experimentally determined. We have then

into 100 parts is attached, and the instrument is 20

then fixed upright by a little cement on a wooden a = N1.165 = 1.0077.

sole. If the finger, or any body warmer than

the ambient air, be applied to one of these bulbs, It only remains, in order to verify the ac- the air within will be heated, and will of course curacy of this law, to compare it with the expand, and, issuing in part from the bulb, dedifferent series contained in the table inserted press before it the tinged liquor. The amount above. In that case, in which the surround- of this depression observed upon the scale will ing medium was 0°, it is necessary to make denote the difference of temperature of the two m = 2.037, for m = and n is an

balls. But if the instrument be merely carried, log. a,

without touching either ball, from a warmer to a intermediate number; we have then V = cooler, or from a cooler to a warmer air, or me

dium of any kind, it will not be affected; be2.037 (a 1).

cause the equality of contraction or expansion,

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in the enclosed air of both bulbs, will maintain it, prevents it from receiving their calorific emathe equilibrium of the liquid in the stem. Being nations in return. On this principle we can unthus independent of the fluctuations of the sur- derstand how a metallic mirror, placed before a rounding medium, it is well adapted to measure fire, should scorch substances in its focus, whilo the calorific emanations of different surfaces, itself remains cold; and, on the other hand, how successively converged, by a concave reflector, a mirror of darkened, or even of silvered glass, upon one of its bulbs.

should become intolerably hot to the touch while Dr. Howard has described, in the sixteenth it throws little heat before it. From this absorbnumber of the Journal of Science, a differential ent faculty it comes that a thin pane of glass inthermometer of his contrivance, which he con- tercepts almost the whole heat of a blazing fire, ceives to possess some advantages. Its form is while the light is scarcely diminished across it. an imitation of Mr. Leslie's; but it contains By degrees indeed itself, becoming heated, conmerely tinged alcohol, or ether; the air being ex- stitutes a new focus of emanation; but still the pelled by ebullition previous to the hermetical energy of the fire is greatly interrupted. · Hence closure of the instrument. The vapor of ether, also we see why the thinnest sheet of bright tinor of spirit in vacuo, affords, he finds, a test of foil is a perfect fire-screen; so impervious superior delicacy to air. He makes the two legs indeed to heat that, with a masque coated with of different lengths; since it is in some cases it, our face may encounter without inconvenience very convenient to have the one bulb standing the blaze of a glass-house furnace. quite aloof from the other.

Since absorption of heat goes hand in hand In Mr. Leslie's, when they are on the same with radiation, we perceive that the inverse of level, their distance asunder varies from one-third absorption, that is reflection, must be possessed of an inch to an inch or upwards, according to the in inverse powers by the different substances size of the instrument. The general length of composing the list. Thus bright metals reflect the legs of the syphon is about five or six inches. most heat, and so on upwards in succession. Ilis reflecting mirrors, of about fourteen inches Mr. Leslie is anxious to prove that elastic fluids, diameter, consisted of planished tin-plate, ham- by their pulsatory undulations, are the media of mered into a parabolical form by the guidance the projection or radiation of heat; and that thereof a curvilinear gauge. A hollow tin vessel, six fore liquids, as well as a perfect vacuum, should inches cube, was the usual source of calorific obstruct the operation of this faculty. But the laws emanation in his experiments. He coated one of the cooling of bodies in vacuo, experimentally of its sides with lamp-black, another with paper, established by MM. Dulong and Petit, are fatal a third with glass, and a fourth was left bare. to Mr. Leslie's hypothesis, which indeed was not Ilaving then filled it with hot water, and set it tenable against the numerous objections which in the line of the axis and four or six feet in had previously assailed it. The following beaufront of one of the mirrors, in whose focus the tiful experiment of Sir H. Davy seems alone to bulb of a differential thermometer stood, he settle the question. He had an apparatus made noted the depression of the colored liquid pro- by which platina wire could be heated in any duced on presenting the different sides of the elastic medium or in vacuo; and by which the tube towards the mirror in succession. The effects of radiation could be distinctly exhibited following table gives a general view of the re- by two mirrors, the heat being excited by a Volsults, with these, and other coatings :

taic battery. In several experiments, in which Lamp black.

