1908] on the Ether of Space. 71 



ous. It may mean tlie specific gravity of the dry powder as it lies, 

 like snow ; or it may mean the specific gravity of the particles of 

 which it is composed, like ice. 



So also with regard to the density of matter, we miglit mean the 

 density of the fundamental material of which its units are made — 

 which would be ether ; or we might, and in practice do, mean the 

 density of the aggregate lump which we can see and handle ; that is 

 to say, of water or iron or lead, as the case may be. 



In saying that the density of matter is small, I mean, of course, 

 in the last, tbe usual, sense. In saying that the density of ether is 

 great, I mean that the actual stuff of which these highly porous 

 aggregates are couiposed is of immense, of well-nigh incredible, density. 

 It is only another way of saying tliat the ultimate units of matter are 

 few and far between — i.e. that they are excessively small as compared 

 with the distances between them ; just as the planets of tlie solar 

 system, or worlds in the sky, are few and far between — the inter- 

 vening distances beiug enormous as compared with the portions of 

 space actually occupied by lumps of matter. 



Here it may be noted that it is possible to argue that the density 

 of a continuum is necessarily greater than the density of any discon- 

 nected aggregate : certainly of any assemblage whose particles are 

 actually composed of the material of the continuum. Because the 

 former is " all there," everywhere, without break or intermittence of 

 any kind ; while the latter has gaps in it — it is here, and there, but 

 not everywhere. 



Indeed, this very argument was used long ago by that notable 

 genius Robert Hooke, and I quote a passage which Professor Poynting 

 has discovered in his collected posthumous works, and kindly copied 

 out for me : — 



" As for matter, that I conceive in its essence to be immutable, 

 and its essence being expatiation determinate, it cannot be altered in 

 its quantity, either by condensation or rarefaction ; that is, there 

 cannot be more or less of that power or reality, whatever it be, within 

 the same expatiation or content ; but every equal expatiation contains, 

 is filled, or is an equal quantity of ?nateria ; and the densest or 

 heaviest, or most powerful body in the world contains no more 

 materia than that which we conceive to be the rarest, thinnest, 

 lightest, or least powerful body of all ; as gold for instance, and 

 CBther, or the substance that fills the cavity of an exhausted vessel, or 

 cavity of the glass of a barometer above the quicksilver. Nay, as I 

 shall afterwards prove, this cavity is more full, or a more dense body 

 of aether, in the common seuse or acceptation of the word, than gold 

 is of gold, bulk for bulk ; and that because the one, viz., the mass of 

 ffither, is all aether : but the mass of gold, w^hich we conceive, is not 

 all gold ; but there is an intermixture, and that vastly more than is 

 commonly supposed, of aether with it ; so that vacuity, as it is com- 

 monly thought, or erroneously supposed, is a more dense body than 



