the uiidulatory 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 Eiiler, 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 by 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 no 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 



