498 Dr. Arthur Schuster [Jan. 28, 



however, a line-spectrum. Here, tlieu, a change of spectrum has 

 taken place. The same body at different temperatures gives us a 

 different spectrum, and the change which takes place is the same as 

 that observed in the spectrum of a compound body the moment the 

 temperature has risen sufficiently to decompose that body. I con- 

 clude from spectroscopic observations, therefore, that the molecules 

 of bromine and iodine just above their boiling-point are complex 

 molecules, which are broken up at the temperature of the electric 

 spark. At high temperatures the molecules of these bodies contain a 

 smaller number of atoms, and it follows from this that the gases must 

 be lighter or that their density must be smaller. These conclusions, 

 which on spectroscopic grounds have been definite and clear for some 

 years, have recently, by independent methods, been confirmed by 

 Victor Meyer and others. It has been directly proved that at high 

 temperatures the molecules of iodine and bromine contain a smaller 

 number of atoms than they do just above their boiling-point. In other 

 cases the change of density has not been directly proved, only because 

 these necessary measurements are difficult or even impossible at very 

 high temperatures, but we may be perfectly sure that chlorine, as 

 well as the metallic vapours of silver, sodium, potassium, &c., which 

 show an analogous change of their spectra, will ultimately be proved 

 to undergo a change of density at high temperatures. 



As we can trace the change from a line-spectrum to a band- 

 spectrum taking place simultaneously with an increase of density, 

 so may we follow the change from a band-spectrum to a continuous 

 spectrum indicating the formation of a molecule still more complex. 



Sulphur vapour, at a temi)erature just above its boiling- jDoint, con- 

 tains three times the number of atoms iu one molecule that it does at 

 a temjDerature of a thousand degrees. The spectrum of sulphur vapour 

 observed by absorption is continuous when the heavier molecule only 

 is present. At the higlier temperatures, when each molecule is decom- 

 posed into three, the spectrum belongs to the type of fluted band- 

 spectra. From the cases in which we can thus prove the change in 

 the spectra and in the densities to go on simultaneously, we are justi- 

 fied in concluding that also in other cases, where no such change of 

 density has yet been observed, it yet takes place ; and it is not a very 

 daring generalisation to believe that a change in spectra is always 

 due to a change in molecular arrangement, and generally, perhaps 

 always, accompanied by a change in the number of atoms which are 

 bound together into one molecule. 



With regard to the well-known statement, that solids and liquids 

 give continuous spectra, while gases give line-spectra, it must be 

 remarked that metallic vapours show in nearly all cases a continuous 

 spectrum before they condense. Oxygen gives a continuous sj)ectrura 

 at the lowest temperature at which it is luminous. Examining liquids 

 and solids by the method of absorption, we find that many of them 

 show discontinuous spectra, presenting fairly narrow bands. It is not 

 denied that the nearness of molecules does not affect the spectrum. It 



