488 LECTURE LI. 



by the mixture of their particles : hence a fluid heated on its surface 

 transmits the heat very slowly downwards, since the parts which are first 

 heated, being rendered specifically lighter, retain their situation above the 

 colder and heavier parts ; while, on the contrary, any cause of heat, 

 applied at the bottom of a vessel, very soon reduces all its contents to a 

 uniform temperature. It appears also, from some late experiments, that 

 the immediate transmission of heat within the internal parts of solids is 

 much slower than has commonly been supposed ; and it has been found 

 almost impossible to keep a thermometer, at the centre of a large and solid 

 globe of metal, at the same temperature with that of its superficial parts.* 



Besides the communication of heat by contact, it is usually, if not 

 always, emitted from the surfaces of bodies in the form of radiant heat, 

 which is thrown off in all directions, wherever it meets no obstacle from a 

 substance impervious to it, and is propagated nearly in the same manner 

 as light, and probably with the same velocity, without producing any 

 permanent effect on the temperature of the medium transmitting it. Thus, 

 a thermometer, suspended by a fine thread under the receiver of an air 

 pump, or in the Torricellian vacuum, will continue to vary its temperature 

 with that of the surrounding bodies : and in this case the whole of the heat 

 must be communicated by radiation. Mr. Leslie has discovered that the 

 quantity of heat thus emitted depends not only on the temperature, but 

 also on the nature of the surface concerned, a polished surface of tin 

 emitting only T W> or less than one eighth part as much, as the same surface 

 blackened. A surface of tin scraped with a file in one direction has its 

 powers of radiation more than doubled ; but by crossing the scratches, 

 they are reduced nearly to their original state ; and a coating of isinglass, 

 resin, or writing paper, or a glassy surface of any kind, produces an effect 

 nearly approaching to that of black paint. This radiation from a heated 

 surface, like that of light, takes place in almost equal degrees in every 

 direction ; and its magnitude is nearly independent of the nature of the 

 fluid in contact with the surface, provided however that it be an elastic 

 fluid ; for water does not seem to transmit every kind of radiant heat with 

 freedom. It appears that the radiant heat emitted by a surface of glass, or 

 of black paint, is about one third greater than that which is at the same 

 time carried off by the atmospheric air ; but that the radiation from a 

 metallic surface is only one sixth of that which the air receives. Mr. 

 Leslie has also found that the same surfaces which emit heat the most 

 freely, are also the readiest to receive it from the radiation of other bodies.t 



The solar heat radiates freely through air, glass, water, ice, and many 

 other transparent mediums, without producing any sensible effect on their 

 temperatures, and even when it is concentrated by the effect of a burning 

 mirror, it scarcely affects the air through which it passes, and other trans- 

 parent mediums but little. But the heat of a fire warms a piece of common 



* The law of conduction is not yet correctly defined. See Kelland, on the pre- 

 sent State of our Knowledge of the Laws of Conduction of Heat, Rep. of Brit. 

 Ass. 1841. The law of radiation in vacua has been determined by MM. Dulong and 

 Petit ; their experiments will be found in the Annales de Chimie, vii. 225, &c. 

 Thomson's Annals, vol. xiii. ; or in the art. Heat, in the Encyclop. Metr. 



f Inquiry into the Nature and Propagation of Heat, Lond. 1804. 



