68 Prof. Thomson, On the Electric Theory of Gravitation. 



If we suppose that the atoms of the different elements are 

 electrical systems, and that each atom contains a number of units 

 of positive and also of negative electricity proportional to the 

 atomic weight, then Pj and Pg will be proportional to in^, ma, the 

 masses of unit areas of the slabs J. and P, so that the attraction 

 between the slabs will be proportional to {27 — (a + /3)} miWs. 

 Thus the attraction between two infinite slabs will be proportional 

 to the product of their masses and independent of the distance 

 between them, it follows from this that the attraction between 

 finite masses separated by distances large compared with their 

 linear dimensions, will vary inversely as the square of the distance 

 between them. 



If we take the number of positive units of electricity in the 

 atom as equal to the atomic weight, then using the electrostatic 

 system of units 



P P 



=^ = =^ = 10^ X 3 X 10i«, 



when mi and mo are measured in grammes. 



Thus the attraction between the slabs 



= {27 - (a + y8)} 9 X 10^ mima. 



The gravitational attraction between the slabs is 



27r X 6*6 X 10~^ x miin^, 



hence if the attraction we are considering is the gravitational 

 attraction 



27 - (a + y8) = Q X 10^^ 



But a, /3, 7 are all very nearly equal to I/Stt, hence 



g y-<"+^> ^i-2xio-'. 



a 



If in equation (1) we put N-^^^N^^ 0, we see that the repulsion 

 between these charges is aP^P^, if we put P^ = P^ = 0, the repulsion 

 is ^NiN^, and if we put iV^i = 0, P2 = 0, the attraction is yPilSf^, 

 thus a is proportional to the force between two unit positive 

 charges, /3 to the force between two unit negative charges and 7 to 

 the attraction between a unit positive and a unit negative charge. 

 It is to be noted that unless a = yS = 7, we have to amend the de- 

 finition of unit charges usually given, as in this case, if the force 

 between two units of positive charge at unit distance is the unit 

 force, the force between two units of negative charge will not be 

 so. If we recognise these differences between positive and nega- 

 tive electricity the definition of an unelectrified body is a matter 



