Density of tie Miher. 481> 



instance on the u superficial-charge " experiment of Cavendish 

 (carefully repeated by Clerk Maxwell and the present Prin- 

 cipal of Glasgow University with modern appliances) *; and 

 in the second instance on the phenomenon of gravitation. 



4. Furthermore, of late years, the whole world has become 

 accustomed to think of minute electric charges — configura- 

 tions probably of aether of some kind — moving about freely, 

 and endowed with a definite mass and bulk : and, as is well 

 known, the hypothesis has been mooted that matter itself may 

 consist of bodies of this sort of character. 



But if so, it is apparent how very porous, and so to speak 

 rarefied, a structure matter must exhibit, to everything except 

 electric disturbance : for the interstices must be enormous 

 compared with the regions of definite structure. So, just as 

 the ratio of mass to volume is small in the case of a solar 

 system or a nebula or a cobweb, I have been driven to 

 think that the observed mechanical density of matter is 

 probably an excessively small fraction of the total density of 

 the substance, or aether, contained in the space which it thus 

 partially occupies — the substance of which it may hypo- 

 thetical!}- be held to be composed. 



5. Thus, for instance, consider a mass of platinum, and 

 assume that its atoms are composed of electrons, or of some 

 structures not wholly dissimilar : the space wmich these 

 bodies actually fill, as compared with the whole space which 

 in a sense they 'occupy,' is comparable to 10 -10 th of the whole, 

 even inside each atom ; and the fraction is still smaller if 

 it refers to the visible mass. So that a kind of minimum 

 estimate of aetherial density, on this basis, would be something 

 like ten-thousand-million times that of platinum. 



This, standing alone, would be a wild guess ; but other 

 physicists have probably of late years been led, on various 

 grounds, to suspect that the aetherial density may be large ; 

 though probably it has not occurred to ihem to surmise that 

 it is as great as that. Nevertheless if more detailed consi- 

 deration should urge us to contemplate a density of enormous 

 magnitude for the aether, even greater than the above, there 

 is no need to be shocked or surprised. 



6. With this introduction I will state the argument as 

 briefly as possible ; referring for fuller or more explanatory 

 details to the concluding chapter of a new edition of 

 ' Modern Views of Electricity ': — 



* Maxwell, ' Cavendish Researches/ p. 104, and Note 19, p. 417. See 

 also Proc. Oainb. Phil. Sec. vol. iii. part iii., and Maxwell's Collected 

 Works, p. 612. 



