330 Dr. B. Hasselberg on the 



radiations in acetylene. This circumstance may be regarded 

 as a sufficient reason for a renewed study of the subject, in 

 view of the great importance which attaches to an exact 

 knowledge of the spectroscopic phenomena exhibited by this 

 gas ; the more so since it offers opportunity to give a more 

 accurate description of the spectrum than those which spec- 

 troscopic literature offers at present, and which are in no way 

 fitted to give even a moderately satisfactory idea of the spec- 

 trum in question. It is, indeed, surprising that so little has 

 been done towards the exact knowledge of a subject which 

 has so often given occasion to discussion ; and I should be 

 even inclined to regard this very circumstance as the reason 

 of more than one of the differences in opinion which still 

 exist upon this subject. As complete an examination of the 

 spectrum as possible, as it appears under different conditions, 

 will then, in my opinion, not only supply a gap in the spec- 

 troscopic literature concerning this gas, but will facilitate in 

 no small degree the solution of the problem concerning the 

 origin of the spectrum. In order to make this clearer, I will, 

 in the first place, and before passing to the description of 

 my own experiments, consider somewhat fully the inves- 

 tigations upon this spectrum published up to the present 

 time. 



As already mentioned, the first description of the spectrum 

 is to be found in the paper of Pliicker and Hittorf referred 

 to above. This gives in few words as exact a description of 

 the main features of the spectrum as possible without mea- 

 surements and drawings, and shows at any rate that the 

 subsequent designation of the spectrum as a band-spectrum, 

 belonging to the same category as the band-spectra of 

 nitrogen, sulphur, &c, is to be regarded as altogether in- 

 admissible. The description is as follows : — a Even in the 

 old spectral tubes enclosing highly-rarefied hydrogen, the 

 ground, from which the three characteristic lines rise, did not 

 appear always of the same darkness ; in some instances new 

 bright lines appeared, especially in the neighbourhood of the 

 sodium-line. In resuming the subject we pointed out the 

 existence of a new hydrogen spectrum corresponding to a 

 lower temperature, but having no resemblance at all to the 

 spectra of the first order of nitrogen, sulphur, &c. In this 

 spectrum, of a peculiar character if fully developed, we ob- 

 serve a great number of well-defined bright lines almost too 

 numerous to count and represent by an engraving, but bril- 

 liant enough to be examined with a magnifying power of 72, 

 after the light has passed through four prisms/' It is clear 

 from this that the spectrum possesses a character altogether 



