HEAT. 59 



effects, I mentioned that I considered the fact of the superfi- 

 cial change upon the surface of metals in proximity, and, a 

 fortiori^ in contact, would explain the developement of elec- 

 tricity in Volta's original contact experiment, without having 

 recourse to the contact theory, i. e. a theory which supposes 

 a force to be produced by mere contact of dissimilar metals 

 without any molecular or chemical change. I have seen 

 nothing to alter this view. Mr. Gassiot has repeated and 

 verified my experiment with more delicate apparatus and 

 under more unexceptionable circumstances ; and without say- 

 ing 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 proxim- 

 ity, 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 de- 

 veloped 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 ap- 

 parently familiar to every one, but there may be some rea- 

 son to doubt whether the expression to produce light is cor- 

 rect 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 ap- 

 pear to be rather modifications of the same force than dis- 

 tinct forces mutually dependent. The modes of action of ra- 

 diant heat and of light are so similar, both being subject to 

 the same laws of reflection, refraction, and double refraction, 

 and polarisation, that their difference appears to exist more 

 in the manner in which they affect our senses than in our 

 mental conception of them. 



The experiments of Melloni, which have mainly contrib- 