100

the same powers were employed to produce the Water by estimate

100+

ignition, it was found that the temperature of a

thermometer rose nearly three times as much in

98 Resin

the focus of radiation, when the air in the re

96 Sealing-wax :

95

ceiver was exhausted to so, as when it was in Crown glass .

its natural state of condensation. The cooling

90 China ink

88

power, by contact of the rarefied air, was much Ice

less than that of the air in its common state, for

85 Red lead

80

the glow of the platina was more intense in the Plumbago

75

first case than in the last; and this circumstance Isinglass

75

perhaps renders the experiment not altogether Tarnished lead

45

decisive; but the results seem favorable to the Mercury

20+

idea that the terrestrial radiation of heat is not Clean lead

19

dependent upon any motions or affections of the Iron polished

15 atmosphere. The plane of the two mirrors was 12

placed parallel to the horizon, the ignited body Gold, silver, copper

12

being in the focus of the upper, and the thermo

meter in that of the under mirror. It is evident Similar results were obtained by Leslie and that a diminished density of the elastic medium, Rumford in a simpler form.

amounting to to, should, on Mr. Leslie's views, Coating the bulb of his thermometer with have occasioned a greatly diminished temperature different substances, Mr. Leslie ingeniously dis- in the inferior focus, and not a threefold increase, covered the power of different substances to as happened. The experiments with screens of absorb heat; and he found this to follow the glass, paper, &c., which Mr. Leslie adduced in same order as the radiating or projecting quality. support of his undulatory hypothesis, have been The same film of silver leaf which obstructs the since confronted with the experiments on screens egress of heat from a body to those surrounding of Dr Delaroche, who, by varying them, ob

Writing paper

Tin plate

1

tained results incompatible with Mr. Leslie's upwards, so as to rest against the side of the ves. views.

sel. The best form of the cup is an ellipsoid, The constancy or steadiness of the tempera- whose eccentricity is equal to half the transverse ture of a body will consist in the equality of the axis, and the focus consequently placed at the quantities of radiating caloric which it ernits and third part of the whole height of the cavity; receives in an equal time; and the equality of while the diameter of the thermoscope ball temperature between several bodies which influ- should be nearly the third part of the orifice of ence one another, by their mutual radiation, will the cup.

A lid of the same thin metal, unpoconsist in the perfect compensation of the mo- lished, is fitted to the mouth of the cup, and rementary interchanges effected among one and moved only when an observation is to be made. all. Such is the ingenious principle of a move. The scale attached to the stem of the thermoable equilibrium, proposed by professor Prevost: scove may extend to sixty or seventy millesimal a principle, whose application, directed with degrees above the zero, and about fifteen degrees discretion and combined with the properties below it. This instrument, exposed to the open peculiar to different surfaces, explains all the air in clear weather, will at all times, both durphenomena which we observe in tħe distribution ing the day and the night, “indicate an impresof radiating caloric. Thus, when we put a ball sion of cold shot downward from the higher of snow in the focus of one concave mirror, and regions,' in the figurative language of Mr. Leslie. a thermometer in that of an opposite mirror Yet the effect varies exceedingly. It is greatest placed at somc distance, we perceive the tempe- while the sky has the pure zure hue; it diminrature instantly to fall, as if there were a real ra ishes fast as the atmosphere becomes loaded with diation of frigorific particles, according to the spreading clouds; and it is almost extinguished ancient notion. The true explanation is derived when low fogs settle on the surface. The liqnid from the abstraction of that return of heat which in the stem falls and rises with every passing the thermoscope mirror had previously derived cloud. Dr. Howard's modification of the therfrom the one now influenced by the snow, and moscope would answer well here. now participating in its inferior radiating ten The diffusion of heat, among the particles of sion. Thus, also, a black body placed in the Auids themselves, depends upon their specific focus of one mirror would diminish the light in gravity and specific heat conjunctly, and therethe focus of the other; and, as Sir H. Davy hap- kore must vary for each particular substance. pily remarks, the eye is, to the rays producing The mobility of the particles in a fluid, and their light, a measure, similar to that which the ther reciprocal independence on one another, permit mometer is to rays producing heat.

