284 THE CHEMISTRY OF THE SUN. [CHAP. 



Now to the receptive mind there is nothing " anomalous " 

 about this : a higher temperature brings about simplification, 

 viola tout ! And what says the spectroscope ? On passing from 

 the density of 96 to that of 32, the vapour changes its spectrum 

 from one giving continuous absorption in the blue to one of 

 flutings. 



Here then we have, if vapour density determinations be worth 

 anything, an undoubted change of molecular grouping ac- 

 companied by a spectroscopic change exactly in the direction 

 required by other considerations. 



Have we then similar evidence having regard to the change 

 from the fluted spectrum to the line spectrum ? We have, and 

 it is precisely of the same nature as that afforded by sulphur, 

 except that the temperature employed is higher. 



For many years the sulphur result stood alone, but abundant 

 evidence has now been brought forward to show that iodine and 

 bromine behave exactly in the same way, that is they have 

 two different vapour densities at two different temperatures. 



Vapour density. Atomic weight. 



Bromine 



Above 2000 5 40 \ ^ 



Below 2000 80 j 

 Iodine 



Above 2000 63-5 \ 



Below 2 000 127 j 



This change of vapour density occurs at the temperature at 

 which the fluted spectra give place to line spectra. 



The absence of sodium and potassium from the above lists 

 may have been remarked, since they vaporize at low tempera- 

 tures. Very great difficulties have presented themselves in 

 experiments on these bodies, and various values have been 

 recorded of the density of their vapours. It is the opinion, how- 

 ever, of many, that it changes perhaps twice with the tempera- 



