HOW THE HEAT IS KEPT UP. 39 



and depth of space. If these materials were drawn close 

 together in virtue of their mutual attraction, a quantity 

 of heat would be developed in accordance with the laws 

 we have already explained. It is this quantity of heat 

 for which we desire to find an illustration. Imagine a 

 globe of water so vast that its mass shall be equal to that 

 of the sun. We can attach a perfectly definite idea to the 

 quantity of heat which would be required to raise this 

 globe of water from the temperature of ice to that of 

 boiling water. It can be shown that the quantity of heat 

 generated in the contraction of the nebula from dimen- 

 sions infinitely great down to the dimensions of a sun 

 would be not less than two hundred and seventy thousand 

 times as great as that which would be required to raise to 

 boiling heat an equal mass of water. The result is a 

 remarkable one. It clearly demonstrates that in the 

 transposition of the materials so large a quantity of heat 

 is liberated as to amount to several thousand times as 

 much as the sun now contains. Need we wonder that 

 this mechanical explanation of the source of the sun's 

 heat now finds almost universal acceptance? 



