xl Centennial Atmiversary 



nebulae ; and has thus revealed a substantial identity of chemical 

 constitution throughout the universe. Profoundly interesting, 

 from this point of view, is the recent discovery, in uraninite and 

 some other minerals, of the element helium, previously known 

 only by its line in the spectrum of the sun. Profoundly interest- 

 ing will be, if confirmed by further researches, the still more recent 

 alleged 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, pro- 

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

 Lord Bacon declared heat to be a species of motion. And Huy- 

 ghens, in the seventeenth century, distinctly formulated 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 theory, and laid the 

 foundation 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 succes- 

 sors ignored. They could give no account of the light and heat 

 and electricity so generally associated with chemical transforma- 

 tions. These "imponderable 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 important 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 wei'e 

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

 tion of his experiments, " Anything which any insulated body or 

 system of bodies can continue to furnish without limitation, can- 

 not possibly be a material substance," the fate of the supposed 

 imponderable body, caloric, was sealed ; but it was not till near 

 the middle of our century that Joule completed the work of Rum- 

 ford by the determination of the mechanical equivalent of heat. 

 About the same time, Foucault's measurement of the velocity of 

 light in air and in water afforded conclusive proof of the undula- 

 tory theory of light. In these great discoveries was laid the 



