562 EVENING DISCOURSES. 
tions ; and that each condensation would have something like the same mass as the 
observed nebulx. “Or to put the same thing in another way, the primeval gas would 
break up into condensations at distances apart comparable with the 1,800,000 light- 
years which Dr. Hubble finds for the average distance of the actual nebulz. Thus, 
although, from the nature of things, we can never know the truth for certain, there 
are good reasons for conjecturing that the nebule came into existence as condensa- 
tions formed by a primeval gas which was spread uniformly, or at least with some 
approach to uniformity, throughout space. 
Cosmogony presents us with a picture of the evolution of the universe—a cinemato- 
graph film—in which big bodies continually break into smaller; the film shows the 
one for ever changing into the many. One primeval gas produces millions of nebuilzx, 
each nebula produces millions of stars, each star may, perchance, change into a solar 
system producing millions of planets, comets and meteors. Even this is not the end 
of the story, for planets may break up and form satellites, satellites may break up 
and form rings of miniature moons, such as we see encircling Saturn. The extra- 
galactic nebulz illustrate the first two chapters in this general break-up— 
I. Primitive chaos into nebule. 
If. Nebulz into stars. 
The general breaking-up process of which this is the beginning is in operation 
throughout the universe. It might be thought that the attractive forces of gravitation 
would continually draw all the broken pieces together again. 
The exact reverse appears to be the case. Not only is the substance of the 
universe for ever being broken into smaller pieces, but these pieces for ever tend to 
scatter further and further apart. 
To take the example nearest home, the earth is forever driving the moon further 
away, by the agency known as tidal friction. When we watch the waves of the sea 
being checked by a sea-beach or a headland, we may reflect that their impact is not 
only slowing down the earth’s rotation and so lengthening the day—it is also lengthen- 
ing the month by driving the moon further away from the earth. Incidentally, it is 
also, through the solar tides, driving the earth further from the sun, and so lengthening 
the year as well. 
Again, every ray of sunlight that enters our eye carries mass with it; eight minutes 
previously this mass was part of the mass of the sun. Every second the sun loses 
more than four million tons of mass, in the form of sunlight and sun-heat. As the 
result of this continual loss of mass, the sun’s gravitational hold on its family of 
planets for ever weakens and these are driven further off into space. The earth’s 
orbit round the sun is not so much like a circle or ellipse as like a coiled watch-spring— 
a spiral for ever receding into the cold and dark of space. : 
The same tendency affects the galactic system as a whole. The stars of which it is 
formed continually scatter their mass broadcast in the form of radiation. As they 
do so, their gravitational hold on one another weakens, so that the whole galactic 
system for ever expands. 
And it must be the same with the other star-systems in space. Throughout the 
universe, all the smaller broken pieces, satellites, planets, stars, are scattering away 
from one another in apparent opposition to the laws of gravitation. 
Still more surprising and sensational is the recent discovery that the largest pieces 
of the universe—the great extra-galactic nebule we have been discussing—are to all 
appearances engaged in a similar scattering. They, too, appear to be running away 
from us and from one another. Until recently, it was thought that on the whole the 
nearer nebule were approaching the galactic system, while the more remote were 
receding. We now know that the nearer nebule appeared to be approaching merely 
because they happen to lie mostly in the direction towards which the sun is being 
carried by the rotation of the galaxy ; actually we are approaching them. After the 
sun’s motion in the galaxy has been taken into account, all, or nearly all, of the 
nebule appear to be receding from the galaxy. The nearer nebule have small speeds, 
and the more remote nebule have greater speeds ; in general, speed is approximately 
proportional to distance. This simple law seems to hold to the very furthest of the 
nebulze—Hubble finds that for every million light-years of distance, there is a speed 
of recession of about 105 miles a second. The last nebula to be investigated at 
Mount Wilson shows a speed of recession of 12,300 miles a second; its distance, as 
estimated from its faintness, being about 105 million light-years. 
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