THE ASTRONOMICAL VIEW OF NATURE. 



345 



either exceedingly small or this force is propagated 

 instantaneously through the greatest cosmical distances 

 which come under our notice. Then, again, light and 

 radiated heat spend themselves as they meet with reflect- 

 ing or absorbing bodies, whereas gravitation does not 

 seem to be affected by intervening or screening bodies.^ 



' It is now known that this 



screening effect exists likewise in 



magnetic and electric action. In 



the formula which expresses the 



action at a distance of magnetic, 



electrical, and ponderable masses, 



. m.m! 

 VIZ., /=jii ^^ y the older view 



previous to Faraday's researches 

 considered m and m' the masses 

 (ponderable or imponderable), and 

 the distance r to be variable, fi a 

 constant, corresponding to the gra- 

 vitation constant. As stated above, 

 the gravitation constant is, so far 

 as we know, a real constant i.e., it 

 is not affected by the nature of the 

 medium which fills the space inter- 

 vening between m and rn!, the at- 

 tractive masses. Faraday doubted 

 this; but leaving gravitation "as 

 a relation by some higher quality " 

 aside, he directed his efforts to 

 the testing of the validity of this 

 view as regards electric and mag- 

 netic action. He found that ix. is 

 not a real constant, but dependent 

 on the nature of the medium and 

 the objects which intervene be- 

 tween the magnetic and electric 

 masses. These researches, which 

 are probably the first step in the 

 direction of gaining by observation 

 some notion of the mechanical 

 manner in which action at a dis- 

 tance is brought about, begin with 

 the year 1837 (see 11th series of 

 ' Experimental Researches in Elec- 

 tricity,' No. 1252). The result was 

 that the " specific electric induction 

 for different bodies " was estab- 

 lished, contrary to the ideas of 



Poisson and others ('Exper. Res.,' 

 No. 1167), and the word "dielec- 

 tric " invented to denote the " action 

 of the contiguous particles of the 

 insulating medium" (No. 1168). 

 From this point he was led a 

 step farther, to "expect that all 

 polar forces act in the same general 

 manner" viz., by contiguous par- 

 ticles. Faraday, however, is care- 

 ful to remark that bj' contiguous 

 particles he means those " which 

 are next to each other, not that 

 there is no space between them " 

 (No. 1665). 



In 1838 Faraday was still doubt- 

 ful whether magnetic action was 

 similar in this respect to statical 

 electric action ; but he thought it 

 probable that it was "communi- 

 cated by the action of the interven- 

 ing particles" (No, 1729), and in 

 pursuing this line of thought, in 

 spite of many unsuccessful trials, 

 he at last saw his ideas realised, 

 discovered the magnetisation of 

 light, and invented the term " dia- 

 magnetic" to describe "a body 

 through which lines of magnetic 

 force are passing, and which does 

 not b\' their action assume the 

 usual magnetic state" (1845, 'Ex- 

 per. Res.,' No. 2149). At the end 

 of the 19th series of researches he 

 says: "In former papers (1838) I 

 proposed a theory of electrical in- 

 duction founded on the action of 

 contiguous particles, . . . and I 

 then ventured to suggest that prob- 

 ably . . . magnetic action was also 

 conveyed onward in a similar man- 

 ner. At that time I could discover 



