48 IOWA ACADEMY OP SCIENCES. 



spherical surface containing the point; following the law of all 

 forces expanding in three- fold space, which may be stated thus: 

 Force x area of distribution=a constant. 



From this fact it is evident that the distribution of the force 

 of gravitation is conj&ned to threefold space; for, since the 

 boundary of a fourfold sphere is a solid, a force expanding in 

 all directions from a point in fourfold space decreases in inten- 

 sity in proportion to the increase of the boundary, that is to 

 say, in proportion to the cube of the radius, instead of follow- 

 ing Newton's law. 



Newton's law has been experimentally proved for distances 

 that are very great compared with the diameter of an atom, and 

 to a degree of accuracy limited by errors of experiment. It 

 does not necessarily follow that the law holds with absolute 

 accuracy, or that it holds at all for distances comparable with 

 atomic dimensions. All that we can say is that for distances 

 moderate and great the law expresses the facts as accurately 

 as they have been experimentally determined. 



Gravitation is not, like magnetism, polar. In crystals atoms 

 have an orderly arrangement, yet no difference has been found 

 in the weight of any crystal when it is set on end or laid on its 

 side. This fact, along with the complete independence of elec- 

 tric conditions, show that gravitation is neither an electric nor 

 a magnetic phenomenon. 



The ether, so far as our knowledge goes, is a homogeneous 

 isotropic continuum. In the conveyance of light and of elec- 

 tric strain it shows the properties of an elastic solid. To plane- 

 tary motions and to ordinary motions on the earth it offers no 

 appreciable resistance, and may therefore be called a fluid. 

 Michelson and Morley have shown that the ether close to and 

 in the earth moves with the earth, which indicates that the 

 ether does not move among atoms without some resistance cor- 

 responding to friction. The existence of an ether strain such 

 as that in a leyden jar also shows that there is a resistance on 

 the part of the ether to the kind of motion that takes place in 

 the electric discharge. Ether has mass, since it conveys energy 

 by waves which have a finite velocity. Lord Kelvin has pointed 

 out that the apparently inconsistent properties of the fluid-solid 

 ether are analogous with the properties of ordinary matter. 

 Pitch or taffy, either of which can be bent or moulded easily 

 by a steady pressure, is shattered like glass by a quick blow 

 from a hammer. The ether in like manner yields easily before 



