me that heat and light may be said to be affections, or, according to the 
nndulatory theory, vibrations of matter itself, and not of a distinct ethereal 
fluid permeating it : these vibrations would be propagated just as sound is 
propagated by virbations of wood, or as waves by water.” Professor Grove 
quotes himself (as having used this language before such ideas were publicly 
advanced) in the preface to his great essay on “ The Correlation of Physical 
Forces,” which had reached its fourth edition in 1862. In that part of this 
essay in which the distinguished author treats of light, he says, — “ Light was 
regarded by what was called the corpuscular theory, as being in itself matter, 
or a specific fluid emanating from luminous bodies, and producing the effects 
of sensation by impinging on the retina. This theory gave way to the undu- 
latory one, which is generally adopted at the present day, and which regards 
light as resulting from the undulations of a specific fluid to which the name 
of ether has been given, which hypothetic fluid is supposed to pervade the 
universe and to permeate the pores of all bodies. In a lecture delivered in 
January, 1842, when I first publicly advanced the views advocated in this 
essay, I stated that it appeared to me more consistent with known facts to 
regard light as resulting from a vibration or motion of the molecules of 
matter itself, rather than from a specific ether pervading it.” Mr. Grove 
mentions Euler as having published a somewhat similar theory.* The argu- 
ments advanced by this philosopher, apart altogether from his name, more 
than warrant us in setting this “ ether ” down as a nonentity. At the best, 
besides, it had never more than a hypothetical existence. 
It is not necessary that I should do more than notice Mr. Warington’s 
remarks on the nature of knowledge. My words on this point are to the 
effect that all our “ impressions, outer and inner, are but the raw material, so 
to speak, from which knowledge is manufactured.” Mr. Warington comes 
to the conclusion that, “ the impressions received by our senses constitute the 
whole amount of our knowledge — or (he adds), to speak more accurately, the 
materials for our knowledge.” So far, therefore, as he speaks “ accurately ,” 
Mr. Warington says just what I had said ; and it would be hypercritical in 
me to deal with what are merely his acknowledged inaccuracies. Mr. Greig 
has spoken effectively on the passive and active views of knowledge, as 
argued in this part of my subject. 
As to my remark regarding getting at what ice mean by a mode, Mr. 
Warington mistakes me, as if I had spoken of getting at a concrete idea 
from an abstract, while I speak rather of how we analyse an abstraction 
which we have conceived vaguely. Having risen from the concrete too 
hastily — or having accepted the abstract at second hand — we need to go 
back in order to clear up our thinking. 
Mr. Keddie has exploded the gunpowder element in the criticism ; and I 
need only repeat that a cake from the interior of which atmospheric air is 
excluded, is surely a very different substance from a powder with which it is 
* Grove’s Essay, Preface, p. xi., and pp. 162 and 163. 
