RADIATION. 



BY PROFESSOR G. FORBES. 



AMONG the different physical researches which are illustrated 

 by the collection of instruments at present exhibited, theie is 

 one which has performed a very important part in forward- 

 ing the progress of a special branch of physics. 



This department, which is shown pretty fully by the 

 apparatus on the table, is the department of radiant heat, 

 y esterday we were discussing the question of the undulatory 

 theory to a certain extent, and we saw that by the measure- 

 ment of the velocity of light it was absolutely proved that 

 Newton's corpuscular theory of light was untenable, and 

 that therefore, as far as that theory was opposed to the 

 undulatory theory, the undulatory theory must be accepted 

 as true. There are a vast number of other facts, however, 

 which tend to prove the truth of the undulatory theory, both 

 directly and indirectly. Yet there is no proof so absolutely 

 certain of this theory as others, of the theory of gravitation 

 for example. In fact, in physics there are wonderfully few 

 theories which are absolutely demonstrated with the same 

 certainty as the law of gravitation. The law of gravitation 

 as stated by Newton is simply a statement of the facts 

 which have been observed, assuming the definition of force 

 as he gave it. It is not a theory it is simply a statement 

 of observation, a gathering together of a host of phenomena 

 into one general statement. Just as the whole of the pro- 

 positions included in spherical astronomy and spherical 

 trigonometry are dependent on the simple statement that a 

 sphere is a surface, every part of which is equally distant 



