Page images
PDF
EPUB
[blocks in formation]
[blocks in formation]

The first column contains the excesses of temperature above the walls of the balloon; that is to say, the temperatures themselves, since the balloon was at 0°. The second column contains the corresponding velocities of cooling, calculated and corrected. These velocities are the numbers of degrees that the thermometer would sink in a minute. The first series shows clearly the inaccuracy of the geometrical law of Richmann; for, according to that law, the velocity of cooling at 200° should be double of that at 100°; whereas we find it as 7-4 to 2:3, or more than triple; and in like manner, when we compare the loss of heat at 240° and at 80°, we find the first about six times greater that the last; while, according to the law of Richmann, it ought to be merely triple. From the above, and some analogous experiments, the following law has been deduced: when a body cools in vacuo, surrounded by a medium whose temperature is constant, the velocity of cooling for excess of temperature, in arithmetical progression, increases as the terms of a geometrical progression, diminished by a certain quanty. Or, expressed in algebraic language, the following equation contains the law of cooling in vacuo:

[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small]

The laws of cooling in vacuo being known, nothing is more simple than to separate from air, or with any other gas, the portion of the the total cooling of a body surrounded with effect due to the contact of the fluid. For this, it is obviously sufficient to subtract from the real velocities of cooling, those velocities which would take place if the body, cæteris paribus, were placed in vacuo. This subtraction may be easily accomplished now that we have a formula, which represents this velocity with great precision, and for all possible

cases.

From numerous experimental comparisons the following law was deduced: the velocity of cooling a body, owing to the sole contact of a gas, depends for the same excess of temperature on the density and temperature of the fluid; but this dependence is such, that the velocity of cooling remains the same, if the density and the temperature of the gas change in such a way that the elasticity remains constant.

The influence of the nature of the surface of bodies, in the distribution of heat, was first accurately examined by Mr. Leslie. This branch of the subject is usually called the radiation of caloric. To measure the amount of this influence, with precision, he contrived a peculiar instrument, called a differential thermometer. It consists of a glass tube, bent into the form of the letter U, terminated at each end with a bulb The bore is about the size of that of large thermometers, and the bulbs have a diameter of hermetically closing the instrument one-third of an inch and upwards. Before a small portion of sulphuric acid, tinged with carmine, is introduced. The adjustment of this liquid, so as immediately below the bulb, requires dexterity to make it stand at the top of one of the stems, in the operator. To this stem a scale divided into 100 parts is attached, and the instrument is then fixed upright by a little cement on a wooden sole. If the finger, or any body warmer than the ambient air, be applied to one of these bulbs, the air within will be heated, and will of course expand, and, issuing in part from the bulb, depress before it the tinged liquor. The amount of this depression observed upon the scale will denote the difference of temperature of the two balls. But if the instrument be merely carried, without touching either ball, from a warmer to a cooler, or from a cooler to a warmer air, or medium of any kind, it will not be affected; because the equality of contraction or expansion,

in the enclosed air of both bulbs, will maintain the equilibrium of the liquid in the stem. Being thus independent of the fluctuations of the surrounding medium, it is well adapted to measure the calorific emanations of different surfaces, successively converged, by a concave reflector, upon one of its bulbs.

Dr. Howard has described, in the sixteenth number of the Journal of Science, a differential thermometer of his contrivance, which he conceives to possess some advantages. Its form is an imitation of Mr. Leslie's; but it contains merely tinged alcohol, or ether; the air being expelled by ebullition previous to the hermetical closure of the instrument. The vapor of ether, or of spirit in vacuo, affords, he finds, a test of superior delicacy to air. He makes the two legs of different lengths; since it is in some cases very convenient to have the one bulb standing quite aloof from the other.

In Mr. Leslie's, when they are on the same level, their distance asunder varies from one-third of an inch to an inch or upwards, according to the size of the instrument. The general length of the legs of the syphon is about five or six inches. Ilis reflecting mirrors, of about fourteen inches diameter, consisted of planished tin-plate, hammered into a parabolical form by the guidance of a curvilinear gauge. A hollow tin vessel, six inches cube, was the usual source of calorific emanation in his experiments. He coated one of its sides with lamp-black, another with paper, a third with glass, and a fourth was left bare. Having then filled it with hot water, and set it in the line of the axis and four or six feet in front of one of the mirrors, in whose focus the bulb of a differential thermometer stood, he noted the depression of the colored liquid produced on presenting the different sides of the tube towards the mirror in succession. The following table gives a general view of the results, with these, and other coatings :

[merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

it, prevents it from receiving their calorific ema→ nations in return. On this principle we can understand how a metallic mirror, placed before a fire, should scorch substances in its focus, while itself remains cold; and, on the other hand, how a mirror of darkened, or even of silvered glass, should become intolerably hot to the touch while it throws little heat before it. From this absorbent faculty it comes that a thin pane of glass intercepts almost the whole heat of a blazing fire, while the light is scarcely diminished across it. By degrees indeed itself, becoming heated, constitutes a new focus of emanation; but still the energy of the fire is greatly interrupted. Hence also we see why the thinnest sheet of bright tinfoil is a perfect fire-screen; so impervious indeed to heat that, with a masque coated with it, our face may encounter without inconvenience the blaze of a glass-house furnace.

