74 INORGANIC EVOLUTION. [CHAP. 



density of iodine at changed temperatures and with a change in 

 spectrum. 



The specific gravity of iodine vapour was found by Deville and 

 Troost to be 8'72 (air = 1), which corresponds to the density 125-9, 

 proving that the molecule or two volumes of iodine gas weighs 

 126-53 x 2 = 253-06. When iodine vapour is heated to 700 its specific- 

 gravity begins to diminish until at higher temperatures it becomes 

 constant, and is half that at 700, the vapour consisting of free 

 atoms.* 



Another, but less direct, argument in favour of dissociation, 

 independently of the changes in the intensities of the lines, was based 

 upon some observations I had made in an attempt to work out a 

 spectroscopic method for the detection of impurities. I noted the 

 presence of what I termed "basic lines," that is, short lines which 

 remained common to two or more spectra, after " long lines " had 

 been eliminated as being due to impurities. 



I now refer to these different points seriatim. 



Flutings represent Vibrations of Complex Molecules. 



I take the change of the continuous spectrum successively into 

 flutings and lines first, and in justification of the statement that in 

 this matter my view is now generally accepted, I give the following 

 quotations from Schuster and Eder and Yalenta : 



" That the discontinuous spectra of different orders (line and band 

 spectra) are due to different molecular combination I consider to be 

 pretty well established, and analogy has led me (and Mr. Lockyer 

 before me) to explain the continuous spectra by the same cause ; for 

 the change of the continuous spectrum to the line or band spectrum 

 takes place in exactly the same way as the change of spectra of 

 different orders into each other."! 



"Spater fiihrte Lockyer weiter aus, dass die Gase, solange ihre 

 Molekiile aus mehreren Atomen zerfallen, Linienspectren geberi 

 miissen. Diese Anschauung wurde seither ziemlich allgemein accep- 

 tirt/'| 



The question of flutings was early conceded generally, but special 

 exceptions were made, carbon furnishes one instance. 



Messrs. Liveing and Dewar in 187 9 objected to my hypothesis, 



V Viet. Meyer, Ser. DeutscTi. CJiem. Ges., vol. xiii, pp. 394, 1010, 1103; Meier 

 and grafts, Compt. Rend., rol. xc, p. 690; rol. xcii, p. 39. 



f \chuster, Phil. Trans., 1879, Part I, vol. clxix, p. 39. 



J Eer and Valenta, DenJcschriften der Jcaiserlichen AJcademie der Wissen- 

 schaften, Wien, vol. Ixi, p. 426, 1894. 



Froc. Hoi,'. Soc., vol. xxx, p. 508. 



