24 EVOLUTION 
that under suitable conditions other matter may be 
observed to behave in the same way as radium. More 
than this, Professor Sir J. J. Thomson has been able to 
describe the atoms of the elements as different aggrega- 
tions of a single kind of corpuscles, and to show that 
a progressive change in the number of corpuscles 
making up the atom is accompanied by a progressive 
alteration in the properties of the atom itself, so that 
it has now become possible to establish a theory of 
the evolution of the chemical elements themselves. 
Passing from the almost immeasurably small to the 
almost immeasurably great, we may briefly consider 
the probable mode of origin of the solar system from 
an extremely diffuse cloud of material substance, ac- 
cording to the famous nebular hypothesis of Laplace. 
By a long-continued process of contraction under the 
influence of gravity the nebular substance came to be 
of varying density, and acquired a rotary movement 
in one plane. As the mass continued to contract 
owing to the mutual attraction of its particles, the 
velocity of rotation increased, until at last the increas- 
ingly rapid motion of the outermost ring of the now 
lens-shaped nebula gave rise to a centrifugal force great 
enough to counteract the tendency to contraction, and 
in the further condensation of the mass this ring was 
left behind. The ring next broke down at one point, 
and contracting on itself gave rise to a single spheroidal 
body which acquired a movement of rotation in the 
same direction as that of the parent nebula. This 
body was the outermost planet Neptune, and the rest 
of the planets were produced in a similar manner, 
