HEAT. 45 



under more unexceptionable circumstances ; and without 

 saying that radiant heat is the initial force in this case, we 

 have evidence, by the superficial change which takes place 

 in bodies closely approximated, that some molecular change 

 is taking place, some force is called into action by their 

 proximity, which produces changes in matter as it expends, or 

 rather transmits itself; and, therefore, is not a force without 

 molecular change, as the supposed contact force would be. 

 The force in this, as in all other cases, is not created, but 

 developed by the action of matter on matter, and not annihi- 

 lated, as it is shown by this experiment to be convertible into 

 another mode of force. 



To say that heat will produce light, is to assert a fact 

 apparently familiar to everyone, but there may be some reason 

 to doubt whether the expression to produce light is correct in 

 this particular application ; the relation between heat and light 

 is not analogous to the correlation between these and the other 

 four affections of matter ; heat and light appear to be rather 

 modifications of the same force than distinct forces mutually 

 dependent. The modes of action of radiant heat and of light 

 are so similar, both being subject to the same laws of reflec- 

 tion, refraction, double refraction, and polarisation, that their 

 difference appears to exist more in the manner in which they 

 affect our senses than in their modes of physical action. 



The experiments of Melloni, which have mainly contributed 

 to demonstrate the analogies of heat and light, afford a beau- 

 tiful instance of the assistance which the progress of one 

 branch of physical science renders to that of another. The 

 discoveries of Oersted and Seebeck led to the construction of 

 an instrument for measuring temperature incomparably more 

 delicate than any previously known. To distinguish it from 

 the ordinary thermometer, this instrument is called the tliermo- 

 multiplicr. It consists of a series of small bars of bismuth 

 and antimony, forming one zigzag chain of alternations 

 arranged parallel to each other, and forming together a 

 cylinder or prism; so that the points of junction, which are 

 soldered, shall be all exposed at the bases of the cylinder : the 

 two extremities of this series are united to a galvanometer 

 that is, a coil of wire surrounding a freely-suspended mag- 



