MATTER, MOTION, AND FORCE 



solar spectrum, which manifest themselves 

 chiefly in causing chemical changes, and the 

 lower rays, which are cognized as violet light, is 

 genericallv the same as the difference between 

 these and the still lower rays which are cog-' 

 nized as indigo, blue, green, yellow, orange, or 

 red light ; and the same is true if we descend 

 to those still lower rays which are recognized 

 only by their thermal effects. If we call the 

 energy manifested in the solar beam by the 

 general name of actinism, we may say that ac- 

 tinism is transformable into all the other modes 

 of motion. In Mr. Grove's celebrated experi- 

 ment, where a daguerreotype-plate is ingen- 

 iously connected with a galvanometer, a gridiron 

 of silver wire, and a heat-registering helix, and 

 where actinism is the initial mode of motion, 

 there are obtained " chemical action on the 

 plate, electricity in the wires, magnetism in the 

 coil, heat in the helix, and [molar] motion in 

 the needles." 



In all cases where the disappearance of any 

 given mode of motion is followed by the ap- 

 pearance of some other mode, the proof that 

 there has been an actual transformation of the 

 former mode into the latter is of two kinds. 

 Deductive proof is furnished by the fact that the 

 only alternative supposition is unthinkable, — 

 namely, the supposition that the one kind of 

 motion has been annihilated, while the other 



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