Since absorption of heat goes hand in hand with radiation, we perceive that the inverse of absorption, that is reflection, must be possessed in inverse powers by the different substances composing the list. Thus bright metals reflect most heat, and so on upwards in succession. Mr. Leslie is anxious to prove that elastic fluids, by their pulsatory undulations, are the media of the projection or radiation of heat; and that therefore liquids, as well as a perfect vacuum, should obstruct the operation of this faculty. But the laws of the cooling of bodies in vacuo, experimentally established by MM. Dulong and Petit, are fatal to Mr. Leslie's hypothesis, which indeed was not tenable against the numerous objections which had previously assailed it. The following beautiful experiment of Sir H. Davy seems alone to settle the question. He had an apparatus made by which platina wire could be heated in any elastic medium or in vacuo; and by which the effects of radiation could be distinctly exhibited by two mirrors, the heat being excited by a Voltaic battery. In several experiments, in which the same powers were employed to produce the ignition, it was found that the temperature of a thermometer rose nearly three times as much in the focus of radiation, when the air in the receiver was exhausted to, as when it was in its natural state of condensation. The cooling power, by contact of the rarefied air, was much less than that of the air in its common state, for the glow of the platina was more intense in the first case than in the last; and this circumstance perhaps renders the experiment not altogether decisive; but the results seem favorable to the idea that the terrestrial radiation of heat is not dependent upon any motions or affections of the atmosphere. The plane of the two mirrors was placed parallel to the horizon, the ignited body being in the focus of the upper, and the thermometer in that of the under mirror. It is evident that a diminished density of the elastic medium, amounting to, should, on Mr. Leslie's views, have occasioned a greatly diminished temperature in the inferior focus, and not a threefold increase, as happened. The experiments with screens of glass, paper, &c., which Mr. Leslie adduced in support of his undulatory hypothesis, have been since confronted with the experiments on screens of Dr Delaroche, who, by varying them, ob

tained results incompatible with Mr. Leslie's views.

The constancy or steadiness of the temperature of a body will consist in the equality of the quantities of radiating caloric which it eraits and receives in an equal time; and the equality of temperature between several bodies which influence one another, by their mutual radiation, will consist in the perfect compensation of the momentary interchanges effected among one and all. Such is the ingenious principle of a moveable equilibrium, proposed by professor Prevost: a principle, whose application, directed with discretion and combined with the properties peculiar to different surfaces, explains all the phenomena which we observe in the distribution of radiating caloric. Thus, when we put a ball of snow in the focus of one concave mirror, and a thermometer in that of an opposite mirror placed at some distance, we perceive the temperature instantly to fall, as if there were a real radiation of frigorific particles, according to the ancient notion. The true explanation is derived from the abstraction of that return of heat which the thermoscope mirror had previously derived from the one now influenced by the snow, and now participating in its inferior radiating tension. Thus, also, a black body placed in the focus of one mirror would diminish the light in the focus of the other; and, as Sir H. Davy happily remarks, the eye is, to the rays producing light, a measure, similar to that which the ther mometer is to rays producing heat.

upwards, so as to rest against the side of the ves-
sel. The best form of the cup is an ellipsoid,
whose eccentricity is equal to half the transverse
axis, and the focus consequently placed at the
third part of the whole height of the cavity;
while the diameter of the thermoscope ball
should be nearly the third part of the orifice of
the cup.
A lid of the same thin metal, unpo-
lished, is fitted to the mouth of the cup, and re-
moved only when an observation is to be made.
The scale attached to the stem of the thermo-
scope may extend to sixty or seventy millesimal
degrees above the zero, and about fifteen degrees
below it. This instrument, exposed to the open
air in clear weather, will at all times, both dur-
ing the day and the night, indicate an impres-
sion of cold shot downward from the higher
regions,' in the figurative language of Mr. Leslie.
Yet the effect varies exceedingly. It is greatest
while the sky has the pure azure hue; it dimin-
ishes fast as the atmosphere becomes loaded with
spreading clouds; and it is almost extinguished
when low fogs settle on the surface. The liquid
in the stem falls and rises with every passing
cloud. Dr. Howard's modification of the ther-
moscope would answer well here.

