CHEMISTRY. 



JMptw* tin-l'oil, the rite will be reduced to 20. Hence it 

 follows, that those bodies that radiate heat bet im- 

 bibe it best, and that those which radiate worst im- 

 bibe we i 



The contrary holds with respect to reflectors ; 

 thjic substances reflect Ijftt whicli radiate n-unt, and 

 vice tvisa. The following Table exhibits the com- 

 parative goodness of different substances as reflectors, 

 according to Mr Leslie's experiments. 



Brass, ............. 100 



Silver, ............. X) 



Tin-foil, ............ 85 



Block-tin, ........... 80 



Steel, ............. 70 



Lead, ............. 60 



Tm- fuil, softened by mercury, 10 

 Glass, ............ 



Ditto, coated with wax or oil, 



power. 



10 

 5 



When the polish of the reflector is destroyed by 

 rubbing it with sand paper, the eflect is very much 

 diminished. 



Radiation takes place only in elastic mediums. It 

 is destroyed altogether by plunging the apparatus 

 under water. It is nearly the same in air and in 

 hydrogen gas, and does not seem to be affected by 

 the nature of the elastic medium. It is diminished 

 by rarefying the surrounding air. 



Effect of a When a substance is interposed between the hot 

 vreea. canister and the reflector by way of screen, the ef- 

 fect is varied by its distance from the canister, by its 

 thickness, and by the nature of its surface. The 

 nearer it is to the canister, the less is the radiation af- 

 fected ; at a certain distance all radiation is destroy- 

 ed. The thinner the screen, the less of the heat is 

 interrupted ; the radiation slowly diminishes as the 

 thickness of the screen increases. When the surface 

 of the screen radiates heat well, the radiation is much 

 less interrupted than if it radiate heat ill. Thus, if 

 the screen be glass, the thermometer still rises a cer- 

 tain number of degrees, but if it be tin- foil the ther- 

 mometer does not rise at all. From these phenomena 

 it cannot be doubted that the screen, in all cases, in- 

 tercepts the whole of the heat, that it becomes hot- 

 ter itself, and then radiates heat from its surface. 



Such are the phenomena of the radiation of heat 

 as far as they have been investigated. It follows 

 very different laws from light in its radiation. Mr 

 Leslie has endeavoured to show, that heat is not in 

 reality radiated, but that it is propagated with the 

 velocity of sound by means of undulations or pulses 

 of air. This opinion he has supported with much 

 ingenuity. But as he has brought no other evi- 

 dence for its truth, but its convenience in explaining 

 the phenomena, and ai it is at variance with the direct 

 experiments of other philosophers, it cannot be ad- 

 mitted till direct evidence be brought forward in sup- 

 port of it. 



2. Postage of Caloric through Bodies. 

 Caloric, we have seen, is incapable of radiating 

 '' lrou gl> *d*d bodies, yet it is well known that all 

 bodies are pervious to it. Through them, then, it 



throurh' C 



' 



must make it* way in a different manner. In general EKmtwt 

 it pastes very (lowly through them, and when it pas . f 



ses in this way, it it said to be conducted through ^ 



them. "V" 



Bodies seem to conduct heat in consequence of 

 their affinity for it, and of the property which they 

 have of combining with an indefinite number of doses 

 of it. Hence the reason of the slowness of the pro- 

 cess. Hence also the reason why the temperature 

 of the body through which it passes diminishes equal- 

 ly as we advance from the source of heat to the other 

 extremity. 



Bodies vary in their power of conducting heat. Conducting 

 The metals are the best conductors of heat of alt powtr of 

 known bodies. From the experiments of Ingenhousz <liei. 

 it follows, that silver and gold are the best conductors 

 among the metals. Copper and tin follow next in 

 order, and platinum, iron, steel, and lead are nearly 

 equal among themselves, but much inferior to the- 

 others. Stones come next after the metals, but they 

 are greatly inferior to them. Bricks are still inferior 

 to stones. Glass also is a bad conductor. Henee 

 the facility with which it cracks when suddenly heat- 

 ed or cooled. 



Dried woods are considerably inferior to glass. 

 From the experiments of Meyer, it appears that they 

 differ considerably among themselves. Charcoal in 

 also a bad conductor. According to the experiments 

 of Morvcau, its conducting power is to thr.t of fine 

 sand as 2 to 3. Feathers, silk, wool, and hair are 

 still worse conductors than any of the preceding sub- 

 stances. Hence the reason that they answer so well 

 as articles of clothing. 



It is admitted on all hands, that all solid bodies are 

 conductors, for they allow heat to pass through them. 

 Liquids also allow heat to pass through them. But 

 they differ from solids in the mobility of their par- 

 ticle*. When a particle of a liquid is heated, it be- 

 comes specifically lighter, and therefore rises. Count 

 Rumford has endeavoured to prove, that heat passes 

 through liquids only in consequence of the motion of 

 their particles, and thafif the particles of liquids were 

 immoveable, heat could not pass through them at all. 

 Hence he infers, that liquids are, in reality, non-con- 

 ductors of caloric. But his experiments arc not such 

 as to warrant the conclusions he has drawn. The 

 subject has been investigated by different chemists, 

 with all the requisite care. It has been shown that 

 heat can make its way downwards through liquids, 

 in which case their particles cannot be supposed to 

 move. Hence it follows, that they are all conduc- 

 tors. They are, however, very bad conductor?. 

 Water, for example, conducts heat much- worse than 

 any of the dry woods. 



The gases arc still worse conductors than liquids. 

 They differ a good deal among themselves in their 

 conducting puwer. Hydrogen gas appears to be the 

 best, and carbonic acid the worst conductor. From 

 the experiments of Leslie, it appears, that hydrogen 

 conducts four times as well as common air. The 

 conducting power of gases is diminished by rarefac- 

 tion, by vapours of all kinds, and every thing which 

 has a tendency to dilate air. The following '1 able by 

 Mr Dalton, exhibiting the time taken by a thermo* 



