ON OUR KNOWLEDGE OF SrECTRUM ANALYSIS. 297 



hio-her tunes, stronger molecular shocks may bring out waves of smaller 

 leno-fcli. There are several instances of a regular increase in the relative 

 intensity of the blue rays which may be ascribed to this cause. The 

 most remarkable instance is perhaps seen in the spectrum of phospho- 

 retted hydrogen. If a little phosphorus is introduced into an apparatus 

 generating hydrogen, the flame will .show a series of bands, chiefly in the 

 o-reen. The spectrum gets more brilliant if the flame is cooled. This 

 can be done, according to Salet,^ by pressing the flame against a surface 

 kept cool by means of a stream of water or by surrounding the tube, at 

 the orifice of which the gas is lighted, by a wider tube throngh which 

 cold air is blown. The process of cooling the flame, according to Lecoq,^ 

 changes the relative intensity of the bands in a perfectly regular manner. 

 The almost invisible least refrangible band becomes strong, and the 

 second band, which was weaker than the fourth, now becomes stronger. 



As another example of a similar change we may give the_ spectrum 

 shown to a Bnnsen burner. By charging the burner with an indifferent 

 gas 3 (N", HCl, CO2), the flame takes a greenish colour, and though the 

 spectrum is not altered, the least refrangible of the bands are increased in 

 intensity. 



A similar change takes place, as pointed out by Mr. Lockyer,'' in the 

 two sets of bands of the spectrum ascribed by Profs. Liveing and Dewar 

 to a combination of nitrogen and carbon. (See Report on the Spectra of 

 Metalloids.) 



While in the cases we have just mentioned, the jDhenomena are per- 

 fectly regular, and such as would flow from a general law of a more rapid 

 increase in the intensity of the more refrangible lines by an increase of 

 temperature, there are other cases where the changes are very irregular, 

 as in the spectrum of tin, lithium, magnesiujn, the changes in which 

 spectra we have already noticed. Zinc behaves in the opposite way : a rise 

 in temperature increases the intensity of the less refrangible blue rays. 

 The theory which, as we have suggested, may account for this increased 

 brilliancy of the more refrangible rays is in accordance with the theory 

 which explains the long and short lines by molecular shocks, but the 

 theory of molecular combinations may also account for the facts which 

 are now before us ; for if the low-temperature spectrum is due to a more 

 complicated molecule, it is quite in accordance with our ideas of molecular 

 vibrations that its vibrations should take place in longer periods than 

 those of a simpler molecule. 



We have here again two hypotheses, that of molecular shocks and 

 that of molecular combinations. Both explain the facts satisfactorily. 

 and I do not think that one of them necessarily excludes the other. I 

 believe, on the contrary, that a line can be drawn, and that while the 

 regular changes observed chiefly in band spectra may be due to one cause, 

 the often irregular changes in metallic spectra, where one set of lines 

 disappears and another appears — often ou the violet side, but sometimes 

 towards the red — may be due to another. 



It is often said that we must not ascribe the same phenomenon to two 

 different causes, when one of them is sufficient to explain it ; but tho 

 point at issue is whether the phenomena are the same in all cases. An 



' Ann. Chim. PUys. xxviii. p. 57 (187.S). 



2 Spectres Lvmiiievx, texte p. 188 (1874). 



' Lecoq de Boisbaudran, Spectres Lumineux, p. -43 (187i). 



* Proc. Roy. Soc. xxs. p. 461 (1880). 



