948 
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 
discoyery of universal gravitation two hun- 
dred 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 analysis has taught us some- 
what of the chemical constitution, not only 
of the sun, but also of the distant stars and 
nebule ; and has thus revealed a substantial 
identity of chemical constitution through- 
out the universe. Profoundly interesting, 
from this point of view, is the recent dis- 
covery, in uraninite and some other min- 
erals, 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 cor- 
onium. 
The doctrine of the conservation of en- 
ergy 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 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 successors ignored. They 
SCIENCE. 
[N. 8. Vou. X. No. 261. 
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, be- 
lieved to be material, yet so tenuous as to 
be destitute of weight, haunted like ghosts 
the workshop of the artisan and the labora- 
tory 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 were des- 
tined to be laid. When Rumford declared, 
in his interpretation of his experiments, 
“Anything which any insulated body or sys-- 
tem of bodies can continue to furnish with- 
out limitation, cannot possibly be a material 
substance,”’ the fate of the supposed im- 
ponderable fluid heat was sealed ; but it was 
not till near the middle of our century that 
Joule completed the work of Rumford by 
the determination of the mechanical equiv- 
alent of heat. About the same time, Fou- 
cault’s measurement of the velocity of light 
in airand in water afforded conclusive proof 
of the undulatory theory of light. In these 
great discoveries was laid the strong founda- 
tion for the magnificent generalization of 
the conservation of energy—a generaliza- 
tion which the sagacious intuition of Mayer 
and Carpenter and Le Conte at once ex- 
tended beyond the realm of inorganic na- 
ture to the more subtile processes of vege- 
table and animal life. In this connection, I 
may be pemitted to refer to the work of 
some of my colleagues, with the Atwater- 
Rosa calorimeter, which has given more 
complete experimental proof than had pre- 
viously been given of the conservation of 
energy in the human body. 
But by far the greatest of the intellectual 
achievements of our age has been the de- 
velopment of the idea of the unity of pro- 
cess pervading the whole history of nature. 
The word which sums up in itself the ex- 
