Ill 



ON RADIANT HEAT IN RELATION TO THE COLOR AND 

 CHEMICAL CONSTITUTION OF BODIES 1 



ONE of the most important functions of physical 

 science, considered as a discipline of the mind, is 

 to enable us by means of the sensible processes 

 of Nature to apprehend the insensible. The sensible proc- 

 esses give direction to the line of thought; but this once 

 given, the length of the line is not limited by the bounda- 

 ries of the senses. Indeed, the domain of the senses, in 

 Nature, is almost infinitely small in comparison with the 

 vast region accessible to thought which lies beyond them. 

 From a few observations of a comet, when it comes within 

 the range of his telescope, an astronomer can calculate its 

 path in regions which no telescope can reach: and in like 

 manner, by means of data furnished in the narrow world 

 of the senses, we make ourselves at home in other and 

 wider worlds, which are traversed by the intellect alone. 

 From the earliest ages the questions, "What is light?" 

 and "What is heat?" have occurred to the minds of men; 

 but these questions never would have been answered had 

 they not been preceded by the question, "What is sound?' 1 

 Amid the grosser phenomena of acoustics the mind was 

 first disciplined, conceptions being thus obtained from direct 



1 A discourse delivered in the Royal Institution of Great Britain, January 

 19, 1866. 

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