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 niav be questioned 



The periodic 



few- whether the celebrated periodic law of Xewlands, 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 



Finstrument le plus parfait pour * According to the relation, first 



lea conceptions elevens de la the'orie observed by Christian Wolff and 



et le guide le plus sur pour les Daniel Titius, that the distances of 



recherchesexpe'rimentale8"(p.241). \ the planets from the sun obey ap- 



And quite mournfully does Kopp re- ' proximately the formula Q'4 + 0'3 



port at the close of his historical sur- i x 2", where n for Venus, Earth, 



vey of the development of chemistry Mars, &c., assumes the values 0, 1,2, 



('Entwickelung,'&c.,p.829)how that &c., the planet corresponding to 



science about 1860 again "turned = 3 was missing. When, on the 



into the course which it had tried so \ discovery of Uranus in 1781 , it was 



often, and had so often abandoned found that this planet's distance 



as hopeless, endeavouring to gain also agrees approximately with the 



a knowledge how the elementary formula, Bode and von Zach drew 



atoms are arranged in the smallest attention to this fact, and suggested 



particles of their compounds." a systematic search for the missing 



