564 REPORT- 1901. 



of whicli we are not yet cognisant. If we consider ether to be matter, we 

 postulate that it has rigidity enough for the vibrations of light, but we have no 

 right to say that it is absolutely incompressible. We must admit that sufficiently 

 great pressure all round could condense the ether in a given space, allowing the 

 ether in surrounding space to come in towards the ideal shrinking surface. When 

 I say that ether must be outside the law of gravitation, I assume that it is not 

 infinitely incompressible. I admit that if it were infinitely incompressible, 

 it might be subject to the law of mutual gravitation between its parts ; but to 

 my mind it seems infinitely improbable that ether is infinitely incompressible, 

 and it appears more consistent with the analogies of the known properties of molar 

 matter, which should be our guides, to suppose that ether has not the quality 

 of e.^erting an infinitely great force against compressing action of gravitation. 

 Hence, if we assume that it extends through all space, ether must be outside 

 the law of gravitation — that is to say, truly imponderable. I remember the 

 self-complacent compassion with which sixty years ago — I myself, I am afraid — 

 and most of the teachers of that tiine looked upon the ideas of the elderly 

 people who went before us, who spoke of ' the imponderables.' I fear that in 

 this, as in a great many other things in science, we have to hark back to the 

 dark ages of fifty, sixty, or a hundred years ago, and that we must admit there 

 is something which we cannot refuse to call matter, but which is not subject 

 to the Newtonian law of gravitation. That the sun, stars, planets, and meteoric 

 stones are all of them ponderable matter is true, but the title of my paper implies 

 that there is something else. Ether is not any part of the subject of this paper ; 

 what we are concerned with is gravitational matter, ponderable matter. Ether 

 we relegate, not to a limbo of imponderables, but to distinct species of matter 

 which have inertia, rigidity, elasticity, compressibility, but not heaviness. In a 

 paper I have already published I gave strong reasons for limiting to a definite 

 amount the quantity of matter in space known to astronomers. I can scarcely avoid 

 using the word ' universe,' but I mean our universe, which may be a very small 

 affair after all, occupying a very small portion of all the space in which there is 

 ponderable matter. 



Supposing a sphere of radius 3'09.10"' kilometres (being the distance at which 

 a star must be to have parallax 0"'001) to have within it, uniformly distributed 

 through it, a quantity of matter equal to one thousand million times the sun's 

 mass, the velocity acquired by a body placed originally at rest at the surface 

 would, in five million years, be about 20 kilometres per second, and in twenty-five 

 million years would be 108 kilometres per second (if the acceleration remained 

 sensibly constant for so long a time). Hence, if the thousand million suns had 

 been given at rest twenty-five million years ago, uniformly distributed throughout 

 the supposed sphere, many of them would now have A'elocities of 20 or 30 kilo- 

 metres per second, while some would have less and some probably greater velo- 

 cities than 108 kilometres per second ; or, if they had been given thousands of 

 million years ago at rest so distributed that now they were equally spaced 

 throughout the supposed sphere, their mean velocity would now be about 50 kilo- 

 metres per second.' This is not unlike the measured velocities of stars, and hence 

 it seems probable that there might be as much matter as one thousand million 

 suns within the distance 3-09.10"' kilometres. The same reasoning shows that ten 

 thousand million suns in the same sphere would produce velocities far greater than 

 the known star velocities, and hence there is probably much less than ten thousand 

 million times the sun's mass in the sphere considered. A general theorem dis- 

 covered by Green seventy-three years ago regarding force at a surface of any 

 shape, due to matter (gravitational, or ideal electric, or ideal magnetic) acting 

 according to the Newtonian law of the inverse square of the distance, shows that 

 a non-uniform distribution of the same total quantity of matter would give 

 greater velocities than wovild the uniform distribution. Hence we cannot, by any 

 non-uniform distribution of matter within the supposed sphere of 3"09.10"^ kilo- 

 metres radius, escape from the conclusion limiting the total amount of the matter 

 within it to something like one thousand million times the sun's mass. 



' Phil. 3tag., August 1901, pp. 1G9, 170. 



