36 THE PROGRESS AND SPIRIT OF 



or poles of the circuit, and the still more curious influence 

 upon this discharge in vacuo by a magnetic force acting 

 from without, are facts which carry us deeply into that obscure 

 part of physical science, towards which so many paths of 

 enquiry are now laid open and eagerly pursued. 



Scarcely less remarkable than the discoveries in Elec- 

 trical and Magnetic science, are those which regard the 

 material phenomena of Heat. We have already spoken of 

 solar heat and its connection with light, as transmitted to 

 us from the sun. It is difficult to separate this, even in 

 theory, from the heat unceasingly generated or lost in the 

 mutual actions and changes of all matters surrounding us on 

 earth ; and science is ever tending to annul such distinctions. 

 The main problem before us, as regards both solar and 

 terrestrial heat, is the intimate nature of Heat itself; 

 whether it be a separate element, or simply a state or con- 

 dition of matter ? The latter definition of it, as a specific 

 mode of motion of material molecules, interchangeable with 

 other modes of molecular motion, we believe to furnish the 

 best interpretation of all the various phenomena ; and even 

 of those seeming anomalies of the radiation and focal con- 

 centration of cold, which impugned or perplexed all earlier 

 theory. This view, now adopted by the most eminent natu- 

 ralists, is every day receiving fresh illustrations. The most 

 recent is that furnished by the beautiful experiments of 

 Professor Tyndall, ' On the Absorption and Eadiation of 

 Heat by Grases and Vapours;' a research, the results of 

 which are not limited to this single object, but embrace con- 

 clusions stretching far beyond ; even to the distinction of 

 simple and compound molecules, in determining the internal 

 actions, as well as qualities, of the matter by which we are 

 everywhere surrounded. We cannot do more than allude to 

 the foreign labourers in this great field Ampere, Fresnel, 



