600 Dr. Gr. B. Jeffery on the Identical 



We expect, however, to be able to obtain more definite 

 evidence on this point, by extending the investigations over 

 a larger range of the velocities of agitation u. The pro- 

 portion of the energy of an electron lost in a collision with 

 a molecule of nitrogen diminishes rapidly with the velocity 

 of the electron, so that the value of X for electrons moving 

 in argon would be much less affected by a small proportion 

 of nitrogen, if the determinations were made with velocities 

 of agitation of about 4 x 10 7 cms. per second instead of the 

 velocities given in Table III. 



Note. 



Since this paper was written we have noticed that Mayer* 

 and Ramsauerf have determined the penetration of electrons 

 in gases, and obtained estimates of the free paths of the 

 electrons or cross sections of the molecules. They found that 

 the free path in argon was about 15 times the free path in 

 nitrogen or hydrogen. They also found that the free paths 

 in argon were much increased when the velocity of the 

 electrons was reduced from 6'3 x 10 7 to 5*2 x 10 7 cms. per sec. 

 Ramsauer concluded that there was no noticeable change in 

 the free paths of electrons in the other gases for similar 

 changes in the velocity. The latter result is not in agreement 

 with our determinations. 



LXXI. The Identical Relations in Einstein's Theory. By 

 G. B. Jeffery, M.A., TJ.Sc, Fellow of University College, 

 London %. 



TN Einstein's general theory of relativity it is assumed 

 that the gravitational field is completely defined by the 

 coefficients g in the expression for the interval in the four- 

 dimensional continuum, 



$s 2 =g Ex Ba (1) 



where, with the usual convention, the repetition of a suffix 

 in the same term implies summation for values 1, 2, 3, 4 of 

 that suffix. 



It is further assumed that these coefficients satisfy the 



* H. F. Mayer, Ann. d. Phys. vol. lxiv. p. 451 (1921). 

 f C. Ramsauer, Ann. d. Phys. vol. lxiv. p. 513 (1921). 

 \ Communicated by the Author. 



