40 LECTURES TO SCIENCE TEACHERS. 



of ice. I shall apply a weight in order to press this 

 junction against the ice, and when I give a certain amount 

 of pressure to the wire the pressure lowers the melting 

 point of the ice, and since this ice cannot get cooler it 

 must melt. The consequence of this is that the wire is 

 gradually eating into the ice, and the greater weight we 

 apply the more it will eat into the ice, but as soon as the 

 wire passes through the ice the pressure by the wire is 

 relieved and consequently the water will solidify, and thus 

 we shall have the phenomenon of the piece of wire cutting 

 its way into the ice and the ice being solidified in the 

 channel through which the wire has passed. But more 

 than that, the temperature at the place where this eating 

 is being accomplished is changed, the temperature is 

 lowered by the application of this process, and I hope 

 by the arrangement of wires with this thermo-electric 

 junction that we may just be able to see the alteration of 

 temperature when we exert a considerable pressure upon 

 the wire. 



Now I will show you the experiment of stretching India- 

 rubber, which is another almost as striking an illustration 

 of the second law of thermo-dynamics. This India-rubber 

 is suspended from a point above in the interior of a tube. 

 The weight which stretches it has its base in contact with 

 the short arm of this lever which points to zero on the 

 scale. I can heat the India-rubber by holding a flame at 

 the bottom of the tube in which it is suspended, and we 

 shall find that the India-rubber contracts, as indicated by 

 the motion of the pointer on the scale ; so that there is 

 not the least doubt that the application of heat to stretched 

 India-rubber causes it to contract. In connection with this 

 there are several curious considerations, to which I would call 

 your attention. Here is one point which has not yet been 

 investigated. This stretched India-rubber contracts when 

 you apply heat to it, but if it were not stretched we know 

 it would expand by the application of heat. What then is 

 the amount of stretching required in order that the India- 

 rubber might neither expand nor contract? This would 

 probably depend on the temperature and a variety of other 

 causes ; but it would be extremely interesting if some one 

 would investigate the subject. 



I regret that I have been able to-day to enter only so 



