ORIGIN OF URANIUM AND THORIUM I2l 



metals and metalloids, basing his opinion largely npon the analogous 

 character of triads and other groups of elements. The hypothesis was 

 not unfavorably received by Faraday, Grove, and W. E. Gladstone, who 

 partook in the discussion. 



Various attempts at grouping the elements finally resulted in Mende- 

 leef's famous periodic law (1869), which enabled its author successfully 

 to predict the properties of unknown elements (gallium, scandium, ger- 

 manium) and is now everywhere received as expressing true relationships 

 between their various fundamental properties. Such relationships would 

 of course be inconceivable, were the "simple substances" totally inde- 

 pendent entities. 



Great attention has been excited by Sir IsTorman Lockyer's hypothesis, 

 published in December, 1873. He suggested that the elements might be 

 dissociated by heat. "On this working hypothesis," he writes, ^the so- 

 called elements not present in the reversing layer of a star will be in 

 course of formation in the coronal atmosphere and in course of destruc- 

 tion as their vapor density carries them down." His hypothesis supposes 

 the original atoms of which a star is composed to be possessed of the 

 additional potential energy of combination, or that they are endothermic 

 compounds, and he points out that the liberation of this energy through 

 dissociation would prolong the period during which the star would give 

 light.^" Lockyer thus begins nebular history with complex atoms. He 

 made no attempt to explain imder what conditions they were formed or 

 Avhence they derived their internal energy. His hypothesis has always 

 aroused interest, but, as C. A. Young put it, "is encumbered with great 

 difficulties and has not yet been accepted by physicists and chemists." 



In January, 1873, nearly a year before Sir Norman Lockyer's lecture, 

 Mr F. W. Clarke was daring enough to suggest the possibility of the 

 evolution of elements. "We do not Ivnow but that the evolution of one 

 element from another may be possible," he wrote. "These elements which 

 seem today so diverse in character may be, after all, one in essence. 

 Upon this theory the planets should contain more elements than the sun; 

 the Sim more than some of the more advanced among the fixed stars, and 

 these in turn should be more highly organized than the nebulae." ^° Mr 

 Clarke's hypothesis has not attracted much notice, but it is entirely in 

 line with modern results. Thus Mr J. J. Thomson, in his Silliman lec- 

 tures, 1904, said in discussing the electronic theory of matter: 



1" Nature, vol. 9, 1894, pp. 411 and 429. Philosophical Transactions, vol. 164, 1874, 

 p. 492. 



»* Popular .Science Monthly, vol. 2, 1873, p. 320. For this paper Clarke was roundly 

 denounced by a distinguished analyst, being classed with dreamers and speculators like 

 Charles Darwin. American Association for the .\dvancement of Science, 1874, p. 13. 



