12 SIR ROBERT S. BALL, LL.D., F.R.S., ON 
for a short time, any other star in the northern sky, and then 
there set in a decline and the star got gradually fainter. It 
did not go out entirely, but declined, and then every now 
and then there was a recrudescence. Sometimes the star 
could not be seen and sometimes it could. It suffered 
remarkable fluctuations in brightness and then gradually 
declined, and it had sunk down to a faint star when last seen. 
before the advancing daylight extinguished it. 
Now what is the origin of such a star as that? Let us 
first consider what the stars are. This isa question which 
depends very much on heat, or the degree of capacity for 
radiating heat and light that the stars present. Will 
you imagine this stick to be a thermometer with a long 
graduated scale, and where I[ hold my finger to represent 
the tempe1 rature of the bodies when red hot; when you can: 
just see them. Up about here would be the temperature 
of bodies when they would be as hot as suns; and below 
would be a very cold temperature—the temperature of space. 
Such a temperature as this Professor Dewar has shown 
us by his most remarkable researches to be that at which air 
freezes to a solid lump. Below the point corresponding to 
redness an object sends no light that can make it visible, 
but above that it sends heht which will make it visible. 
according to the degree of brightness and other circum- 
stances. I exclude the moon and the planets for the 
moment. Jupiter is only bright because it reflects back to: 
us a little sunlight. Were” Jupiter to pass out to where 
the stars in that picture are, you would not see it at all; 
and all the objects we see in the heavens, excluding the: 
planets, are objects which shine by their own light. They 
are objects of which the temperature would be on this part 
of the scale, above the red line. But the simplest con- 
sideration shows us that every hot body tends to get cold, 
and every one of those objects that we look at is radiating 
its heat, and is generally tending to cool, and consequently, 
tending to come down and pass this line. When once they 
get down there they may stay at that temperature to all 
cternity, unless some tremendous change takes place to 
bring them up again. There is a general tendency of padied 
in space to come down to the colder temperatures. When 
we look up at the heavens above and their myriads of objects, 
we must remember that we only see the bright objects, the 
dark ones are invisible. The earth only meni a two-thcusand 
millionth part of the light from the sun, and therefore you 
