the Conservation of Force. 89 



effect of attraction diminishes inversely as the squares of the di- 

 stances, we can hy no means refer this to the source of attraction ; 

 we must rather assume that the latter is constant, and that also 

 the sum of the effects neither diminishes nor increases with the 

 distance. An example will make this clear. 



Let a molecule be imagined with the space around it filled 

 with molecules which are scattered at equal distances from each 

 other. For the sake of simplicity, let the latter be regarded as 

 of equal mass, and the mass of each of them as a vanishing 

 quantity in comparison with that of the first molecule, which I 

 will call the central molecule. The central molecule will attract 

 all the others, and the action upon each of them will vary as the 

 inverse squares of the distances. Let us suppose that around the 

 central molecule we have a spherical shell of definite thickness, 

 the sum of the actions proceeding from the central molecule will 

 always be the same within this shell, however great the distance 

 at which it is placed from the central molecule. For the action 

 on the single molecules diminishes as the square of the distance 

 increases, but the number of the molecules which make up such 

 a shell, augments directly as the square of the distance. 



When, therefore, we say that the attracting forces are inversely 

 to each other as the squares of the distances, we wish to say 

 nothing else than that the attracting force which belongs to every 

 mass is constant, and spreads its actions, undiminished in their 

 totality, in all directions. They act the more feebly on the sepa- 

 rate parts the greater the space over which they are spread. We 

 are not hereby forced to assume a propagation in the ordinary 

 sense of the term, that is, a process which requires time for its 

 transmission. For the law of the change of densities (or inten- 

 sities) inversely as the squares of the distances, is totally indepen- 

 dent of this assumption. It is true of a system of an infinite num- 

 ber of straight lines which issue from a point, as well as for a wave- 

 motion which propagates itself on all sides round the same point. 



[The idea of gravity was originally suggested by the attraction of 

 iron by a magnet. A magnetic pole and a mass of soft iron will, if 

 free to move, mutually ajjjjroach each other, in virtue of some quality 

 resident in each, which quality we call, or may call, the force of 

 magnetism. In the case of the soft iron this is a variable quality, 

 for the induced magnetism of the latter augments as it approaches 

 the steel magnet. It would he no illegitimate use of language to say 

 thpc in the casie of tl-e soft iron the force of magnetism varies, while 

 in the steel it rcma'ns constant. Regarding, in the same way, the 

 force of gravivy as a ([uabty resident in the gravitating masses, 

 ir ihis quality, r.s believed by Prof. Briicke, be constant, it would 

 cer.ainly be a contradiction in terms to say that it varied inversely 

 as the squy.rc of the distance. I5ut this is not asserted ; and the dif- 

 ference between Mr. Faraday and his commentators we believe to be 



