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PROCEEDINGS OE THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
afterwards their primaries; that -Jupiter, Saturn, 
Uranus, and Neptune, from their great size, were 
still too hot to support organic beings; and that 
the sun, having been formed by the rushing together of 
enormous masses, and from its great size, was still, and 
would continue for unfathomable ages, inconceivably hot, 
—so hot, indeed, that iron at a white heat was cold com¬ 
pared to it. 
The sun, however, must gradually cool, and in doing 
so diminish in size; but the heat and light evolved by 
its condensation would be sufficient to make up for its 
emission for millions of years. A diminution of one ten- 
thousandth part of its diameter could not be detected by 
our finest instruments, and that would cover its total emis¬ 
sion for 2100 years. It was obvious, however, that the energy 
of the sun must ultimately be exhausted; and although 
this might be partially restored by the fall of meteors and 
planets into its body (as was proved by the motion of 
Encke’s comet they must ultimately do), the sun, having 
the whole bodies of the system welded into its body, must 
at length revolve through space a cold, dark, and lifeless 
mass, until it came into contact with some large body like 
Sirius, when the heat produced by the impact of such 
enormous masses would be sufficient to convert both into 
vapour, which would form a nebula, from which, in the 
course of ages, a greater and more glorious system than 
the present would be evolved. 
Dr Miller afterwards spoke of the velocity of light, 
the best method of calculating which, he said, was 
by making observations on the transit of Venus over 
the sun’s disc. The discovery of Mitchelson (who 
found the velocity of light to be 186,380 miles per 
second) was published in 1879, and he had no doubt 
it would be tested by the observations made an the 
transit of Venus this year; and as astronomical methods 
and instruments had been greatly improved, a correct re¬ 
sult might be reasonably anticipated. In the meantime 
they might assume 186,000 miles per second as the velecity 
of light. That light was not propagated instantaneously 
was proved by Dr Bradley in 1728 by his discovery of the 
aberration of the fixed stars. 
The author next adverted to refraction, which was one of 
the most valuable properties of light, as without it we should 
neither have lenses for correctingour imperfections of vision, 
nor microscopes for scanning the minute arcana of Nature, 
nor telescopes for exploring the distant regions of space, 
nor even the eye itself. Having referred to the many 
ordinary and extraordinary atmospheric phenomena 
which were easily explained by refraction, the lecturer 
proceeded to state that if a ray of the sun were admitted 
into a dark chamber, through a small aperture in a 
window-shutter, and made to pass through a glass 
prism, there would be obtained on a white screen an 
elongated image consisting of seven different colours—red, 
yellow, orange, green, blue, indigo, and violet. This was 
called the solar spectrum. "When the light of the sun 
was properly decomposed, it was found that the coloured 
spaces in the spectrum were not continuous, but that 
they were furrowed by about 2000 dark lines parallel 
to the axis of the analysing prism. These lines did not 
follow each other with any degree of regularity, but they 
always retained the same position for the same kind of 
light, independently of the material of which the prism 
was composed, and the size of the refracting angle. They 
differed, however, in number, size, and arrangement for 
different kinds of light; and Professors Kirchkoff and 
Bunsen had demonstrated that they were the result of 
metals existing in a gaseous form in the solar atmosphere, 
as they produced the same lines in the electric spectrum 
by applying iron, calcium, sodium, magnesium, and 
chromium to the carbon points, and then making the light 
thus produced pass through an atmosphere of the vapour 
of the same metals before it fell on the analysing prism. 
They thus proved that a gas or vapour absorbed those 
precise rays which it could itself produce. 
The discovery of spectrum analysis, made by Pro¬ 
fessors Kirchkoff and Bunsen, was next, after the dyna¬ 
mical theory of heat, the most important of the present 
century. By it we were made acquainted, inter alia, 
with the constitution of the sun, planets, fixed stars, 
and nebulae. Of the thirty-six known metals, seven¬ 
teen had been discovered in the sun’s atmosphere; and 
were it not that our atmosphere was so unsettled and 
the lines of the rest so faint, there was no doubt 
the rest would be discovered also. The sun was 
thus proved to be of the same matter as the earth; and 
as the spectra of the planets contained the same dark 
lines as that of the sun, it followed that they were 
opaque dark bodies shining by reflecting the sun’s 
light. Those fixed stars also on which correct observa¬ 
tions had been made were proved to consist of nuclei in a 
state of intense incandescence, surrounded by atmospheres 
in which, like that of the sun, metals existed in a state of 
vapour. By spectrum analysis also we were enabled to 
discover the constitution of nebulae, and to ascertain 
whether a star was approaching or receding from the 
earth, and with what velocity. 
But it was not only in researches in astronomy that 
spectrum analysis had proved so valuable. Several metals, 
the existence of which was not even dreamed of, had 
