ItAPIATION OK H 



RADIATION OF HEAT. 



KX> 



focus, may there produce tlie eflect of absolute cold, by causing the 

 thrnnomi'trr to send out radiant heat to the ice, and thus n-n.l.-r th> 

 mercury in it lower than it \v,.uld otherwise be. Thin interchange of 

 caloric between the thermometer and the ice may be conceived to go 

 on, the former on the whole losing, and the latter acquiring it, till an 

 >inilil<riiim U established between their temperatures. 



From the results of Leslie's experiment.-, it is apparent that the 

 power by which bodies absorb heat U directly pro]>ortin&l to that by 

 which tliry allow it to radiate from them ; :in.l that both are inversely 

 so to the power of reflecting it. When the rays of heat full "ii poli-hed 

 glaw, a Urge portion of them is absorbed ; these raise the temperature 

 of the elan, and from the surface of the latter that heat U afterward* 

 radiated in abundance ; whercsw, when the rays strike the surface of 

 polichcd metal, nearly all of them It was found how- 



ever that different metals. indi>|H 'inlrntly of their polish, have different 

 degrees of reflecting and initiating power. A mirror of tin being 

 rubbed with mercury, the reflection was increased in the ratio of 12 to 

 Jo, though the degree of polish was the same ; and a metal having lost 

 some of it* brilliancy by oxidation, the radiating power was propor- 

 tionally augmented. 



Leslie's canister was found, moreover, to produce different effects, 

 according to the thickness of the covering material on its side*. One 

 coat of jelly on the tin side produced an elevation of the thermometer 

 equal to 38, and four such coats produced an effect equal to 54 ; but 

 after a certain thickness the difference was insensible. A plate of 

 metallic leaf rendered however the radiating power aa great as a thick 

 plate of the same metal. 



Kxpcrimenta made by Count Rumford, soon after the publication of 

 Leslie's work, produced nearly the same results aa the latter ha 1 ob- 

 tained ; and the Count draws from them several useful conclusions. 

 He observes ('Phil. Trans.', 1804) that when we would confine heated 

 substances, solid or fluid, in a vessel, the surface of the latter should 

 be highly polished ; on the other hand, if the object be to cool the 

 substances, the surface should be painted or varnished, or be covered 

 with a soft coating which is not metallic. Also, in warming apart- 

 ments by steam, the intention being to promote radiation as much 

 as possible, the tubes conveying the steam should be unpolished or 

 painted. 



In pursuing the views of Scheele concerning the transmission of 

 heat through screens, Leslie found that a sheet of tin interposed 

 between the canister and the mirror entirely intercepted the heat, and 

 a plate of glass nearly so. Employing two sheets of tin, each of which 

 had one side covered with black varnish, while the other side was 

 polished, the following were the results : when the .varnished sides 

 were in contact, little heat passed through, evidently because one of 

 the exterior sides was not well adapted to receive the caloric, nor the 

 other to nuliate the little whieh might have been acquired ; but when 

 the varnished sides were placed exteriorly, the quantity whieh passed 

 through was considerable, the varnish enabling one plate easily to 

 absorb and the other as easily to radiate the heat. 



The reality of the transmission of heat through glass plates, whieh 

 both Leslie and Brewster appeared to doubt, from the difficulty of 

 distinguishing between heat BO transmitted and that which radiates 

 from the.glass after having been for a time absorbed in it, is now sup- 

 posed to be sufficiently established by the experiments of MM. Prevost 

 and De la Roche (' Annals of Philos.,' 1S03), and also by the researches 

 of M. Melloui. (' Annales de Chimie,' zlviii., &c.) The first, in order 

 to ascertain the fact, received on a thermometer the rays from a heated 

 body after passing through screens of glass which were renewed ao 

 - often that they had not time to become heated. The second suffered 

 a thermometer to rise to ita maximum by the rays from a heated 

 body when a transparent glass screen was interposed ; and again, when 

 there was interposed a glass screen blackened so as to prevent the 

 radiant heat from passing, the excess of the rise of the thermometer in 

 the former case above that in the latter gave evidently the effect pro- 

 duced by the radiant heat alone. The conclusion at which the experi- 

 menters arrive is, that the quantity of heat which radiates I 

 glass is so much the greater as the temperature of the source of the 

 heat is higher. M. Mclloni found that of 100 rays incident on the 

 same plate of glass from an oil-lamp, from red-hot platinum, from 

 copper heated to 734, and from the same heated to 212, the numbers 

 were 77, 57, 34, and 12, reqxxtivcly. He has also ascertained that all 

 bodies which have the power of transmitting heat are in general more 

 or leas transparent ; and that rock-salt is the only known substance in 

 which all the radiant heat falling on it U cither reflected or transmit tc< I, 

 whether the temperature of the heated body be low or high. 



licrmacy does not depend upon transparency, either in * 1 

 in liquid*. An iron ball heated to 400, placed midway between the 

 blackened balls of a thcnnoscope, will not cause a rise in the liquid. 

 since each bulb receives an equal share of heat ; but if a plate of rock- 

 salt be interposed between the iron and one of the bulbs, and a plate 

 of glass of equal thickness between the hot ball an 1 the other bulb, 

 ilb next the rock-salt will rise in temperature much more rapidly 

 than the one next the glass, although both screens are e< 

 |ually transparent to light. So aNu, if four li 

 transparent receive each 100 rays from an argand lamp. \vm 

 transmit only 11, siilphiuic acid 17, ether 21,:>n<l oil of turpi-in 

 while chloride of sulphur, which \ f a reddish colour, allow* 



to pas*. The diathermacy of different solids and liquids for heat from 

 different sources is stated in a tabulated form in such books as M 

 ' Chemical Physics.' We do not repeat them here ; but state a i 

 the more modem results given in that work in connection with tin- 

 subject 



Knoblauch has found that very thin metallic films are diatl,. 

 thus presenting an analogy to their limited transparency to light. 

