IX. 



ON RADIANT HEAT IN RELATION TO THE COLOR 

 AND CHEMICAL CONSTITUTION OF BODIES. 



ONE of the most important functions of physical sci- 

 ence, considered as a discipline of the mind, is to enable us by 

 means of the tangible processes of Nature to apprehend the 

 intangible. The tangible 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 

 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 tele- 

 scope can reach ; and in like manner, by means of data 

 furnished in the narrow world of the senses, we make our- 

 selves at home in other and wider worlds, which can be 

 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 ? " 

 Amid the grosser phenomena of acoustics the mind was 

 first disciplined, conceptions being thus obtained from 

 direct observation, which were afterward applied to phe- 

 nomena of a character far too subtle to be observed directly. 

 Sound we know to be due to vibratory motion. A vibrating 



