474 '^HE POPULAR SCIENCE MONTHLY. 



fied in concludinnr that also in other cases, where no such chans^e of 

 density has yet been observed, it yet takes j^lace ; and it is not a very 

 daring generalization 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 vapors show in nearly all cases a continuous 

 spectrum before they condense. Oxygen gives a continuous spectrum 

 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 sj^ectra, presenting fairly narrow bands. It is not 

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

 may render the bands more wide and indistinct at their edges, but its 

 influence is more of a nature which in gas spectra is sometimes ob- 

 served at high pressures when the lines widen, and does not consist of 

 an alteration in type. Though in a solid or liquid body the molecules 

 are much nearer together, they are less mobile ; and hence the number 

 of actual collisions need not be necessarily much increased. The fact 

 that a crystal may show a difference in the absorption spectrum ac- 

 cording as the vibrations of the transmitted light take place along or 

 across the axis, shows, I think, that mutual imj^acts can not much 

 affect the vibrations, but that each molecule, at least in a crystal, must 

 be kept pretty well in its place. 



We have divided spectra into three types, but in all attempts at 

 classification we are met by the same difficulty. The boundaries be- 

 tween the different types are not in all cases very well marked. Every 

 one will be able to distinguish a well-defined band-spectrum from a 

 line-spectrum, but there are spectra taking up intermediate positions 

 both between the line- and band-spectra and between band-spectra 

 and continuous spectra. With regard to these it may be difficult to 

 tell to which type the spectrum really belongs. It may happen that a 

 change of spectrum takes place, the spectrum retaining its type ; but 

 in these cases, as a rule, the more complex molecule will have a spec- 

 trum approaching the lower type, although it may not actually belong 

 to that lower type. To be perfectly general, we may say that a com- 

 bination of atoms always produces an alteration in the spectrum in 

 the direction of the change from the line-spectrum, through the band- 

 spectrum to the discontinuous spectrum. 



If we accept the now generally received opinion as to the cause of 

 the different types of spectra, we may obtain information on molecular 

 arrangement and complexity vrhere our ordinary methods fail. At 

 high temperatures^ or under much diminished pressure, measures of 

 density become difficult or impossible ; and it is just in these cases 

 that the spectroscope furnishes us with the most valuable information. 



