July 19, 1921 clarke : evolution of matter 291 



formed, and their rate of decay, as shown between uranium and lead, 

 is also relatively slow. The formation and decomposition of com- 

 pounds, on the other hand, is rapid; and in some cases their rate is 

 measurable. The distinction is not absolutely definite, for some of 

 the short-lived products of radioactive decay seem to be exceptions to 

 the rule, which in general may be stated as follows: The process of 

 evolution is characterized by progressive acceleration, being slow at 

 first, and becoming gradually more and more rapid. Its rate of 

 acceleration may not be uniform but the general drift is clear. It 

 follows the line from the simplest substances to the most complex. 

 In all vital processes the ease and rapidity with which compounds are 

 formed and developed is evident, and some of these substances are 

 extremely complicated. 



In any attempt to discuss the evolution of the chemical elements 

 we have for guidance some facts and many analogies. That the most 

 complex elements are unstable we have already seen, and it is sus- 

 pected that all the others follow the same rule. Potassium and rubid- 

 ium are feebly radioactive, which is an evidence of instability, and 

 other confirmatory evidence will be cited later. Stability, however, 

 is a relative term, and a substance which is stable under certain con- 

 ditions becomes unstable under others. The prime factors which 

 determine external stability are temperature, pressure, and chemical 

 environment. For example, some compounds which are stable in 

 anhydrous surroundings are decomposed in presence of water. Calcium 

 carbonate, under ordinary conditions, is divided at high temperatures 

 into carbon dioxide and lime, but heated in a steel bomb it not only re- 

 mains undecomposed, but it may even be melted, to form upon cooling 

 a crystalline marble. Examples like these might be multiplied in- 

 definitely. As a rule stability diminishes with increasing temperature, 

 but ,is favored by increased pressure. We may also assume that the 

 more symmetrical an atom or compound is, the more stable it is likely 

 to be. We are dealing now with compounds; but to the evolution 

 of the elements the same general rules must apply. 



Now, returning to our main problem, was the evolution of the ele- 

 ments a regular progression, such as might be represented by a smooth 

 curve or a straight line ; or was it irregular and quite independent of 

 their order in the scale of atomic weights? To answer this question 

 we must try to imagine what happened in the development of the 

 larger masses, the nebulae and the stars. On this subject there is a 

 plausible hypothesis which has been favored by many astronomers; 



