PHYSICAL SCIENCE IN THE NINETEENTH CENTURY 359 



sipated heat. All workers in this field, from Carnot to Thom- 

 son, had appreciated the impossibility of "perpetual motion." 

 Helmholtz expresses his appreciation of Thomson's contri- 

 bution to the theory in a striking passage : 



We must admire the acumen of Thomson, who could read between 

 the letters of a mathematical equation, for some time known, which 

 spoke only of heat, volume and pressure of bodies, conclusions which 

 threaten the universe, though indeed only in infinite time, with eternal 

 death. 



Thomson, more than any other thinker, put the problem into 

 common-sense language. . . . He saw at once, when adopting Joule's 

 doctrine of the convertibility of heat and mechanical work, that, if 

 all processes in the world be reduced to those of a perfect mechanism, 

 they will have this property of a perfect machine, namely, that it 

 can work backward as well as forward. It is against all reason and 

 common sense to carry out this idea in its integrity and completeness. 

 If then, the motion of every particle of matter in the universe were 

 precisely reversed at any instant, the course of nature would be simply 

 reversed forever after. The bursting bubble of foam at the foot of a 

 waterfall would reunite and descend into the water; the thermal 

 motions would reconcentrate their energy and throw the mass up the 

 fall in drops, re-forming into a close column of ascending water. Heat 

 which had been generated by the friction of solids and dissipated by 

 conduction and radiation with absorption, would come again to the 

 place of contact and throw the moving body back against the force 

 to which it had previously yielded. Boulders would recover from the 

 mud the materials required to rebuild them into their previous jagged 

 forms, and would become re-united to the mountain-peak from which 

 they had formerly broken away. And also, if the materialistic hypoth- 

 esis of life were true, living creatures would grow backwards with 

 conscious knowledge of the future, but with no memory of the past, 

 and would become again unborn. But the real phenomena of life 

 infinitely transcend human science; and speculation regarding con- 

 sequences of their imagined reversal is utterly unprofitable. Far 

 otherwise, however, is it in respect to the reversal of the motions 

 of matter uninfluenced by life, a very elementary consideration of 

 which leads to the full explanation of the theory of dissipation of 

 energy. 



