THE TEACHINGS OF MODERN SPECTROSCOPY. 473 



ble to low temperatures. If I have a transparent body and allow light 

 sent out by a body giving a continuous spectrum to fall through it, I 

 often observe that the transparent body sifts out of the light falling 

 through it certain kind of rays. Spectra are thus produced which are 

 called absorption spectra, because the body which is under examination 

 does not send out any light, but absorbs some vibrations which are 

 made to pass through it. It is an important fact that a molecule 

 absorbs just the rays which it is capable itself of sending out. I can 

 therefore investigate the spectrum of a body just as well by means of 

 the absorption it produces as by means of the light which it sends out. 



Vapors like bromine or iodine examined in this way give us a spec- 

 trum of fluted bands. A powerful spark in these gases gives, how- 

 ever, a line-spectrum. Here, then, 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 ob- 

 served in the spectrum of a compound body the moment the tempera- 

 ture has risen sufficiently to decompose that body. I conclude 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 tem- 

 peratures 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 spectro- 

 scopic grounds have been definite and clear for some years, have re- 

 cently, 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 tempera- 

 tures, but we may be perfectly sure that chlorine, as well as the metal- 

 lic vapors of silver, sodium, potassium, etc., 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 -spec- 

 trum 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 inore complex. 



Sulphur-vapor, at a temperature just above its boiling-point, con- 

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

 a temperature of 1,000 Centigrade. The spectrum of sulphur-vapor 

 observed by absorption is continuous when the heavier molecule only 

 is present. At the higher temperatm*es, when each molecule is decom- 

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

 spectra. From the cases in which -wq can thus prove the change in 

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



