472 THE POPULAR SCIENCE MONTHLY. 



distant. No one who has seen a spectrum of fluted bands can ever fail 

 to distinguish it from the other types of spectra which I have described. 



What, then, is the cause for the existence of these different types ? 

 The first editions of text-books in which our science was discussed 

 stated that a solid or liquid body gave a continuous spectrum, while a 

 gaseous body had a spectrum of lines ; the spectra of bands were not 

 mentioned. The more recent editions give a few exceptions to this 

 rule, and the editions which have not appeared yet, will so I hope, at 

 least tell you that the state of aggregation of a body does not directly 

 affect the nature of the spectrum. The important point is not whether 

 a body is solid, liquid, and gaseous, but how many atoms are bound 

 together in a molecule, and how they are bound together. This is one 

 of the teachings of modern spectroscopy. A molecule containing a 

 few atoms only gives a spectrum of lines. Increase the number of 

 atoms, and you will obtain a spectrum of fluted bands ; increase it 

 once more, and you will obtain a continuous spectrum. The scientific 

 evidence for the statements I have made is unimpeachable. In the 

 first place, I may examine spectra of bodies which I know to be com- 

 pound. Special i^recautions often are necessary to accomplish this 

 purpose, for too high a temperature would invariably break up the 

 compound molecule mto 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 supply a sufiicient quantity of luminous undecomposed matter to 

 allow the light to be analyzed 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 continuous spectrum. The spectra of the ox- 

 ides, 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, bromine, iodine, or oxygen. 



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

 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 spectrum 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 mole- 

 cule need not be necessarily a compound of an atom with an atom of 

 different kind, for it may be a compound of an element with itself. 

 We have ample proof that this is the true explanation of the different 

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

 of the view which is now nearly unanimously adopted by spectroscopists. 



I have hitherto left unmentioned one important method of investi- 

 gating the periods of molecular vibrations, a method which is applica- 



