28 LECTURES TO SCIENCE TEACHERS. 



was filled to a certain line with water, and above that was 

 placed another cylinder containing a freezing mixture. 

 Through the centre of this a hole was pierced which 

 allowed a rod to descend with a cylinder on its base, and 

 by means of raising the rod we could bring the cylinder in 

 contact with the ice which was formed below, and so by 

 reading the position of the rod on the scale could actually 

 measure at different intervals the amount of ice which had 

 been formed. The experiment proved rather laborious, and 

 sometimes extended over a period of twenty-four hours, 

 almost unceasing attention being required the whole time ; 

 but the results were extremely accordant, and it was found 

 that the formation of the ice was exactly in accordance with 

 theory, and, what was very fortunate, by employing fresh 

 snow it was found possible to keep the temperature per- 

 fectly constant ; simply changing the snow every hour or 

 two, the temperature was found not to alter at all. You 

 may notice the material point of this is the way in which 

 the temperature is kept always constant at the lower 

 surface. There it is always 0., because it is always at 

 the boundary between the ice and the water. The result 

 of this experiment was to show that the conductivity of 

 ice in the same units which we have employed before, is 

 00223. M. Lucien de la Rive, by an extremely ingenious 

 method, but which was not quite so correct, arrived at the 

 result of the conductivity of ice as '00230, and I think the 

 remarkably close accord between these two results and the 

 complete accordance between a large number of experiments 

 made with this apparatus may make us feel certain that 

 the conductivity of ice is one of the best determined quan- 

 tities we know. Consequently, we have as standards of 

 the conductivity of heat the kinds of soil I have spoken 

 of, also the conductivity of iron bars and the conductivity 

 of ice, and these are the substances about which we know 

 most. 



A large number of experiments have been made by 

 different philosophers to determine the conductivity of 

 a number of bad conductors, but it must be confessed that 

 none of their methods have been entirely unobjectionable, 

 and in few can very great confidence be placed. Experi- 

 ments have been tried following out this idea of freezing 

 water through some bad conducting siibstance, and measur- 



