90 Attraction of Gravitation 



The second law has a greater significance than this. The 

 attraction of one body on another varies inversely as the 

 square of the distance between them. If r be the distance 

 between the two bodies, then one of the factors of the ex- 

 pression for their attraction is r^. Now r is a surface 

 quantity, and if gravitation were a simple force acting in a 

 simple straight line from a particle of one body to a particle 

 of the other, then it would be difficult to conceive of any ex- 

 planation for the entrance of such a factor. 



But in the case of magnetic attraction, or of any other 

 form of radiation, we can see easily enough the origin 

 of this term. For in all cases of waves propagated from 

 a centre, the square of the distance naturally enters. As 

 the wave moves forward, it expands equally in two 

 directions, and the expansion in each direction being 

 proportional to the distance traversed, the intensity of the 

 wave is lessened in proportion to the square of the distance 

 traversed. Hence the inverse square is the law for light, 

 heat, magnetism, and electricity. If we find the same law 

 in the case likewise of gravitation, it strengthens to a certain 

 extent the supposition that the internal energy of matter is 

 radiated through space in spherical waves which obey the 

 ordinary law of such waves, and decrease in intensity in 

 proportion to the squares of distances they have travelled. 



In conclusion, it may be observed that of all the possible 

 explanations that could be given of gravitation, the simplest 

 and most likely is that the power of attracting lies in the 

 mass of matter itself ; and if we ask what it is in matter 

 that gives it this power, we can scarcely have any other 

 answer than that it is some form of energy due to the motion 

 of the constituent molecules. It certainly would be a 

 step in the establishment of that conformity of nature, 

 to which all science tends, if it could be shown for gravi- 

 tation, as it has recently been shown for electricity and 

 magnetism — that it is the effect of molecular vibrations 

 propagated through the same omnipresent medium which 

 conveys the vibrations of light, heat, and actinism. Of 

 course, no real advance will be made in such a theory 

 until, by fresh experiments, or by mathematical investi- 

 gations, founded on previous experiments, something like 

 a reasonable explanation shall be given for the nature 

 of the connection that binds the two together ; till we 

 shall be able to say how it is that a molecular dis- 