The diffusion of heat, among the particles of fluids themselves, depends upon their specific gravity and specific heat conjunctly, and therefore must vary for each particular substance. The mobility of the particles in a fluid, and their reciprocal independence on one another, permit them to change their places whenever they are expanded or contracted by alternations of temperature; and hence the immediate and inevitable effect of communicating heat to the under stratum of a fluid mass, or of abstracting it from the upper stratum, is to determine a series of intestine movements. The colder particles, by their superior density, descend in a perpetual current, and force upwards those rarefied by the heat. When, however, the upper stratum primarily acquires an elevated temperature, it seems to have little power of imparting heat to the subjacent strata of fluid particles. Water may be kept long in ebullition at the surface of a vessel, while the bottom remains ice cold, provided we take measures to prevent the heat passing downwards through the sides of the vessel itself. Count Rumford became so strongly persuaded of the impossibility of communicating heat downwards, through fluid particles, that he regarded them as utterly destitute of the faculty of transmitting that power from one to another, and capable of acquiring heat only in individual rotation, and di

This interchange of heat is finely exemplified in the relation which subsists between any por tion of the sky and the temperature of the subja cent surface of the earth. In the year 1788 Mr. Six of Canterbury mentioned, in a paper transmitted to the Royal Society, that on clear and dewy nights he always found the mercury lower in a thermometer laid upon the ground, in a meadow in his neighbourhood, than it was in a similar thermometer suspended in the air six feet above the former: and that upon one night the difference amounted to 5° of Fahrenheit's scale. And Dr. Wells, in autumn 1811, on laying a thermometer upon grass wet with dew, and suspending a second in the air two feet above the surface, found, in an hour afterwards, that the former stood 8° lower than the latter. He at first regarded this coldness of the surface to be the effect of the evaporation of the moisture; but subsequent observations and experiments convinced him that the cold was not the effect, but the cause of deposition of dew. Under a cloudless sky the earth projects its heat, without re-rectly from a foreign source. The proposition turn, into empty space; but a canopy of cloud is a concave mirror, which restores the equilibrium by counter-radiation. See DEW:

On this principle Dr. Wollaston suggested the construction of an instrument, which professor Leslie has called an athrioscope, whose function it is to denote the clearness and coolness of the sky. It consists of a polished metallic cup, of an oblong spheroidal shape, very like a silver porter-cup, standing upright, with the bulb of a differential thermometer placed in its axis, and the stem lying parallel to the stalk of the cup. The other ball is gilt, and turned outwards and

thus absolutely announced is absurd, for we know that by intermixture, and many other modes, fluid particles impart heat to each other; and experiments have been instituted, which prove the actual descent of heat through fluids by communication from one stratum to another. But unquestionably this communication is amazingly difficult and slow. We are hence led to conceive, that it is an actual contact of particles which in the solid condition facilitates the transmission of heat so speedily from point to point through their mass. This contact of certain poles in the molecules is perfectly consistent

with void spaces, in which these molecules may slide over each other in every direction; by which movements or condensations heat may be excited. The fluid condition. reverts or averts the touching and cohering poles, whence mobility results. This statement may be viewed either as a representation of facts, or an hypothesis to aid conception.

The transmission of heat through solids was made the subject of some popular experiments by Dr. Ingenhousz. He took a number of metallic rods of the same length and thickness, and, having coated one of the ends of them for a few inches with bees' wax, he plunged their other ends into a heated liquid. The heat travelled onwards among the matter of each rod, and soon became manifest by the softening of the wax. The following is the order in which the wax melted; and according to that experiment, therefore, the order of conducting power relative to heat:

[ocr errors]
[blocks in formation]

"In my repetition of the experiment, I found,' says Dr. Ure, silver by much the best conductor, next copper, then brass, iron, tin, much the same, then cast iron, next zinc, and, last of all, lead. Dense stones follow metals in conducting power, then bricks, pottery, and, at a long interval, glass. A rod of this singular body may be held in the fingers for a long time, at a distance of an inch from where it is ignited and fused by the blowpipe. It is owing to the inferior conducting power of stone, pottery, glass, and cast-iron, that the sudden application of heat so readily cracks them. The part acted on by the caloric expands, while the adjacent parts, retaining their pristine form and volume, do not accommodate themselves to the change; whence a fissure must necessarily ensue. Woods and bones are better conductors than glass; but the progress of heat in them, at elevated temperatures, may be aided by the vaporisation of their juices. Charcoal and saw-dust rank very low in conducting power. Hence the former is admirably fitted for arresting the dispersion of heat in metal furnaces. If the sides of these be formed of double plates, with an interval between them of an inch filled with pounded charcoal, an intense heat may exist within, while the outside is scarcely affected. Morveau has rated the conducting power of charcoal to that of fine sand as two to three, a difference much too small. Spongy organic substances, silk, wool, cotton, &c., are still worse conductors than any of the above substances; and the finer the fibres, the less conducting power they possess. The theory of clothing depends on this principle. The heat generated by the animal powers is accumulated round the body by the imperfect conductors of which clothing is composed.'

