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the undulatory theory now very generally received assumes 
only that vibratory motion is transmitted by matter of some 
kind, and the inferences drawn from it are not invalidated by 
any hypotheses as to the precise nature of the transmitting 
medium. The writer, differing from many eminent physicists 
and mathematicians, is inclined to adopt the view that long 
since had the able support of Leonard Euler, and was first 
prominently put forward in this country by Grove, that the 
hypothesis of the presence of ether instertitially in all kinds of 
matter is gratuitous. 
38. It may, however, be desirable to consider a little more in 
detail the means by which the various kinds of energy are 
transmitted. Sonorous vibrations are freely transmitted bv all 
kinds of homogeneous matter, whether in the gaseous, fluid, or 
solid state ; in solid matter not homogeneous the amount of 
transmission depends upon structure. Thus, the transmission 
of sound through wood is much less perfect in the traverse than 
in the longitudinal direction ; it is much more impeded by cork, 
and almost intercepted by cotton-wool and similar substances. 
Electric energy is more or less freely transmitted by most kinds 
of matter, except glass, silk, and the resinous products of the 
vegetable kingdom. Since the transmission of the vibrations 
of light and heat through an absolute vacuum is obviously 
impossible, because the transmission of motion implies the 
presence of matter to be moved, it becomes a necessity that 
infinite space must be pervaded by some highly elastic and 
attenuated kind of matter, as the medium of the transmission 
of light and heat from the central luminaries of all existing 
solar systems to their attendant satellites. This, in entire and 
probably unavoidable ignorance of its nature, has been termed 
“ ether,” and the existence of ether has been assumed to be 
demonstrated by the periodic retardation of Enke’s comet. 
But it has been further assumed that ether alone is capable of 
transmitting the vibrations of light and heat, and must there- 
fore exist interstitially in all kinds of translucent and transcalent 
matter. 
39. The only basis on which this interstitial ether hypothesis 
rests is the assumed incapacity of ordinary matter, whether in the 
solid, liquid, or gaseous state, to transmit the extremely rapid 
vibrations of light and heat, for ho more valid reason than this : 
that the only vibrations of ordinary matter of which any actual 
knowledge exists — namely, those of sound — are almost im- 
measurably slower than those of light and heat, the former 
being numbered by at most a few thousands, the latter by 
hundreds of millions of millions in one second of time. But it 
must be borne in mind that sonorous vibrations are always 
