EQUILIBRIUM OF TEMPERATURE. 35 



But the power of transmission, in the case of rock- salt, is 

 the same for heat from all these sources, or for heat of all 

 intensities ; 92 per cent of the incident heat being transmitted 

 by that body, whether it be the heat radiated from the hand 

 or from a bright Argand-lamp. Rock-salt stands alone in this 

 respect among diathermanous bodies. This substance may be 

 cut into lenses or prisms, and be used in concentrating heat 

 of the very lowest intensity, or in decomposing it by double 

 refraction, in the same manner as glass is employed in the case 

 of the light of the sun. Indeed, rock-salt has become quite 

 invaluable in researches upon the transmission of heat. 



It thus appears that a body at different temperatures emits 

 different species of rays of heat, which may be sifted or separated 

 from each other by passing them through certain transparent 

 media. They are all emitted simultaneously, and in different 

 proportions by flame ; but in heat from sources of lower 

 intensity, some of them are always absent. The calorific rays 

 of the sun are chiefly of the kind which passes through glass ; 

 but Melloni shows that the other species are not altogether 

 wanting. The rays of heat emitted by the sun and other 

 luminous bodies are quite different rays from the rays of light 

 with which they are accompanied. 



Of the Equilibrium of Temperature. When several bodies 

 of various temperatures, some cold and some hot are placed 

 near each other, their temperatures gradually approximate, 

 and, after a certain period has elapsed, they are found all to be 

 of one and the same temperature. To account for the produc- 

 tion and continuation of this equilibrium of temperature, it is 

 necessary to assume, that all bodies are at all times radiating 

 heat in great abundance in all directions, although their 

 temperature does not exceed or even falls below the temperature 

 of the atmosphere. Hence, there is an incessant interchange 

 of heat between neighbouring bodies ; and a general equalisation 

 of temperature is produced, when every object receives as much 

 radiated heat as it emits. 



This theory, which was first proposed by Prevost of Geneva, 

 enables us to account for the apparent radiation of cold. Cold, 

 we know, is a negative quality, being merely the absence of heat, 

 and cannot therefore be radiated. Yet, when we place a lump 

 of ice in the focus of a reflecting mirror, a thermometer in 

 the focus of the opposite conjugate mirror is chilled. To 



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