THERMAL CONDUCTIVITY. 27 



that are the best conductors of heat are the best conductors 

 of electricity. This, so far as we can be certain, is true 

 without exception. He was now able to show this still 

 more remarkable fact, that the analogy between heat and 

 electricity holds good in this matter also, the greater the 

 temperature the less is the conductivity of metals for heat, 

 and also, as Mr. Mathieson has shown, the less is the 

 conductivity of these metals for electricity, and the varia- 

 tion is very much in the same, proportion. This simply 

 stands as a remarkable fact which has not been explained. 

 Professor Tait has been led from theoretical considerations 

 to conclude that the resistance which is opposed to the 

 passage of heat or electricity is directly proportional to the 

 absolute temperature of the substance. 



Here I will draw your attention to a similar bar of 

 German silver which has since been employed by Professor 

 Tait, also at Edinburgh, and he has made a vast series of 

 observations of the same kind as those made on iron, and 

 it is only the difficulty and labour of reduction that has 

 prevented us, as yet, from having results. These results 

 are looked forward to with great interest, because German 

 silver is a substance whose conductivity for electricity 

 varies very little, and if it were shown to vary very 

 little for heat, that would be another very great step 

 in our knowledge. 



I should have liked to have mentioned some of the 

 methods which have been employed for determining the 

 conductivity of bad conductors besides those made on the 

 conductivity of the soil ; but I shall have very little time 

 for that, and will only devote a short time to explaining the 

 method that I employed some years ago for determining, 

 as a very important quantity, the thermo-conductivity of 

 ice. The idea of the method was originally suggested by 

 Sir "William Thomson, namely, to artificially freeze water 

 and see the rate at which the ice is formed. That will 

 give you a means of measuring the quantity of heat which 

 is passed through from the water to the freezing mixture, 

 and you will have, therefore, a measure of it in the quantity 

 of ice which is formed. There was a large tin vessel made 

 to contain a quantity of snow in order to keep the instru- 

 ment at an uniform temperature. In the interior on a 

 tripod stand is a cylinder, also tin, about halfway up. It 



