EVOLUTION AND THE PRESENT AGE 261 
in 1861 has shown that the whole stellar universe is 
made up of the same chemical materials as those with 
which we are familiar upon the earth. A part of the 
dazzling brilliance of the noonday sun is due to the 
vapour of iron floating in his atmosphere, and the faint 
luminosity of the remotest cloud-like nebula is the glow 
of just such hydrogen as enters into every drop of water 
that we drink. But this is not quite the whole story. 
The study of spectrum analysis has shown that the 
most deeply individual and characteristic attribute of 
any substance whatever is the number and arrange- 
ment of the lines and bands which it makes in the 
spectrum. You cannot say of iron that it is always 
black, for you have often seen it red, and occasionally, 
perhaps, white; nor can you say that it is always cold 
or hard; and if it has weight invariably, that is no more 
than can be said of other things besides iron. But 
whether black or white, hot or cold, smooth or rough, 
hard or soft, iron is that substance which when heated 
till it is luminous, always throws upon the spectrum 
the same elaborately complicated system of lines and 
bands, which are different from those that are thrown 
by any other substance. The revelations of the spec- 
troscope therefore show that in all parts of the universe 
the interior constitution of matter is the same, and that 
_ its manifestations in the forms of light and heat are of 
the same character and conformable to the same physi- 
cal laws. There is not one science of mechanics for 
the earth, or one kind of optics for Sirius, or one law 
of radiation for Jupiter, but from end to end of the visi- 
ble universe the same laws hold sway and the funda- 
mental principles of action are the same. 
Not only is it true that the same physical laws hold 
