108 INORGANIC EVOLUTION. [CHAP. XI. 



a second smaller disc, which can be screwed lightly to the first. The 

 diameters of the two discs are 33 and 22-2 cm., the photographs being 

 taken in the annular space of 10'8 cm., left uncovered by the smaller 

 disc. An electric motor drives the disc, and we have obtained velocities 

 of 170 turns per second, though in our experiments the number of 

 revolutions was generally about 120, giving a linear velocity of about 

 100 metres/second for that part of the film on which the photograph 

 was taken." 



Now the curvature of the metallic lines must depend upon the 

 rate of diffusion of the vapours in opposite directions from the metallic 

 poles ; and if the spectrum of each metal used as a pole be due to the 

 vibrations of one set of molecules, there will be equal curvature in 

 all the lines of that metal. 



The photographs however, so far taken, show that the curvature 

 is not equal ; so in this work as in the other I have referred to in the 

 previous chapters, and shall refer to in subsequent ones, we are driven 

 to the conclusion that the spectrum has a complex origin. The results 

 of the investigation, so far as it has gone, have not yet been completely 

 published, but Dr. Schuster in a letter to me states that he has " no 

 doubt as to great differences in inclination [curvature] of the bismuth 

 lines. I also believe the difference to be real in the case of the zinc lines 

 (the green doublet being different from the blue triplet), but this I do not 

 consider established with the same certainty as in the case of bismuth." 



In order to give an example of the magnitude of the differences in 

 velocity determined by the unequal curvature of the lines, Dr. A. 

 Schuster allows me to print the following numbers : - 



Wave- Velocity 



Metal. length. metres/ second. 



Zinc 4925 1 41g 



4912J 



481 H 545 



4722 / 



Cadmium 5379 1 435 



5339 J 

 5086 1 



4800 L 559 



4416 | 



3613- 1 



Bismuth 5209 i 



4561 I- 1420 



3696 J 



4302 1 533 



4260 J 



3793 394 



Mercury . 4359 481 



3663 383 



