422 SCIENTIFIC THOUGHT. 



This statement implies that even as late as the end of the 

 third quarter of the century, foremost thinkers hesitated to 

 attach a more than provisional importance to chemical sym- 

 bolism and the various elaborations of the atomic theory, 

 as chemical text-books then exhibited them. Similar 

 " merely provisional theories have existed in other branches 

 of science. The theory of the two fluids in electricity 

 did good service for a long time in enabling philosophers 

 to define their ideas, to describe, calculate, and predict 

 phenomena. In optics, the so-called corpuscular theory 

 of light is still used with advantage as a convenient 

 means of summarising the laws of reflexion and re- 

 fraction ; similarly, in treatises on the conduction of heat, 

 the old caloric theory still holds a place alongside of the 

 26. more modern dynamical views. It may be questioned 



The periodic 



law. whether the celebrated periodic law of Newlands, Lothar 



Meyer, and Mendeleeff, which has brought some order 

 into the atomic and other numbers referring to the dif- 

 ferent elements, and has even made it possible to predict 

 the existence of unknown elements with definite pro- 

 perties, stands really in a firmer position than the once 

 well-known but now forgotten law of Bode, 1 according to 



I'instrument le plus parfait pour 

 lea conceptions eleve'es de la th^orie 

 et le guide le plus sur pour les 

 recherches experimen tales " (p. 241). 

 And quite mournfully does Kopp re- 

 port at the close of his historical sur- 

 vey of the development of chemistry 

 ('Entwickelung,'&c.,p.829)howthat 

 science about 1860 again " turned 

 into the course which it had tried so 

 often, and had so often abandoned 

 as hopeless, endeavouring to gain 

 a knowledge how the elementary 

 atoms are arranged in the smallest 

 particles of their compounds." 



1 According to the relation, first 

 observed by Christian Wolff and 

 Daniel Titius, that the distances of 

 the planets from the sun obey ap- 

 proximately the formula 0'4 + - 3 

 x 2, where n for Venus, Earth, 

 Mars, &c., assumes the values 0, 1, 2, 

 &c., the planet corresponding to 

 7i = 3 was missing. When, on the 

 discovery of Uranus in 1781, it was 

 found that this planet's distance 

 also agrees approximately with the 

 formula, Bode and von Zach drew 

 attention to this fact, and suggested 

 a systematic search for the missing 



