398 SCIENTIFIC THOUGHT IN THE NINETEENTH CENTURY. 



borg; yet the credit of the discover}^ belongs not so much to the first 

 conception of the idea as to its development into a thoroughly scientitic 

 theory. Our century, moreover, has added to those evidences of the 

 nebular theory which Laplace derived from the analogies of movement 

 in the solar svstem, the evidence furnished by the spectroscope, which 

 finds in the nel)uhe matter in some such condition as that from which 

 the solar system is supposed to have been evolved. 



But by far the most important contribution of this century to the 

 intellectual life of man is the share which it has had in developing the 

 idea of the unity of nature. The greatest step prior to this century in 

 the development of that idea (and probably the most important single 

 discovery in the whole history of science) was Newton's disco^'ery of 

 universal gravitation two hundred years ago; but the investigations 

 of our century have revealed with a fullness not dreamed of before a 

 threefold unity in nature — a unity of substance, a unity of force, and 

 a unity of process. 



Spectrum anal3^sis has taught us somewhat of the chemical constitu- 

 tion, not only of the sun, but also of the distant stars and nebuhe; 

 and has thus revealed a substantial identit}' of chemical constitution 

 throughout the universe. Profoundly interesting from this point of 

 view is the recent discover}^ in uraninite and some other minerals of 

 the element helium, previously known only by its line in the spectrum 

 of the sun. Profoundly interesting will be, if confirmed by further 

 researches, the still more recent discovery- of terrestrial coronium. 



The doctrine of the conservation of energy formulates a unity of 

 force in all physical processes. In this case, as in others, prophetic 

 glimpses of the truth came to gifted minds in earlier times. Lord 

 Bacon declared heat to be a species of motion. And Huyghens, in the 

 seventeenth century, distinctly fonnulated the theory of light as an 

 undulation, though the mighty influence of Newton maintained the 

 emission theory in general acceptance for a century and a half. 



When Lavoisier exploded the phlogiston theor}^ and laid the foun- 

 dation of modern chemical philosophy, it was seen that in every 

 chemical change there is a complete equation of matter. But there 

 was in the phlogiston theory a distorted representation of a truth 

 which the chemical theory of Lavoisier and his successors ignored. 

 They could give no account of the light and heat and electricity so 

 generally associated with chemical transformations. These " impon- 

 derable agents," as they were called, believed to be material, yet so 

 tenuous as to be destitute of weight, haunted like ghosts the workshop 

 of the artisan and the laboratory of the scientist, wonderfully impor- 

 tant in their effects, but utterly unintelligible in their nature. It was 

 almost exactly at the beginning of our century that the researches of 

 Rumford discovered the first words of the spell by which these ghosts 

 were destined to be laid. When Rumford declared, in his interpreta- 



