328 POPULAR SCIENCE MONTHLY 



is no right and left to gravitation. Two pieces of matter always fit in 

 the gravitational sense. 



The bald statements of the laws of gravitation and electric force 

 bear a strong resemblance to each other. The laws tell us how the 

 forces vary, but reveal no hint of the machinery by which they act. 



Gravitation was the first force man encountered and it is still the 

 one he knows least about, for we have got no farther than where Newton 

 left it two and a half centuries ago. We have some inkling of the pos- 

 sible machinery by which one electric charge acts upon another at a 

 distance and we feel nearly as sure that the push or pull is carried by 

 the ether as that the pull of a horse on a cart is through the traces 

 which bind him to it. With gravitation the case is very different, for 

 we have not as yet the slightest valid conception of how the pull of one 

 mass upon another is conducted across the intervening space, nor what 

 conducts it. We can get no farther until the speed with which gravita- 

 tional disturbances travel has been measured, and no one at present 

 seems to know how to go about making such an experiment. 



One further difference between gravitation and electric force. The 

 force of attraction or repulsion between two charges of electricity is 

 diminished by replacing the free ether between them with any material 

 medium, but the force of gravitation between two bodies remains con- 

 stant as long as the distance remains constant, and intervening masses 

 are powerless to shield or to alter it. Hence we can not yet attribute 

 the gravitation of matter to any electricity which may be contained in 

 it, nor prove the ether to be the medium through which the force acts. 



Gravitation is still unconnected, unattached to anything else in 

 nature ; as independent as Mr. Kipling's " cat that walked by himself, 

 and all places were alike to him." It is still the stumbling block to the 

 physicist which it has been these many years. How can he explain a 

 universe when he is unable to give a reasonable account of the cement 

 which holds it together? 



Of the intimate association of electricity with matter we have 

 learned much from careful study of the processes of electric conduction 

 in solutions and gases. 



When a simple chemical compound (and it should here be borne in 

 mind that the molecule of a compound is built up of atoms of at least 

 two different kinds) — when a simple chemical compound, hydrochloric 

 acid for example, is dissolved in water and an electric current is passed 

 through the solution, the products hydrogen and chlorine of the decom- 

 posed acid appear in definite proportions at the points where the current 

 enters and leaves the liquid — the chlorine where the current enters, 

 the hydrogen where it leaves. We know this current to consist of 

 processions of single charged atoms, a disorderly march, perhaps, with 

 a crowd of bystanders obstructing the way, but the movement is always 

 forward, each constituent of the broken molecule carrying a definite 



