CHAPTER ITI. 



HOW THE HEAT IS KEPT UP. 



IT will be necessary for me, at this part of my subject, to 

 recall to mind a few points in the doctrine of heat. In 

 ordinary language we occasionally employ the words heat 

 and temperature indifferently, but when we are considering 

 such a question as that now before us we must be more 

 particular about our words. The heat contained in a 

 body is one thing, and the temperature shown by that 

 body is a different thing ; in fact, it may well be that of 

 two bodies, the one with the higher temperature contains 

 less heat than that which shows the lower temperature. 

 You might take a pound of iron and a pound of water, 

 each at the same temperature, and then give them both 

 equal additions of heat. The iron will rise to a tempera- 

 ture far higher than the water. A foot-warmer filled with 

 hot water will give out far more heat than if it contained 

 merely the same weight of iron heated to the same tem- 

 perature. A pound of mercury and a pound of water at 

 the same temperature, as shown by a thermometer, would 

 contain very different quantities of heat, the difference being 

 very much in favour of the water. If equal weights of 

 mercury and water had very unequal temperatures, and 



