ii2 CORRELATION OF PHYSICAL FORCES. 



transmitted, reflected, refracted, or polarised, can we view 

 that as an affection of ordinary matter, and when the same 

 effects take place with light, view the phenomena as produced 

 by an imponderable ether, and by that alone ? To account 

 for the phenomena of radiant heat the existence of an ether 

 has been considered as necessary as for those of light, but we 

 find that even with radiant heat the medium through which it 

 is transmitted exercises an all-important influence on its ab- 

 sorption and transmission. In the year 1848 I showed that 

 the variation in the cooling effect of different gases in 

 which an ignited platinum wire was immersed was very re- 

 markable, and appeared to depend more on their chemical and 

 molecular constitution than on their physical characteristics, 

 such as density, &c. Dr. Tyndall, in an elaborate series of 

 papers, has shown that there are remarkable differences in the 

 degree of absorption and radiation of heat by gases and 

 vapours. These results are, to my mind, strongly corrobora- 

 tive of the view which dispenses with the hypothetic ether, 

 and regards the imponderables as modes of motion of matter 

 in different states of attenuation or of aggregation. 



An objection that immediately occurs to the mind in re- 

 ference to the ethereal hypothesis of light is, that the most 

 porous bodies are opaque ; cork, charcoal, pumice-stone, dried 

 and moist wood, &c., all very porous and very light, are all 

 opaque. This objection is not so superficial as it might seem 

 at first sight. The theory which assumes that light is an 

 undulation of an ethereal medium pervading gross matter, 

 assumes the distances between the molecules or atoms of mat- 

 ter to be very great. Matter has been likened by Democritus, 

 and by many modern philosophers, to the starry firmament, 

 in which, though the individual monads are at immense dis- 

 tances from each other, yet they have in the aggregate a 

 character of unity, and are firmly held by attraction in their 

 respective positions and at definite distances. Now, if matter 

 be built up of separate molecules, then, as far as our know- 

 ledge extends, the lightest bodies would be those in which the 

 molecules are at the greatest distances, and those in which any 

 undulation of a pervading medium would be the least inter- 

 fered with by the separated particles such bodies should 



