546 Mr. F. B. Jewett on a New Method of 



It may also be noted that the reciprocal relations between 

 torsion and magnetization, as found by the actual experiments, 

 will be found to be of paramount importance in the theory of 

 magnetostriction. We may conveniently place the results 

 of our experiment with those of previous investigators under 

 the following parallel statements : — 



Magnetization to Twist. Twist to Magnetization. 



(a) The magnetization of iron («') The torsion of iron de- 

 decreases by twisting for all mag- creases in all magnetizing lields. 

 netizing fields. 



(b) The magnetization of nickel (b') The torsion of nickel in- 

 increases by twisting in weak fields. creases in weak fields. 



(c) The magnetization of nickel (c) The torsion of nickel de- 

 decreases by twisting in strong creases in strong fields. 



fields. 



A similar reciprocal relation would probably exist in the 

 case of cobalt. The actual verification of the relation will be 

 undertaken in the near future. 



In conclusion we have to express our best thanks to 

 Prof. H. Nagaoka and also to Prof. A. Tanakadate for many 

 valuable suggestions. 



LXI. A new Method of determining the Vapour -Density of 

 Metallic Vapours, and an Experimental Application to the 

 Cases of Sodium and Mercury. By Frank B. Jewett *. 



IN all investigations on the composition and distribution of 

 light in the spectral lines it is of prime importance that 

 the lines themselves be as narrow and sharply defined as 

 possible ; this is especially true in those cases where the 

 analysis is carried on by means of interference phenomena, 

 for here the difference of path over which interference takes 

 place decreases as the width of the line increases. 



There are in general two causes which may affect the 

 breadth of the lines : {a) motion of the light-producing 

 molecules in the line of sight, and (b) change in the period 

 of the source caused by frequent collisions of the molecules f. 

 To these might be added a third cause, suggested by Lommel j, 

 in which an inhomogeneity is produced in the source by 

 forced changes in the period of ionic vibration, thus putting 

 an upper limit on the power to produce interference-fringes ; 

 this latter supposition is, however, yet to be verified, and irom 

 the present experimental data it seems probable that any 



* Communicated by the Author. 



t Michelson, Phil. Mag. (5) xxxiv. p. 293. 



X Lommel, Wied. Ami. iii. p. 251 ; Drude, Lehrbuch der Optik, p. 498. 