them to change their places whenever they are This interchange of heat is finely exemplified expanded or contracted by alterations of tempein the relation which subsists between any por rature; and hence the immediate and inevitable tion of the sky and the temperature of the subja effect of communicating heat to the under stracent surface of the earth. In the year 1788 Mr. tum of a fluid mass, or of abstracting it from the Six of Canterbury mentioned, in a paper trans- upper stratum, is to determine a series of intesmitted to the Royal Society, that on clear and tine movements. The colder particles, by their dewy nights he always found the mercury lower superior density, descend in a perpetual current, in a thermometer laid upon the ground, in a and force upwards those rarefied by the heat. meadow in his neighbourhood, than it was in a When, however, the upper stratum primarily acsimilar thermometer suspended in the air six feet quires an elevated temperature, it seems to have above the former: and that upon one night the little power of imparting heat to the subjacent difference amounted to 50 of Fahrenheit's scale. strata of fluid particles. Water may be kept And Dr. Wells, in autumn 1811, on laying long in ebullition at the surface of a vessel, thermometer upon grass wet with dew, and sus while the bottom remains ice cold, provided we pending a second in the air two feet above the take measures to prevent the heat passing downsurface, found, in an hour afterwards, that the wards through the sides of the vessel itself. Count former stood 8° lower than the latter. He at first Rumford became so strongly persuaded of the regarded this coldness of the surface to be the impossibility of communicating heat downwards, effect of the evaporation of the moisture; but through fluid particles, that he regarded them as subsequent observations and experiments con- utterly destitute of the faculty of transmitting that vinced him that the cold was not the effect, but power from one to another, and capable of acthe cause of deposition of dew. Under a cloud- quiring heat only in individual rotation, and diless sky the earth projects its heal, without re- rectly from a foreign source. The proposition turn, into empty space; but a canopy of cloud is thus absolutely announced is absurd, for we a concave mirror, which restores the equilibrium know that by intermixture, and many other by counter-radiation. See Dew:

modes, fluid particles impart heat to each other; On this principle Dr. Wollaston suggested the and experiments have been instituted, which construction of an instrument, which professor . prove the actual descent of heat through fluids Leslie has called an athrioscope, whose function by communication from one stratum to another. it is to denote the clearness and coolness of the But unquestionably this communication is sky. It consists of a polished metallic cup, of amazingly difficult and slow. We are hence led an oblong spheroidal shape, very like a silver to conceive, that it is an actual contact of partiporter-cup, standing upright, with the bulb of a cles which in the solid condition facilitates the differential thermometer placed in its axis, and transmission of heat so speedily from point to the stem lying parallel to the stalk of the cup. point thro'ıgh their mass. This contact of certain The other ball is gilt, and turned outwards and poles in the molecules is perfectly consistent

3. Copper, }nearly equal.

with void spaces, in which these molecules may iron, three feet long, and with a drill form three slide over each other in every direction ; by cavities in one of its sides, at ten, twenty, and which movements or condensations heat may be thirty inches from its end, each cavity capable of excited. The fluid conditior. reverts or averts receiving a little mercury, and the small bulb of the touching and cohering poles, whence mobility a delicate thermometer. Cut a bole fitting exresults. This statement may be viewed either actly the prismatic bar, in the middle of a sheet as a representation of facts, or an hypothesis to of tin plate, which is then to be fixed to the bar, aid conception.

to screen it and the thermometers from the focus The transmission of heat through solids was of heat. Immerse the extremity of the bar obmade the subject of some popular experiments liquely into oil or mercury heated to any known by Dr. Ingenbolisz. He took a number of me- degree, and place the thermometers in their cavitallic rods of the same length and thickness, and, ties surrounded with a little mercury. Or the having coated" one of the ends of them for a few bar may be kept horizontal, if an inch or two at inches with bees' wax, he plunged their other its end be incurvated, at right angles to its length. ends into a heated liquid. The heat travelled Call the thermometers A, B,C. Were there no onwards among the matter of each rod, and soon dissipation of the heat, each thermometer would became manifest by the softening of the wax. continue to mount till it attained the temperature The following is the order in which the wax of the source of heat. But, in actual experiments, melted; and according to that experiment, projection and aërial currents modify that result

