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5 
in opinion as to the direction in the series in which evolution is 
proceeding—that is to say, whether the red or orange coloured 
stars are younger, and become white in old age, or whether the 
white stars are in the earlier stage, and pass, in the course of ages, 
into coloured stars. Professor Higgins himself inclines to the latter 
theory, and to the classification of stars suggested by Vogel in 
1814, in which the white stars (by far the most numerous) 
represent the younger and longest stage of stellar life, and the 
orange and red stars, the period of decline and old age, According 
to some authorities, the sun himself is of about middle age, but 
it must always be remembered that we do not see him as he really 
is, his colour being, to our view, materially modified by the inter- 
vention of the earth’s atmosphere. Some experts contend that a 
star passes successively from the orange and red to the white stage, 
and then returns by slow degrees to the red and orange. Another 
theory suggests that the divergence in colour of the stars does not 
mark any stage of evolution in their existence, but is due to a 
difference in their original constitution. 
- Although the w rk of interpreting the nature of the stars or 
nebulx is comparatively easy so long as their spectra agree with 
the solar spectrum, it becomes a task of great difficulty when their 
spectra differ widely from chat of the sun, which, as [ have already 
shewn, contains so large a number of terrestrial elements. It must 
also be borne in mind that the lines or waves which the spectro- 
scope reveals were set in motion many hundreds, or perhaps 
thousands, of years ago, and the suns whence they proceed may 
nw be dark burnt-out orbs. In the case then of those stars whose 
spectra differ from that of the smn, there remain but the tests of 
the laboratory to determine their nature, density, and temperature. 
A spectrum of a remarkable nature is presented by the Aurora 
Borealis, and although we have reason to believe that certain gases 
are present in it, no similar spectrum has yet been obtained from 
them by artificial means. It has recently been suggested that the 
Aurora was produced by the dust of meteors and falling stars, which 
contained traces of manganese, lead, barium, thallium, iron, and 
other substances, Exhaustive experiments on this point have been 
made by Dewar and Liveing with the minute dust thrown off from 
the surfaces of electrodes of various metals, carried forward under 
the tube of observation by a rapid current of air or gas. It 
was then shown that metallic dust does not become luminous in an 
eleccric discharge with the characteristic spectrum of the Aurora. 
It is now, I believe, generally accepted that the Aurora is a 
manifestation of the earth’s magnetism, and that there is some 
connection between the display of the northern lights and the 
maximum period of sun spots. Indeed if the spots or macule are 
