EXPENDITURE AND SUPPLY. 19 



this globe. Suppose that enough of this coal were to be 

 burned every day to supply a quantity of heat equal to 

 that poured forth by the sun in the same time. It is then 

 a matter of simple calculation to find how long a sun so 

 constituted could last as a source of light and heat. It can 

 be easily shown that on this supposition and at the present 

 rate of expenditure all power of radiation would be totally 

 exhausted in about three thousand years. We have, how- 

 ever, the best reasons for knowing that the sun has already 

 dispensed his beams for a far longer period than that just 

 stated. Hence we learn that the supply of the sun's heat 

 cannot be due to the combustion of any substance with a 

 composition similar to coal. 



It is, however, a reasonable question whether there 

 may not be some extraordinary, and to us unknown, 

 elements present in the sun which in the course of their 

 chemical union generate far more heat than those elements 

 with which we are acquainted, so that by the combustion 

 of these materials the solar energies might be recuperated. 

 From all we know of the composition of the celestial 

 bodies, it does not seem in the least degree probable that 

 any materials exist which possess the necessary qualities. 

 In fact, it is one of the triumphs of modern science to have 

 identified to a considerable extent the materials of which 

 the sun is composed with the familiar elements in our 

 earth. By the aid of spectrum analysis it has been dis- 

 covered that upwards of thirty of the elements on the 

 earth are present also in the sun. Among these we may 

 specially draw attention to iron, which appears to be one 

 of the most widely distributed of all the elementary bodies. 

 Some hundreds of lines in the solar spectrum have been 



