CH. XXXV. HE A T A VI BRA TION. 353 



them together made them melt without any warmth being 

 brought near them. In this case, as he said, no one could 

 think that the heat came out of the ice, for ice holds less 

 heat than water; and in order to be quite sure that it did 

 not come out of the air, he made a second experiment. He 

 took a small piece of ice and put it in a machine under an 

 air-pump, by -means of which he drew out all the air; then 

 he set his machine to work so that it rubbed against the ice, 

 and in this way he melted the whole lump, without any air 

 being present. 



Heat a Vibration. From these experiments Davy came 

 to the conclusion 'that heat is a peculiar motion, probably a 

 vibration of the corpuscles (that is the little particles) of 

 bodies, tending to separate them.' Thus for example, when 

 you put a saucepan full of water on the fire, the quivering 

 motion which is going on in coals as they burn passes into 

 the iron of the saucepan, and through it to the water. Im- 

 mediately all the little particles of which the water is com- 

 posed are pushed asunder as if they were trying to get away 

 from each other ; but as they are still held together by the 

 force of attraction, they vibrate to and fro, struggling more 

 and more to get free, and it is this motion which causes in 

 us the feeling of heat when we come in contact with it. 

 Then, if a thermometer be placed in the water, the vibration 

 passes on through the glass of the tube into the mercury, and 

 the particles of mercury are also set in motion, and so the 

 mercury swells and rises in the tube. 



Tho Cause of Latent Heat explained. And now, if 

 you will look back for a moment to Chapter XXVIIL, and 

 read again about the ' latent heat ' which puzzled Dr. Black 

 so much, you will see how beautifully it can be explained 

 by this theory that heat is a kind of motion. You will 

 remember that, however much heat he put under a piece 



