43 o THE POPULAR SCIENCE MONTHLY. 



THE DISSIPATION OF ENEKGY. 1 



By H. F. WALLING. 



THE dissipation of energy is a continuous process, quite familiar to 

 mankind in its main features and results, since the days of the 

 ancient philosophers. It was recognized by them that all mechanical 

 motions, being dissipated by friction, gradually diminish, and must 

 finally cease unless maintained by external power. In the language 

 of modern science, the motion which thus disappears is converted from 

 molar into molecular motion. 



It may be added that molecular energy, existing mainly in the form 

 called heat, tends to equalization of dynamic equilibrium, after the 

 attainment of which it is powerless to produce molar or mechanical 

 motion, a reconversion from the condition of equilibrium being impos- 

 sible. 



Accordingly, the power to produce mechanical motion, exerted by 

 the heat of tbe sun, which is being lavished with such prodigious 

 prodigality, can only last while the sun continues to be hotter than the 

 other bodies in space. At present it is well understood that all ter- 

 restrial motive power is derived from this source with the single 

 unimportant exception of that obtained from the tides, at the expense 

 of the earth's energy of rotation. Among the more obvious processes 

 of conversion of the sun's molecular into terrestrial molar motion, are 

 the expansion and contraction of the atmosphere, the evaporation and 

 condensation of water, and the less direct method by restoration of 

 potential chemical energy accomplished in vegetation, whence are 

 produced food and fuel. 



But it is supposed that the sun will finally grow cold, and that the 

 resistance of the ethereal medium, the evidence of whose existence is 

 found in the demonstration of the undulatory theory of light, will 

 cause satellites to fall into planets, planets into suns, and suns into one 

 common centre, after which, unless by special interposition of divine 

 power, darkness, silence, and death, will forever prevail. 



This gloomy prediction is of course inconsistent with the theory of 

 continuous evolution, which obviously excludes from cosmical economy 

 catastrophes or extensive destructive effects. 



A careful consideration, however, of the circumstances which will 

 be likely to accompany the falling of a satellite into its planet may 

 lead to the conclusion that this occurrence will not necessarily be 

 catastrophic. The process must certainly be an exceedingly slow one, 

 no progress in it having been detected throughout all the recorded 

 observations of the moon's motion extending over thousands of years. 



1 The Relation of the Dissipation of Energy to Cosmical Evolution. Read at the 

 Portland meeting of the American Association for the Advancement of Science. 



