328 IIUMBOLDT'S COSMOS : 



place, demonstrable only in their effects ; but through these 

 various effects demonstrable with mathematical certainty. 

 The sciences of Optics, of Chemistry, and of Electricity, 

 furnish us with numerous examples of this nature, had we 

 room to quote them. Apart, indeed, from all direct proof, 

 we see no difficulty in admitting an ethereal medium (we use 

 this term in default of a better) as occupying the regions of 

 space. Without such admission, in fact, it is hard to con- 

 ceive how the physical forces or actions which we know to be 

 transmitted to us from other bodies in the universe around, 

 should have existence, or be capable of this transmission. 

 Whether gravity be a force per se or, as some have sought 

 to prove, a power residual upon the balance of other atomical 

 attractions and repulsions equally must we reason upon it 

 as a material transmission of power, if we reason at all. 

 The phenomena of light, on the emission theory, presume a 

 luminous matter capable of pervading space from its utter- 

 most depths. On the undulating theory, now generally 

 admitted, they require and are thoroughly consistent with 

 the notion of an ethereal medium capable of transmitting 

 those waves, the relative magnitude, velocity, and interference 

 of which produce all these wonderful results. Heat and light 

 have close kindred in their physical conditions, and future 

 research will probably render this association closer, in rela- 

 tion to a common cause. Though we have some observations 

 from Bessel and Sir J. Herschel on the apparent subjection 

 of the tails of certain comets to a polar force independent of 

 gravitation, which may be electrical, we do not yet possess any 

 absolute evidence of electricity under its magnetic or other 

 forms being transmitted to us through space. Recent en- 

 quiry, however, has brought us closely upon this fact ; and 

 the intimate relation, by mutual convertibility, of all these 

 great agents, makes it probable that time will disclose their 

 common connection with some intervening medium among 



