302 MODERN SCIENCE READER 



visible cosmos is, in fact, as much rarer than what we call 

 a high vacuum (say, the hundred-millionth of an atmo*- 

 phere) as that vacuum is rarer than lead. 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 

 ambiguous. It may mean the 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 might 

 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 this last, the usual, sense. In saying that the 

 density of ether is great, I mean that the actual stuff of 

 which these highly porous aggregates are composed is of 

 immense, of well-nigh incredible density. It is only another 

 way of saying that 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 the solar system, or worlds in the sky, are few 

 and far between the intervening distances being 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 dens- 

 ity of any disconnected aggregate : certainly of any assem- 

 blage whose particles are actually composed of the material 

 of the continuum. Because the former is "all there," 



