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tions to these assumptions. Mr. Spencer argues very strongly 
against both, and arrives at the conclusion “ that matter 
acts upon matter through absolutely vacant space ” (p. 60). 
And in opposition to it, Dr. C. F. Winslow writes in stronger 
terms still. He says,—' “ There was probably never a grosser 
error introduced into physical science than the ethereal theory, 
and its influence in retarding solid progress .... has been 
greater than at first appears.” “All opinions upon the 
conditions of infinite space are the merest hypotheses ; and in 
the midst of conjectures, that would be the most probably 
correct which presumed space to be a perfect vacuum.”* 
30. The fact that a mna of Dr. Tyndall’s very high character 
and culture can permit himself to affirm so positively what are 
merely shadowy conjectures, should teach all to weigh very 
accurately every scientific hypothesis, and would amply justify 
us in saying that we are not called upon to discuss the per- 
sistence of energy, while so important an element in the 
discussion is so confused and undecided. Prove a universal 
plenum, and even then the continuity of motion is only 
rendered possible ; but till that is done, we are warranted in 
asserting its impossibility, and that this grand discovery of the 
nineteenth century is not a discovery at all, or even a fact. 
31. It may be objected by some that the decrease in the 
periodic time of Encke’ s comet almost demonstrates the existence 
of such an ethereal medium. Undoubtedly the decrease of the 
time is a fact ; but the explanation was only a suggestion by 
Encke, who was not aware of any other force that could act in 
the interplanetary spaces. M. Faye has, however, shown that 
this hypothesis is, if not wholly untenable, at least very im- 
probable. He attributes the decrease to solar repulsion ; and 
we think he proves his point very satisfactorily. It is not neces- 
sary to give here all the steps of his reasoning ; it will suffice to 
state the general conclusions at which he arrives, showing, as 
they do, that even Encke’s comet does not overturn our former 
objections to this medium. “ This theory,” he states, “ puts in 
action only known forces : the attraction of the sun, — that 
which the comet exercises on its own particles, the heat of the 
sun, and the repulsion due to this heat.” Again, “ My last 
work had for its object to remove all doubts on this subject 
in showing that the resisting medium could not exist, but on 
condition of circulating round the sun according to the laws of 
Kepler .... and that its action was not constantly resistant, 
as M. Encke supposed.” He also states most truly, “That it 
* “ Force and Nature,” pp. 36, 37. 
