122 
The Physical and Chemical 
[March, 
instead of such instantaneous extinction, a very slow de- 
crease were to set in, the possibility of life might not be 
appreciably lessened, say, for a century to come. So little 
connection has the persistence of energy with the sum total 
of people which this earth can support at any one time ! 
But the quantity of such energy, especially in the forms of 
light and heat, is a matter of much greater importance in 
this connection. Did the earth receive a considerably larger 
supply of heat, Greenland, Spitsbergen, the vast Antarctic 
continent, and many other portions of the frigid zones now 
waste, might be covered with luxuriant crops. Hence it is 
not so much the persistence, as the present quantity of light 
and heat, which gives the physical limit of population. 
We may go further : the invariability of the total amount 
of energy in the universe is not to be called in question. 
But that our earth will continue for ever to receive the same 
share of light and heat as she now does is, in the present 
state of knowledge, not to be supposed. The sun, which 
took its origin in time, must in time cool down, and cease 
to irradiate the planets. The planets themselves will have 
long ago dissipated their stock of internal heat, and will 
therefore either continue to roll round the extinCt sun as 
dark, dead bodies, — the corpses of worlds, — or will, one by 
one, have fallen into the decaying luminary, and for a time 
have revived his power. 
The latter case is, of course, instant destruction. The 
former must mean a gradual decrease of the light and heat, 
which even now is insufficient to maintain the whole globe 
in a condition fit for the maintenance of higher plant and 
animal life. Consequently the time must come — though it 
may be yet far remote, and may approach very slowly — 
when the lifeless and useless portions of the globe will 
broaden, and when death and sterility will creep from the 
Poles into what are now temperate regions. Surely the 
persistence of energy affords a shadowy basis for a faith in 
the possibility of a population of a thousand billions ! 
We come now to the second truth upon which Mr. George 
builds his extravagant estimate. Matter is, as far as we can 
perceive, indestructible. Be it so. But matter, as every 
school-boy is in these days supposed to know, consists of 
some seventy distinct kinds, not by any human agency con- 
vertible one into another. Of some of these kinds the supply 
is practically indefinite, whilst that of others is very limited. 
Now it .happens that some of the elements which are em- 
ployed in building up the bodies of plants and animals, and 
without which they cannot exist, are comparatively scarce. 