, therefore, the order of conducting power relative making the thermometers rise more slowly, and to heat:

preventing them from ever reaching the tempera1. Silver.

ture of the end of the bar. Their state becomes 2. Gold.

indeed stationary whenever the excess of temperature, each instant communicated by the preced

ing section of the bar, merely compensates what Platinum,

they lose by the contact of the succeeding secIron, much inferior to

tion of the bar, and the other outlets of heat. Steel, the others.

The three thermometers now indicate three Lead,

steady temperatures, but in diminishing progres“In my repetition of the experiment, I found,' sion. In forming an equation from the exsays Dr. Ure, silver by much the best conductor, perimental results, M. Laplace has shown, that next copper, then brass, iron, tin, much the same, the difficulties of the calculation can be removed then cast iron, next zinc, and, last of all, lead. only by admitting, that a determinate point is Dense stones follow metals in conducting power, influenced not only by those points which touch then bricks, pottery, and, at a long interval, glass. it, but by others at a small distance before and A rod of this singular body may be held in the behind it. Then the laws of homogeneity, to fingers for a long time, at a distance of an inch which differentials are subject, are re-established, from where it is ignited and fused by the blow- and all the rules of the differential calculus are pipe. It is owing to the inferior conducting observed. Now, in order that the calorific ins power of stone, pottery, glass, and cast-iron, that fluence may thus extend to a distance in the inthe sudden application of heat so readily cracks terior of the bar, there must operate through the them. The part acted on by the caloric expands, very substance of the solid elements a true radiawhile the adjacent parts, retaining their pristiné tion, analogous to that observed in air, but form and volume, do not accommodate them- whose sensible influence is bounded to distances selves to the change; whence a fissure must ne- incomparably smaller. This result is in no cessarily ensue. Woods and bones are better respect improbable. In fact, Newton has taught conductors than glass; but the progress of heat us that all bodies, even the most opaque, become in them, at elevated temperatures, may be aided transparent when rendered sufficiently thin, and by the vaporisation of their juices. Charcoal the most exact researches on radiating caloric and saw-dust rank very low in conducting prove, that it does not emanate solely from the power. Hence the former is admirably fitted for external surface of bodies, but also from material arresting the dispersion of heat in metal furnaces. particles situated within this surface, becoming If the sides of these be formed of double plates, no doubt insensible at a very slight depth, with an interval between them of an inch" filled which probably varies in the same body with its with pounded charcoal, an intense heat may temperature. exist within, while the outside is scarcely affect MM. Biot, Fourier, and Poisson, three of the ed. Morveau has rated the conducting power most eminent mathematicians and philosophers of charcoal to that of fine sand as two to three, of the age, have distinguished themselves in this a difference much too small. Spongy organic abstruse investigation. The following is the substances, silk, wool, cotton, &c., are still worse formula of M. Biot, when one end of the bar is conductors than any of the above substances; maintained at a constant temperature, and the and the finer the fibres, the less conducting other is so remote as to make the influence of power they possess. The theory of clothing de- the source insensible. Let y represent, in depends on this principle. The heat generated by grees of the thermometer, the temperature of the the animal powers is accumulated round the air by which the bar is surrounded ; let the body by the imperfect conductors of which cloth- temperature of the focus be y + Y; then the ining is composed. To discover the exact law of the distribution tegral becomes, log. y = log. Y

M of heat in solids we may take a prisinatic bar of

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Olefiant gas

.

