70 Sir Oliver Lodgp [Feb. 21, 



be generated by it when the radius was 10' light years. But hetero- 

 geneity would enable these values to be reached more easily. 



It is noteworthy how exceedingly small is the average or aggregate 

 density of matter in the visible region of space ; and Lord Kelvin 

 has shown that throughout space in general it inust be smaller still — 

 in fact ultimately infinitesimal. 



The estimated density of 10 ~^^ c.g.s. means that the visible 

 cosmos is as much rarer than a vacuum of a hundred millionths of 

 an atmosphere, as that vacuum is itself rarer than lead. 



It is, of course, because ordinary masses of matter likewise consist 

 of separated particles, with great intervening distances in proportion 

 to their size, that we are able to assert the minute aggregate density 

 of ordinary stuff, such as water or lead, as compared with the con- 

 tinuous medium of which all particles are supposed to be really 

 composed. The fundamental medium itself must be of uniform 

 density everywhere, whether materiahsed or free. 



Appendix II. Explanatory Eemarks concerning the 

 Density of Ether. 



I observe that it is surmised by at least one thoughtful and friendly 

 critic — C. W. S. in the Westminster Gazette — that in speaking of the 

 immense density or massiveness of ether, and the absurdly small 

 density or specific gravity of gross matter by comparison, I intended 

 to signify that matter is a rarefaction of the ether. That, however, 

 was not my intention. The view I advocate is that the ether is a 

 perfect continuum^ an absolute 7;/^?^/??/, and that therefore iio rarefac- 

 tion is possible. The ether inside matter is just as dense as the ether 

 outside, and no denser. A material unit — say an electron — is only a 

 peculiarity or singularity of some kind in the ether itself, which is of 

 perfectly uniform density everywhere. What we sense as matter is an 

 aggregate or grouping of an enormous number of such units. 



How then can we say that matter is millions of times rarer or less 

 substantial than the ether of which it is essentially composed ? Those 

 who feel any difficulty here, should bethink themselves of what they 

 mean by the average or aggregate density of any discontinuous system, 

 such as a powder, or a gas, or a precipitate, or a snowstorm, or a cloud, 

 or a milky way. 



If it be urged that it is unfair to compare an obviously discrete 

 assemblage like the stars, with an apparently continuous substance 

 like air or lead, the answer is that it is entirely and accurately fair ; 

 since air, and every other known form of matter, is essentially an 

 aggregate of particles, and since it is always their average density 

 that we mean. We do not even know for certain their individual 

 atomic density. 



The phrase " specific gravity or density of a powder " is aml)igu- 



