III. 



ON RADIANT HEAT IN RELATION TO THE 

 COLOUR AND CHEMICAL CONSTITUTION OF 

 BODIES* 



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 Na- 

 ture to apprehend the insensible. The sensible pro- 

 cesses 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 as- 

 tronomer 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 ques- 

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



* A discourse delivered in the Royal Institution of Great 

 Britain, Jan. 19, 1866. 



74 



