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^ 



