THERMAL CONDUCTIVITY. 



BY PKOFESSOR FORBES. 



THIS morning it is my duty to explain to you as far as 

 possible the facts that have already been learnt as to the 

 nature of the conduction of heat, and to illustrate the 

 progress of our knowledge in this subject by the original 

 apparatus which has been used by different experimenters. 

 You are all aware that the general equilibrium of the 

 temperature is maintained in virtue of three different 

 processes. Different bodies at different temperatures have 

 a general tendency gradually to arrive at a certain uniform 

 temperature, and this is arrived at by the process of conduc- 

 tion, by that of convection, or by that of radiation. Radia- 

 tion consists in a transmission of the state which we 

 call heat through space without affecting any material 

 object intermediate between the warm and the cold object. 

 That is, the sun heats the earth by radiation, and so a 

 general equilibrium of temperature is gradually being 

 established between " the sun and the planet. Secondly, 

 if we have a bar of any metal, and place one end of it to 

 a fire, the other end will gradually become hot. This is 

 quite a different means of equalising the temperature. In 

 this case the particles of the body intermediate between 

 the one end and the other have gradually got warmer and 

 warmer, and thus the heat is gradually transmitted from 

 point to point of the metal. This is the nature of true 

 conduction. In the third place, if the hot and cold bodies 

 be in a liquid whose parts are capable of moving about 

 freely amongst themselves, then, by the increase of tem- 

 perature which the liquid which surrounds "the hot body 

 acquires, that part of the liquid is rendered lighter, so 



