ADDRESS. 29 
Now we know that the earth contains radio-active materials, and it 
is safe to assume that it forms in some degree a sample of the materials 
of the solar system. Hence it is almost certain that the sun is radio- 
active also ; and besides it is not improbable that an element with so 
heavy an atom as radium would gravitate more abundantly to the central 
condensation than to the outlying planets. In this case the sun should 
contain a larger proportion of radio-active material than the earth. 
This branch of science is as yet but in its infancy, but we already see 
how unsafe it is to dogmatise on the potentialities of matter. 
It appears, then, that the physical argument is not susceptible of a 
greater degree of certainty than that of the geologists, and the scale of 
geological time remains in great measure unknown. 
I have now ended my Foe of the solar sn and must pass 
on to the wider fields of the stellar universe. 
Only a few thousand stars are visible with the unaided eye, but 
photography has revealed an inconceivably vast multitude of stars and 
nebule, and every improvement in that art seems to disclose yet more 
and more. About twenty years ago the number of photographic objects 
in the heavens was roughly estimated at about 170 millions, and some 
ten years later it had increased to about 400 millions, Although Newcomb, 
in his recent book on ‘The Stars,’ refrains even from conjecturing any 
definite number, yet I suppose that the enormous number of 400 millions 
must now be far below the mark, and photography still grows better year 
by year. It seems useless to consider whether the number of stars has 
any limit, for infinite number, space, and time transcend our powers of 
comprehension. We must then make a virtue of necessity, and confine 
our attention to such more limited views as seem within our powers. 
A celestial photograph looks at first like a dark sheet of paper splashed 
with whitewash, but further examination shows that there is some degree 
of method in the arrangement of the white spots. It may be observed 
that the stars in many places are arranged in lines and sweeping 
trains, and chains of stars, arranged in ante parallel curves, seem 
to be drawn round some centre. A surface splashed at hazard might 
present apparent evidence of system in a few instances, but the frequency 
of the occurrence in the heavens renders the hypothesis of mere chance 
altogether incredible. 
Thus there is order of some sort in the heavens, and, although no 
reason can be assigned for the observed arrangement in any particular 
case, yet it is possible to obtain general ideas as to the succession of 
events in stellar evolution. 
Besides the stars there are numerous streaks, wisps, and agglomera- 
tions of nebulosity, whose light we know to emanate from gas. Spots of 
intenser light are observed in less brilliant regions ; clusters of stars are 
sometimes imbedded in nebulosity, while in other cases each individual 
