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error has crept in through confusion and misuse of terms. In the Introductory 

 Chapter of the last edition of his valuable treatise on Natural Philosophy, Mr. 

 Brooke refers to the numberless facts which, since the publication of the 

 fifth edition, had been observed and recorded, " all tending to confirm the 

 opinion that the various 'physical agents ' are not forms of matter, but ' modes 

 of motion.' " Mr. Justice Grove tells us that if we attempt to analyze our con- 

 ception of force, viewed as the cause of any perceived motion, we can get 

 nothing beyond some antecedent motion. And Professor Tyndall asserts that 

 " the cause of motion itself must be motion." No wonder that Mr. Mill 

 has so readily accepted the doctrine of the conservation of energy ! In 

 the eighth edition of his " Logic," published within the last month, he gives 

 us his own statement of it with marvellous but, in this case, fatal 

 clearness. Stated in a few words, the theory is as follows : — " That the 

 conservation of force is really the conservation of motion ; that in the 

 various interchanges between the forms of force, it is always motion that is 

 transformed into motion.'' (" Logic," vol. i., p. 404.) Now, to the theory of 

 the conservation of energy, I oppose the conservation of power ; the power or 

 force in the universe is a constant quantity, but the amount of motion is not 

 the same for two successive moments, while for the theory of the transmuta- 

 tion of energy I substitute that of the correlation of powers. Powers are 

 often correlated in the sense that the action of one supplies the condition of 

 the action of another. I will to move my hand, and the motion immediately 

 follows ; this is an instance of correlation. "I am the cause of my volition : 

 the voJition itself is not the cause of the action of the physical powers which 

 immediately determined the movement of the hand, but merely a remote 

 condition. The conscious volition and the observed movement of the hand 

 are merely the first and last members in a series of an unknown number of 

 efi'ects. In a conversation with Dr. Carpenter on these subjects a few weeks 

 ago, I put the question whether, in a case like the above, the motion of the 

 hand is to be considered as a transmuted volition. " Certainly not," he 

 replied, and agreed with me that the volition is merely a condition, not the 

 cause, not even a remote cause of the movement. Some of Mr. Brooke's 

 remarks on my opinions have raised another question, to which I can discover 

 no satisfactory answer. Why should he and other physicists constantly de- 

 nominate as " material " those theories which they wish to contradistinguish 

 from their own, the so-called dynamical ? Even Tyndall admits that we 

 cannot have motion without some form of " matter " moving. Hence, having 

 thrown overboard the imponderables, physicists have been compelled, with 

 the aid of the scientific imagination, to seek for some kind of material basis 

 which shall take their place ; and now we have an " ether " filling stellar 

 space, and permeating all ponderable bodies. From Professor Tyndall we 

 learn that this ether is a jelly-like substance, and is marvellously elastic ! 

 Mr. Justice Grove, however, regards the assumption of any such material 

 basis as unnecessary, for, in his opinion, it requires no great stretch of imagi- 

 nation to conceive light and electricity as motion, and not as things moving. 

 Once more, I regard the introduction of the term potential energy into the 



