AND MODERN PHYSICS. 141 



that tho kinetic energy is ultimately equally divided 

 among all the variables which determine the system. 



With regard to (1) Maxwell showed that his error 

 law was one possible condition of permanence. If at 

 any -moment tho velocities aro distributed according 

 to the error law, that distribution will be a permanent 

 one. He did not prove that such a distribution is tho 

 only one which can satisfy all tho conditions of the 

 problem. 



The proof that this law is a necessary, as well as a 

 sufficient, condition of permanence was first given by 

 Boltzmann, for a single monatomic gas in 1872, for a 

 mixture of such gases in 188G, and for a polyatomic gas 

 in 1887. Other proofs have been given since by Watson 

 and Burbury. It would bo quite beyond the limits of 

 this book to go into the question of the completeness 

 or sutliciency of the proofs. The discussion of the 

 question is still in progress. 



The British Association Report for 1894 contains 

 an important contribution to the question, in the 

 shape of a report by Mr. G. H. Bryan, and tho dis- 

 cussion ho started at Oxford by reading this report 

 has l)con continued in the pages of Xature and else- 

 where since that time. 



Mr. Bryan shows in the first place what may be 

 the nature of the systems of molecules to which the 

 results will apply, and discusses various points of 

 difficulty in the proof. 



The theorem in question, from which the result (1) 

 follows as a simple deduction, has been thus stated by 

 Dr. Larinor.* 



* JVfforr, vol. 1., p. 152 (DccemWr 13th, 1804). 