& is the distance from the hot end of the bar, a Table I.—Of the Specific Heats of Gases, by and b are two co-efficients, supposed constant for MM. BERARD and DELA ROCHE. the whole length of the bar, which serve to accommodate the formula to every possible case,

Equal

Equal Specified and which must be assigned in each case, agree

volumes, weights. gravity. ably to two observations. M is the modulus of the ordinary logarithmic tables, or the number

Air

1.0000 1.0000 1.0000 2:302585. M. Biot presents several tables of Hydrogen

0.9033 12:3401 0·0732 observations, in which sometimes eight, and Carbonic acid. 1.2583 0·8280 1.5196 sometimes fourteen thermometers, were applied Oxygen .

0.9765 0.8848 1:1036 all at once to successive points of the bar; and Azote

1'0000 1.0318 0.9691 then he computes by the above formula what Oxide of azote 1.3503 0.8878 1.5209 ought to be the temperature of these successive

1:5530 1.5763 | 0-9885 points, having given the temperature of the Carbonic acid. 1.0340 1.0805 0-9569 source; and vice versâ, what should be the temperature of the source, from the indications of the thermometers. A perfect accordance is shown

To reduce the above numbers to the standard to exist between fact and theory. Whence we of water, three different methods were employed; may regard the view opened up by the latter, as

from which the three numbers, 0·2498, 0.2697, a true representation of the condition of the bar. and 0.2813, were obtained for atmospheric air. With regard to the application of this theorem, The experimenters have taken 0.2669 as the to discover, for example, the temperature of a mean, to which all the above results are referred, furnace, by thrusting the end of a thermoscopic as follows :iron bar into it, we must regret its insufficiency. M. Biot himself, after showing its exact coinci

TABLE II. dence at all temperatures, up to that of melting lead, declares that it ought not to apply at high Water

1.0000 heats. But we see no difficulty in making a Air

0.2669 very useful instrument of this kind by experi Hydrogen gas

3.2936 ment, to give very valuable pyrometrical indi Carbonic acid

0.2210 cations. The end of the bar which is to be ex Oxygen

0.2361 posed to the heat, being coated with fire-clay, or Azote

0-2754 sheathed with platinum, should be inserted a Oxide of azote

0.2369 few inches into the flame, and drops of oil being

0.4207 put into three successive cavities of the bar, we Carbonic oxide

0.2884 should measure the temperatures of the oil, when Aqueous vapor

0.8470 they have become stationary, and note the time elapsed to produce this effect. A pyroscope of this kind could not fail to give useful information The following are the results given by MM. to the practical chemist, as well as to the manu- Clement and Desormes, for equal volumes at facturers of glass, pottery, steel, &c.

temperatures from 0° to 60° centigrade, or 32° We shall insert here, from Dr. Ure, tabular to 140° Fahrenheit. views of the specific heats determined by the recent researches of the French chemists. MM.

TABLE III. Petit and Dulong remark, that, the attempts hitherto made to discover some laws in the spe

Inches Clement & Delaroche cific heats of bodies have been entirely unsuccess

Barom. Desormes. & Berard. ful. We shall not be surprised at this, if we attend to the great inaccuracy of some of the Atmospheric air at 39.6 1.215 1.2396 measurements; for if we except those of Lavoisier Ditto

29.84 1.000 1.0000 and Laplace (unfortunately very few), and those

Ditto

14.92 0.693 by Laroche and Berard for elastic fluids, we are

Ditto

7.44 0.540 forced to admit, that the greatest part of the

Ditto

374 0.368 others are extremely inaccurate, as our own ex Do.charged with

29.84 1.000 periments have informed us, and as might indeed ether, be concluded from the great discordance in the Azote

29.84 1.000 1.0000 results obtained for the same bodies by different Oxygen

29.84 1.000 0.974 experimenters.' From this censure we must ex Hydrogen

29.84 0.664 0.9033 cept the recent results of MM. Clement and Carbonic acid 29.84 1.300 1.2583 Desormes on gases, which may be regarded as entitled to equal confidence with those of Berard and Delaroche.

The relative specific heat of air to water is, by MM. Clement and Desormes, 0.250 to 1.000, or exactly one-fourth. The last table, which is extracted from the Journal de Physique, gives the specific heat of oxygen by Delaroche and Berard, a little different from their own number, Table I. from the Annales de Chimie, vol. 85. The most remarkable result given by MM. Cle

Olefiant gas

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