602 SCIENCE PROGRESS 



that expansion is equal to the work spent by gravity in the 

 compression. If, then, the work gained from gravity in Process 

 2A, namely MgC'C, is all spent in compression, there is in the 

 complete cycle zero work spent. 



But if, as I think is the case, part only of the work MgC'C 

 is spent in compression, and part converted into heat, i.e. 

 dissipated (see Raleigh's remarks, pp. 317-8), we have in the 

 whole spent work equivalent to the dissipated energy. 



9. Is not the theory this ? If we had suitable machinery to 

 act on individual molecules, e.g. if we could employ Maxwell's 

 demons for the purpose, we might effect the separation of the 

 oxygen and hydrogen molecules mixed in the horizontal tube 

 without doing any work, no work being essential to the process, 

 unless molecules of the same gas appreciably attract or repel 

 each other, which is understood not to be the case. As we have 

 no such machinery, and cannot employ the demons, we have to 

 effect the separation by indirect processes, as in the example 

 given. And these indirect processes generally involve dissi- 

 pation. Work is spent in the separation of the gases, but 

 the energy is in no sense available. Energy dissipated is 

 unavailable. 



Professor Bryan's Argument 



10. I come now to Prof. Bryan's argument, which is mainly 

 theoretical. I will give his proof of the law (for there is a 

 logical proof) as I understand it. I begin with his definition of 

 " entropy," the second of p. 58. 



(a) If the unavailable energy of any system receives any 



positive or negative increase 8Q, the entropy is increased by 



SO 



^, T being the lowest available temperature. I think we may 



* o 



say, at least by way of example, without affecting the generality 

 of this definition, that BQ is the heat which passes into the 

 refrigerator in the lower isothermal of a Carnot cycle in which 

 the system is the working substance. 



(b) This being the definition of entropy, it follows that if the 

 entropy increases, the unavailable energy increases. There- 

 fore, since energy is either available or unavailable, if while the 

 entropy increases no energy enters or leaves the system from or 

 to external bodies, the available energy diminishes. 



(c) The entropy of a system is a function of the physical 



