~~) 2 
> ae 
1 oe 
14 THE EVOLUTION OF LIFE 
applied. By proto-iron, proto-copper, and _proto- 
nickel, for example, is meant the constituents of iron, 
copper, and nickel as they exist in the sun or hottest 
stars. A very important proto-element is proto- 
hydrogen, discovered some time ago by Professor 
Pickering of Harvard University, in the star named 
zeta-Puppis in the constellation Argo. The spec- 
tral lines representing this substance Pickering at 
first supposed to signify a new element; but he 
was able to show, later, that they belonged to a 
new series of hydrogen lines constituting a form of 
hydrogen unknown on earth. Subsequently, this 
same proto-hydrogen was discovered in the stars 29 
Canis Major and gamma-Argus. It is interesting 
to note that this broken down or proto-hydrogen is 
confined to the very hottest stars known... . If 
we accept the dissociation hypothesis, these curious 
proto-spectra are naturally, simply, and sufficiently 
explained. If we reject it, we shall look in vain 
for any other present-day explanation which will 
co-ordinate and harmonise the observed results.” 
Looked at from the point of view of the elements 
entering into their composition, Sir Norman Lockyer 
divides the stars into three main groups, namely : 
(1) gaseous stars, (2) metallic stars, and (3) carbon 
stars— each representing different grades of 
temperature. In the former, or hottest stars, there 
are found, almost exclusively, the gases hydrogen, 
helium, and the gas asterium, which is as_ yet 
unknown on earth. In the stars of medium 
temperature, these gases become replaced by metals 
in the dissociated state in which they exist in an 
