1881.] on the Teachings of Modern Sjn'ctroscopy. 497 



iiiolccnlc containing a fo.w atoms only gives a ppcctrnm of lines. 

 Incrciise tlic number of atoms and you will obtain a spectrum of 

 fluted bands ; increase it once more, and you will oljtain a continuous 

 spectrum. The scientitio evidence for tlie statements I have made is 

 unimpeacliable. In the first place, I may exann'ne spectra of bodies 

 wliieh I know to be compound. Sjiecial precautions often are neces- 

 sary to accom})lish tliis purpose, for too higli a temperature would 

 invariably breuk uj) the compound molecule into its more elementary 

 constituents. For some bodies I may employ the low temperature 

 of an ordinary Bunsen burner. With others, a weak electric spark 

 taken from their liquid solutions will supjily a sufficient quantity of 

 luminous undccomi)osed matter to allow the light to be analysed by a 

 spectroscope of good power. The spectrum of a compound body is 

 never a line spectrum. It is either a spectrum of bands or a con- 

 tinuous si^ectrum. The spectra of the oxides, chlorides, bromides, or 

 iodides of the alkaline earths, for instance, are spectra of fluted bands. 

 All these bodies are known to contain atoms of different kinds, the 

 metallic atoms of calcium, barium, or strontium, and the atoms of 

 chlorine, brcmiine, iodine, or oxygen. 



But to obtain these spectra of bands we need not have necessarily 

 recourse to molecules containing different kinds of atoms. Elementary 

 bodies show these spectra, and we must conclude therefore that the 

 dissimilarity of the atoms in the molecule has nothing to do with the 

 appearance of the fluted bands. Similarity in the sj)ectrum must 

 necessarily be due to a similarity in the forces which bind the atoms 

 together, and this at once suggests that it is the compound nature of 

 the molecule which is the true cause of the bands, but that the 

 molecule need not be necessarily a compound of an atom wdth an atom 

 of diflicrent kind, for it may be a ccmipound of an element with itself. 

 We have ample proof that this is the true exj^lanation of the different 

 types of spectra. I shall presently give you a few examples in 

 6upi)ort of the view which is now nearly unanimously adopted by 

 spectroscopists. 



I have hitherto left unmentioned one important method of 

 investigating the periods of molecular vibrations, a method which 

 is aj)plicable to low temperatures. If I have a trans2)arent body and 

 allow light sent out by a body giving a continuous spectrum to fall 

 througli it, I often observe that the trans])arent body sifts out of the 

 liglit falling through it certain kind of rays. iSpectra 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 wdiich 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 liglit which it smds out. 



Vapours like bromine or iodine examined in this way give us 

 a spectrum of fluted bands. A powerful spark in these gases gives, 



