m. i.] MATTER, MOTION, AND FORCE. 293 



that actinism is transformable into all the other modes of 

 motion. In Mr. Grove's celebrated experiment, where a 

 daguerreotype-plate is ingeniously connected with a galvano- 

 meter, 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 appearance 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 sup- 

 position is unthinkable, — namely, the supposition that the 

 one kind of motion has been annihilated, while the other 

 kind has been created for the occasion. Inductive proof is 

 furnished by the fact that wherever it is possible to measure 

 both the amount of motion that disappears and the amount 

 that appears in its place, the two quantities are always found 

 to be equal. Thus the molar motion implied in the fall 

 of 772 pounds of matter through one foot of space, will 

 always raise the temperature of a pound of water just one 

 degree of Fahrenheit. And similar quantitative correlations 

 have been established among other modes of motion. 



The second corollary from the persistence of force asserts 

 that the direction of motion in any case is always the 

 resultant between the lines representing respectively the 

 greatest traction and the least resistance exerted by the 

 forces upon which the motion depends. In any plexus of 

 forces whatever, the resultant of all the tractive forces in- 

 volved will be the line of greatest traction ; the resultant of 

 all the resisting forces will be the line of least resistance ; 

 and the direction of motion in the resultant of this final pair 

 of resultants follows directly from the persistence of force. 



