106 THE PROGRESS OF SCIENCE n 



electric energy, and they open up a vast field for 

 future inquiry into some of the deepest problems 

 of physics. Other important steps, in the same 

 direction, have been effected by investigations into 

 the absorption of radiant heat proceeding from 

 different sources by solid, fluid, and gaseous bodies. 

 And it is a curious example of the interconnection 

 of the various branches of physical science, that 

 some of the results thus obtained have proved of 

 great importance in meteorology. 



The existence of numerous dark lines, constant 

 in their number and position in the various regions 

 of the solar spectrum, was made out by Fraun- 

 hofer in the early part of the present century, but 

 more than forty years elapsed before their causes 

 were ascertained and their importance recognised. 

 Spectroscopy, which then took its rise, is probably 

 that employment of physical knowledge, already 

 won, as a means of further acquisition, which most 

 impresses the imagination. For it has suddenly 

 and immensely enlarged our power of overcoming 

 the obstacles which almost infinite minuteness on 

 the one hand, and almost infinite distance on the 

 other, have hitherto opposed to the recognition of 

 the presence and the condition of matter. One 

 eighteen-millionth of a grain of sodium in the flame 

 of a spirit-lamp may be detected by this instru- 

 ment ; and, at the same time, it gives trustworthy 

 indications of the material constitution not only of 

 the sun, but of the farthest of those fixed 



