Polytechnic Association. 869 



sider it in a still more expanded state, as the universally diffused 

 medium of light, heat and actinism ; consequently this conception of 

 the minute ponderable globule does not bring us very near the 

 minima naturae, for a difference in size cannot be less marked between 

 ponderable atoms and those infinitesimal particles forming the lumini- 

 ferous ether or seth which fills the interstellar spaces, and which, in a 

 more condensed state, probably forms the interatomic medium. 

 Assuming that all forces generating wave motions in elastic fluids fol- 

 low the same law of propagation, I endeavored some years ago to 

 estimate the density of this inconceivably attenuated substance.* 



In that calculation the density of air is the unit of measure. If 

 instead hydrogen be taken as the unit, the density of luminiferous 

 ether is expressed by the decimal .00000000001653. Whatever may 

 be its actual density, its reality must be admitted, until the positions 

 established by the investigations of Huyghens, Young, Fresnel, Fou- 

 cault and Fizeau are shown to be untenable. A very able American 

 metaphysician, in meeting an objection brought by Huxley against 

 the views of Comte, has strongly expressed his unqualified dissent.f 

 Nevertheless, the hypothesis that light, heat and actinism are propa- 

 gated by the undulations of a subtle, all-pervading fluid, is the only 

 one which satisfactorily accounts for a certain class of phenomena, 

 and it is accepted by all the prominent experimental physicists of the 

 present day. 



The vast difference in density indicated cannot be apprehended, 

 because numerical comparisons utterly fail to raise in the mind any 

 clear conception regarding a flnid so attenuated ; yet it naturally sug- 

 gests the idea that there must be many intervening conditions of 



* Sound would be propagated, with exactly the velocity of light through a fluid, 

 under the standard pressure, 874,094,104,900 times rarer than air. Therefore, if the 

 density of air be one, the density of teth is represented by the decimal .000,000,000,- 

 001,144 + . 



It will not be inferred from this view that the aim has been to reach 

 "The first of things, quintessence pure," 

 for the elastic quality of seth involves the hypothesis of a still more subtle fluid. "We 

 have raised one curtain only to find another to be raised. As the unfathomed 

 vaults of heaven recede before the sweep of a more powerful refractor, and nebulae 

 resolved reveal nebulae beyond, so the most diminutive germ that springs from the 

 Creator's touch discloses, through the lens of higher power, new signs of more won- 

 derful mechanism within. Each nucleus has its nuclei ! Each entoblast is but the 

 boundai-y of a microcosm ! Each particle a galaxy of atoms revolving in the all- 

 pervading aeth ! Thus, before every far-reaching human advance, Circumference and 

 Center will forever retreat. [Transactions of the American Institute, 1864, p. 539- 

 " Clydonics," No. 1. 



f Eleventh Harvard lecture, by Prof. John Fiske, Cambridge, Mass., 1869. 



