PHYSICAL SCIENCE. 31 



the surface of our own globe; with the strong presumption 

 that the same methods will multiply these or other ana- 

 logous results. Discovery of this kind cannot lie dormant, 

 and is indeed already pressing forwards. It is prolific of 

 present inferences, and of promise for the future ; and affords 

 the best illustration of that progress and spirit of physical 

 science, which it is our especial object to describe. 



Having dwelt thus long on solar light, we may speak more 

 briefly of that other form of elemental force, the Solar Heat, 

 if indeed we can define this as distinct from the former, 

 with which it is so closely blended in passing through space. 

 Endless questions press upon us here, and discovery resolves 

 one only to bring others into view. We have obtained innu- 

 merable facts regarding Heat in its various forms, and yet 

 are ignorant of its intimate nature or even existence, apart 

 from the matter by which its action is expressed. In all 

 these questions we have still continually before us the ab- 

 stract conception of forces or powers, acting in constant 

 relation to each other in the material world a relation 

 capable in every case of being numerically expressed. The 

 phenomena of light and heat stand peculiarly in this close 

 connection, both in their origin in the Sun, and in all their 

 secondary relations. We may seemingly separate the elements 

 thus named by the intervention of certain kinds of matter, as 

 in the familiar example of the glass screen, or in Melloni's 

 more delicate and complex experiments. But such separation 

 really tells us little more than does the disseverment of light 

 into different colours by refraction through a prism ; while all 

 recent enquiries, especially those of Forbes and Melloni, have 

 shown us that radiant heat, in common with light, is strictly 

 subject to those great physical laws which are recognised as 

 the basis of the undulatory theory; the connection being 

 such that heat has not only been polarised and depolarised, 

 according to the terms of this theory, but the phenomena of 



