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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 4 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 volition 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 
effects. 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. 11 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 