 Hold and silver transmit certain calorific rays more freely than others, 

 while plat ilium appears to transmit all the rays with equal facility. 

 Tyndall has found that the gases exert different degrees of absorptive 

 'ii the rays of heat; thus, coal gas Is more abs. 

 heric air. and the vapour of ether considerably moi 

 of the bisulphide of carbon. Certain rays are more powerfully absorbed 

 by the colourless gases than others. Thus coal gas arrests th> 

 rays from a source below a visible red heat much more p< 

 it absorbs the rays of the lime-light after they have traversed a thin 

 layer of water. 



Diathennacy is not dependent on transparency ; thus block g] 

 plates of smoked quartz, so opaque that the midday sun is 

 visible through them, are much more diathermanous than plates of 

 alum; and plates of quartz smoked so as to be opaque arc more 

 diathermanous than when clean and transparent. Diathermacy 

 seems to be more influenced by mechanical arrangement than by 

 chemical composition. Common table salt is adiathermic : a solution 

 of rock-salt is scarcely superior to pure water in diathermacy. 

 solution of alum is equal to one of rock-salt. The latter subst;: 

 the solid colourless form approaches perfection in diathermacy as 

 already noticed. But even this has lately been found to absorb c 

 of the rays of heat somewhat more freely than others. All 

 bodies examined by Melloni transmit a quantity of heat which 

 with the nature of the source, just as coloured media t: 

 quantities of light depending on the nature of the colour. Thus if a 

 pencil of solar light fall upon red glass, red rays alone will b> 

 mitted, the rest being absorbed. This property, whereby different 

 partially diathermanous media absorb different constituents of the 

 thermic pencil, has been termed thermo-chrosit or "calorific tint. 

 as a pencil of solar light incident first on a red plate, and then . 

 of bluish green, will be totally absorbed, the second plate ab> 

 what the first transmitted, so a ray of heat may be entirely al>- 

 by causing it to pass through two media, one of which 

 rays transmitted by the other. 



By letting the calorific rays pass first through one screen and then 

 through two, M. De la Roche found that, in passing through the 

 second screen, the rays suffered less diminution of intensity than in 

 passing through the first; and the fact is considered :; ]>i"v:, 

 some calorific rays experience more difficulty in ]is-inx through glass 

 than others ; consequently that, like light, radiant hc.it i> of <ii 

 kinds. He also observed that a thick plate of glass allows a smaller 

 quantity of radiant heat to pass through than a thin one, and t! 

 difference is so much the less as the temperature of the heated body is 

 higher : and it is inferred that, since radiant heat becomes more 

 capable of penetrating glass as the temperature increases, till tli 

 becomes luminous, heat is only a modification of light. 



The theory of radiant heat is intimately connected with t 

 the cooling of bodies; and the first effort to determine the laws 

 n I.-itin^ to this subject was made by Sir Isaac Ncv.t .n. who, from 

 theoretical considerations, inferred that when a heated body is c 

 to a constant cooling cause, as the uniform action of a current 

 it ought to lose at each instant a quantity of heat proportional to the 

 excess of its temperature above that of the surrounding air, and 

 consequently that its losses of heat in equal intervals of time should 

 form a decreasing geometrical progression. But it is no\v known that 

 this law holds good only when the differences of temperature do not 

 exceed 40 or 5U degrees, and its inaccuracy at high tempi'nvtur 

 first pointed out by Martine (174"). 



From the experiments of MM. Dulong and Petit, it U found that, if 

 it were possible to obtain the absolute loss of heat w Inch a body in 

 vacuo experiences (or that loss which would take place if there v. 

 restoration of heat from surrounding bodies), the velocities of < 

 estimated by the diminutions of temperature indicated by 

 thermometer, would increase inn geometrical progression when the 

 temperature of the heated body increases in an arithmetical pro- 

 gression ; and further, that the ratio of the former 

 ( = 1'0077) would lie the same for all bodies, whatever might 

 state of their surfaoes f and whether the temp 

 remained constant or increased in an arithmetical progression 

 on taking account of the quantity of heat Bent back at every instant by 

 the surrounding medium (a quantity which will bo constant if the 

 temperature of that medium does not vary), it is found that the 

 velocities of cooling in vacuo increase, for equal increments of tempe- 

 rature, in a geometrical progression whose terms are d by a 



it, quantity, which quantity varies in a geometrical prop 

 \\hentlie temperature of the medium varies in an arithmetical pro- 

 gression. Hy direct, experiments on the cooling of heai 

 air ami hydrogen gas, Dulong and Petit determined what 



sccrtained by an indirect [; iy, that tin) loan of he,; 



when a body is in contact with a ga 



"ly. Tlify found also, by experiments on dilated air ami 



