1893.] President's Address. 383 



efficiency of air and of liquid and solid insulators, resuscitated the 

 idea of a medium through which, and not only through which but 

 by which, forces of attraction or repulsion, seemingly acting at a dis- 

 tance, are transmitted. The long struggle of the first half of the 

 eighteenth century was not merely on the question of a medium to 

 serve for gravific mechanism, but on the correctness of the Newtonian 

 law of gravitation as a matter of fact however explained. The 

 corresponding controversy in the nineteenth century was very short, 

 and it soon became obvious that Faraday's idea of the transmission 

 of electric force by a medium not only did not violate Coulomb's law 

 of relation between force and distance, but that, if real, it must give 

 a thorough explanation of that law.* Nevertheless, after Faraday's 

 disco very f of the different specific inductive capacities of different 

 insulators, twenty years passed before it was generally accepted in 

 Continental Europe. But before his death, in 1867, he had succeeded 

 in inspiring the rising generation of the scientific world with some- 

 thing approaching to faith that electric force is transmitted by a 

 medium called ether, of which, as had been believed by the whole 

 scientific world for 40 years, light and radiant heat are transverse 

 vibrations. Faraday himself did not rest with this theory for elec- 

 tricity alone. The very last time I saw him at work in the Royal 

 Institution was in an underground cellar, which he had chosen for 

 freedom from disturbance ; and he was arranging experiments to test 

 the time of propagation of magnetic force from an electro-magnet 

 through a distance of many yards of air to a fine steel needle polished to 

 reflect light ; but no result came from those experiments. About the 

 same time or soon after, certainly not long before the end of his 

 working time, he was engaged (I believe at the shot tower near 

 Waterloo Bridge on the Surrey side) in efforts to discover relations 

 between gravity and magnetism, which also led to no result. 



Absolutely nothing has hitherto been done for gravity either by 

 experiment or observation towards deciding between Newton and 

 Bernoulli, as to the question of its propagation through a medium, and 

 up to the present time we have no light, even so much as to point a 

 way for investigation, in that direction. But for electricity and mag- 

 netism, Faraday's anticipations and Clerk-Maxwell's splendidly de- 

 veloped theory have been established on the sure basis of experiment 

 by Hertz's work, of which his own most interesting account is this 

 year presented to the world in the German and English volumes to 

 which I have referred. It is interesting to know, as Hertz explains 

 in his introduction, and it is very important in respect to the experi- 



* ' Electrostatics and Magnetism,' Sir W. Thomson, Arts. I (1842) and II (1845), 

 particularly 25 of Art. II. 



f 1837, ' Experimental Kesearches,' 11611306. 



