HOW THE HEAT IS KEPT UP. 27 



you stirred the two together, the mixture would assume 

 nearly the temperature of the water, whatever that of the 

 mercury may have been. If the pound of water was cold 

 and the pound of mercury hot the mixture would be nearly 

 cold. If it were the other way, and the pound of water 

 was hot and the pound of mercury cold, the mixture would 

 be nearly as hot as the water. 



These illustrations suffice to show that the quantity of 

 heat in a body and the temperature which that body 

 exhibits are not related in any very obvious manner. 

 Especially is this true in the case of gaseous bodies ; 

 indeed, the connection between the temperature of a gas 

 and the heat it contains presents sometimes the startling 

 anomaly that while the gas is losing its heat it may be, 

 nevertheless, gaining in temperature. Once we take into 

 view that the sun is, to a large extent at all events, com- 

 posed of gaseous material, the difficulties as to the supply 

 of the heat necessary for its daily radiation vanish. 



The other suggested sources of sun heat which we have 

 already discussed had to be rejected, for the very simple 

 reason that they failed to explain why the sun did not 

 seem to get colder. Now we shall be able to account for 

 the fact the strange fact, some may certainly think 

 that the loss of heat does not necessarily involve a fall of 

 temperature. Indeed, it seems just as likely that if the 

 sun's temperature is changing at all at the present time it 

 may be rising instead of falling, though no doubt the 

 alteration must be extremely slow. 



A fundamental doctrine in the theory of heat tells us 

 that when heat is imparted to a body it expands. There 

 are no doubt certain exceptions, but they need not here 



