ON THE AGE OF THE SUN'S HEAT. 355 



substance is very much like the earth's. Stokes's 

 principles of solar and stellar chemistry have 

 been for many years explained in the University 

 of Glasgow, and it has been taught as a first result 

 that sodium does certainly exist in the sun's 

 atmosphere, and in the atmospheres of many of 

 the stars, but that it is not discoverable in others. 

 The recent application of these principles in the 

 splendid researches of Bunsen and Kirchhof (who 

 made an independent discovery of Stokes's theory) 

 has demonstrated with equal certainty that there 

 are iron and manganese, and several of our other 

 known metals, in the sun. The specific heat of 

 each of these substances is less than the specific 

 heat of water, which indeed exceeds that of every 

 other known terrestrial body, solid or liquid. It 

 might, therefore, at first sight seem probable that 

 the mean specific heat l of the sun's whole sub- 



1 The " specific heat " of a homogeneous body is the quantity of 

 heat that a unit of its substance must acquire or must part with, to 

 rise or to fall by i in temperature. The mean specific heat of a 

 heterogeneous mass, or of a mass of homogeneous substance, under 

 different pressures in different parts, is the quantity of heat which 

 the whole body takes or gives in rising or in falling i in temper- 

 ature, divided by the number of units in its mass. The expression, 



A A 2 



