III. 



ON RADIANT HEAT IN RELATION TO THE 

 COLOUR AND CHEMICAL CONSTITUTION OF 

 BODIESt 



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 processes 

 give direction to the line of thought; but this once 

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

 boundaries of the senses. Indeed, the domain of the 

 senses, in Nature, is almost infinitely small in com- 

 parison 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 e 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 ques- 

 tion, ' What is sound ? ' Amid the grosser phenomena 

 of acoustics the mind was first disciplined, conceptions 



% 



1 A discourse delivered in the Royal Institution of Great 

 Britain, Jan. 19, 1866. 