To discover the exact law of the distribution of heat in solids we may take a prismatic bar of

iron, three feet long, and with a drill form three cavities in one of its sides, at ten, twenty, and thirty inches from its end, each cavity capable of receiving a little mercury, and the small bulb of a delicate thermometer. Cut a hole fitting exactly the prismatic bar, in the middle of a sheet of tin plate, which is then to be fixed to the bar, to screen it and the thermometers from the focus of heat. Immerse the extremity of the bar obliquely into oil or mercury heated to any known degree, and place the thermometers in their cavities surrounded with a little mercury. Or the bar may be kept horizontal, if an inch or two at its end be incurvated, at right angles to its length. Call the thermometers A, B, C. Were there no dissipation of the heat, each thermometer would continue to mount till it attained the temperature of the source of heat. But, in actual experiments, projection and aerial currents modify that result, making the thermometers rise more slowly, and preventing them from ever reaching the temperature of the end of the bar. Their state becomes indeed stationary whenever the excess of temperature, each instant communicated by the preceding section of the bar, merely compensates what they lose by the contact of the succeeding section of the bar, and the other outlets of heat. The three thermometers now indicate three steady temperatures, but in diminishing progression. In forming an equation from the experimental results, M. Laplace has shown, that the difficulties of the calculation can be removed only by admitting, that a determinate point is influenced not only by those points which touch it, but by others at a small distance before and behind it. Then the laws of homogeneity, to which differentials are subject, are re-established, and all the rules of the differential calculus are observed. Now, in order that the calorific influence may thus extend to a distance in the interior of the bar, there must operate through the very substance of the solid elements a true radiation, analogous to that observed in air, but whose sensible influence is bounded to distances This result is in no incomparably smaller. respect improbable. In fact, Newton has taught us that all bodies, even the most opaque, become transparent when rendered sufficiently thin; and the most exact researches on radiating caloric prove, that it does not emanate solely from the external surface of bodies, but also from material particles situated within this surface, becoming no doubt insensible at a very slight depth, which probably varies in the same body with its temperature.

MM. Biot, Fourier, and Poisson, three of the most eminent mathematicians and philosophers of the age, have distinguished themselves in this abstruse investigation. The following is the formula of M. Biot, when one end of the bar is maintained at a constant temperature, and the other is so remote as to make the influence of the source insensible. Let y represent, in degrees of the thermometer, the temperature of the air by which the bar is surrounded; let the temperature of the focus be y+Y; then the in

tegral becomes, log. y=log. Y―

پاس

is the distance from the hot end of the bar, a and b are two co-efficients, supposed constant for the whole length of the bar, which serve to accommodate the formula to every possible case, and which must be assigned in each case, agreeably to two observations. M is the modulus of the ordinary logarithmic tables, or the number 2:302585. M. Biot presents several tables of observations, in which sometimes eight, and sometimes fourteen thermometers, were applied all at once to successive points of the bar; and then he computes by the above formula what ought to be the temperature of these successive points, having given the temperature of the source; and vice versa, what should be the temperature of the source, from the indications of the thermometers. A perfect accordance is shown to exist between fact and theory. Whence we may regard the view opened up by the latter, as a true representation of the condition of the bar. With regard to the application of this theorem, to discover, for example, the temperature of a furnace, by thrusting the end of a thermoscopic iron bar into it, we must regret its insufficiency. M. Biot himself, after showing its exact coincidence at all temperatures, up to that of melting lead, declares that it ought not to apply at high heats. But we see no difficulty in making a very useful instrument of this kind by experiment, to give very valuable pyrometrical indications. The end of the bar which is to be exposed to the heat, being coated with fire-clay, or sheathed with platinum, should be inserted a few inches into the flame, and drops of oil being put into three successive cavities of the bar, we should measure the temperatures of the oil, when 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 to the practical chemist, as well as to the manufacturers of glass, pottery, steel, &c.

We shall insert here, from Dr. Ure, tabular views of the specific heats determined by the recent researches of the French chemists. MM.

Petit and Dulong remark, that, the attempts hitherto made to discover some laws in the specific heats of bodies have been entirely unsuccessful. We shall not be surprised at this, if we attend to the great inaccuracy of some of the measurements; for if we except those of Lavoisier and Laplace (unfortunately very few), and those by Laroche and Berard for elastic fluids, we are forced to admit, that the greatest part of the others are extremely inaccurate, as our own experiments have informed us, and as might indeed be concluded from the great discordance in the results obtained for the same bodies by different experimenters.' From this censure we must except the recent results of MM. Clement and Desormes on gases, which may be regarded as entitled to equal confidence with those of Berard and Delaroche.

[blocks in formation]
« PreviousContinue »