336 Dr. B. Hasselberg on the 



scopists, we regard Swan's spectrum as that of carbon, we 

 might ascribe the third spectrum appearing in the above 

 gaseous mixture to the third component of the mixture, viz. 

 acetylene, as Berthelot and Richard do. But the matter is 

 not so. Against such an assumption we have not only 

 the arguments already brought, but also the circumstance 

 that the spectrum does not at all bear the character usual to 

 spectra of chemical compounds, since it does not consist, as 

 Berthelot and Richard maintain, of a system of channelled 

 spaces, but, on the contrary, must be placed in every respect 

 in the category of the line-spectra of the elements. Since, 

 moreover, it appears always in those cases where not only has 

 no acetylene been mixed with the gas, but where the greatest 

 care has been bestowed upon the purification of the gas, 

 it seems much more reasonable to ascribe it to hydrogen 

 itself than to assume for a compound body such as acetylene 

 spectral peculiarities having no analogy with those in any 

 other known case. Against what has here been said, the 

 objection may possibly be raised that even the greatest care in 

 the preparation of the hydrogen does not afford any complete 

 guarantee that the gas may not, after all, be rendered impure 

 by the presence of hydrocarbons, since it must of necessity 

 come repeatedly into contact with the different parts of the 

 air-pump. That carbon compounds may actually be intro- 

 duced into the gas in this way is not to be denied ; but it is 

 not necessarily the case, and I shall presently adduce experi- 

 ments which completely prove this. 



It need scarcely be mentioned that there is no doubt of the 

 identity of the spectrum mentioned by Berthelot and Richard 

 under No. 3 with Pliicker's second hydrogen spectrum. Since, 

 on the scale of their spectroscope, the hydrogen-lines H a , H^, 

 and H y fell respectively upon the divisions 13*5, 144*5, 

 and 229*0, the divisions 25 and 65, which the authors give 

 as the limits upon the same scale of the most conspicuous 

 part of the spectrum, correspond to the wave-lengths X=633 

 and X=570; and these are in fact, according to my measure- 

 ments, the limits within which the part of the spectrum in 

 question is included. 



The views of Berthelot were at the time shared by Ang- 

 strom*, which may appear surprising, since it was from his 

 investigations, made conjointly with Thalen, that the identity 

 of Berthelot' s second spectrum with that of acetylene appears. 

 But one or two remarks must be made upon this point. In 

 the first place, Angstrom has not altogether taken it as settled 



* C. R. lxxiii. p. 372 (1871) ; Togg. Ann. cxliv. p. 300. 



