ELECTRONS AND ATOMS 43 



energy, reference may be made to the volume 

 in this library entitled " Matter and Energy," 

 by Soddy. 



These periodic returns to similar properties 

 indicate rhythmically recurring points of 

 stability in atomic evolution. In each series 

 forming a chemical family, instability mounts 

 up and the power of reactivity varies. The 

 members of a group as a whole may be more 

 stable than those of the group above them 

 possessing molecular weights a unit or two 

 higher for each corresponding member, but at 

 length a point is reached in each group where 

 instability prevails, and no higher member 

 is found. Evolution has reached the highest 

 limit to which the stability of the atom will 

 carry it, and the tale of evolution is then 

 taken up by the chemical compound, until 

 this in turn loses its efficiency, and the next 

 stage, namely, that of the colloid, arises. 



Although it may only be a superficial 

 analogy, all this bears a curious resemblance 

 to the events which occur quite at the other 

 end of the scheme of molecular architecture. 

 It will be shown later that the unit of living 

 organisms, called the cell, consists of large 

 groups of chemical molecules, built up into a 

 delicately balanced mechanism upon which 

 energy types from the environment play, in 



