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REPLY BY PROFESSOR KIRK. 
In briefly replying to the remarks offered on my paper, I must, first of all, 
acknowledge the extreme kindness of the Honorary Secretary in doing greater 
justice to the essay than I could have done myself, and also the great kindness 
of the noble President and others in speaking of it as they have done. But 
I must specially thank Mr. Warington for giving occasion to a discussion 
every way gratifying to me, and for indirectly adding so much to the force of 
the argument which I have endeavoured to advance. 
As to my long introduction, I must plead that it is only in metaphysical 
discussions that we meet with questions respecting the nature of knowledge. 
Chemists, for example, do not trouble themselves as to whether they really 
know the substances with which they experiment ; nor do astronomers inquire 
whether they see the stars or only their own sensations when observing ; but 
metaphysicians encounter such questions everywhere in their investigations 
and discussions ; and little, indeed, can be understood in the relations of 
their science until we have somewhat settled ideas as to the nature of 
knowledge. 
I must confess that I am rather astonished at Mr. Warington’s remarks on 
what he calls my “ scientific errors.” As to light being “ only a move- 
ment in the atmosphere,” my words are — “ The light is but a state of move- 
ment in the atmosphere : ” that is the light of the lighthouse of which, in the 
words referred to, I am speaking, as a movement passing over many miles of 
ocean. Light is a movement of the substance which is illuminated : it passes 
through transparent solids and liquids as it passes through transparent air. 
As to its passing easily through a vacuum, that is a matter more easily 
asserted than proved. If Mr. Warington means by “ a vacuum ” a space 
from which air is excluded, while it is full of some other substance, his state- 
ment is no doubt true as he means it ; but he will, I suspect, find it very 
difficult to secure a real vacuum by means of which to show how easily light 
passes through it Should he mean to assert that light passes easily through 
a space which is empty of all matter I fear his statement is self-destructive — 
and that, too, whether we regard the light as a movement or as a substance. 
If it is merely a movement, it cannot be where there is nothing to move ; and 
if it is a substance, that cannot be a vacuum where a substance is, even if 
only “ passing through.” The “ erroneous idea ” is in Mr. Warington’s logic 
in this case ; but I am confident that it is not “ ingrained ” there ! Nor is the 
“ ether,” which he fancies, so “ingrained.” My words in alluding to this are, 
— “ the ether, imagined as filling up the spaces between the atoms of matter.” 
This is distinct enough from so-called “ ether ” which is supposed to exist in 
the spaces between the celestial bodies, and the positions of the two stand 
wide apart in philosophy. As far back as 1842, Grove said : — “ It appears to 
