Modes of Physical Force. i o i 



thing imponderable, be this luminous ether, or any other 

 agent of like nature. Or perhaps the residual idea is 

 that of motion rather than matter, in which case the 

 view taken approximates to that which supposes light 

 to be some peculiar rare matter emitted from, that is, 

 put in motion by, luminous bodies, as well as to that 

 other view, which is now in favour, and which differs in 

 this, that this peculiar rare matter is supposed to be, 

 not emitted; but put into a state of vibration or undula- 

 tion, that is motion, by these bodies. Between these 

 views there is agreement so far as the resolution of 

 light into motion is concerned, and no great disagree- 

 ment even beyond this point, for the material basis of 

 the more materialistic view now under consideration is in 

 fact rarefiable until it is all but matterless. Be this as 

 it may, however, there is one argument in favour of the 

 materialistic view which can scarcely be shaken, and 

 this is to be found in the facts which go to show that 

 light is inseparably connected with the other modes of 

 physical force by ties about the existence of which there 

 can be no doubt, even those of correlation. Upon this 

 point much might be said in the way of proof, but dne 

 proof which Grove himself supplies, must suffice. " A 

 prepared daguerreotype plate is inclosed in a box filled 

 with water, having a glass front with a shutter over it. 

 Between this glass and the plate is a gridiron of silver 

 wire ; the plate is connected metallically with one 

 extremity of a galvanometer coil, and the gridiron of 

 wire with one extremity of a Brequet's helix an 

 elegant instrument formed by a coil of two metals, the 

 unequal expansion of which indicates slight changes in 

 temperature the other extremity of the galvanometer 

 and helix are connected by a wire, and the needles 

 brought to zero. As soon as a beam of either daylight, or 



