526 Prof. J. Trowbridge on the Gaseous Constitution 



employed, the spectrum of calcium cannot be produced in the 

 capillary of the glass vessels. A direct test of the question 

 whether the reversed lines observed by me are due to the 

 glass is afforded by the use of quartz vessels, of which the 

 ends were closed by metal plates. There being a complete 

 absence of glass, and previous investigation having shown 

 that the metallic plates or terminals, and the luting employed, 

 gave no metallic spectra. In the case of quartz the powerful 

 disruptive sparks produced absolutely no corrosion of the 

 walls of the quartz capillaries ; and the reversed line at ap- 

 proximately wave-length 4227 and wave-lengths 3968, 3963 

 (H.H.) of the solar spectrum came out with the same in- 

 tensity as in the case when glass was employed. Moreover, 

 the strong calcium lines towards the ultra-violet, besides 

 those which apparently coincide with the H.H. lines of the 

 solar spectrum, were conspicuously absent. 



The reversed lines which I described in my previous 

 article, and which are shown on the plates of that article, 

 are not due to calcium. 



These lines may arise from an electrical decomposition of 

 residual air. It seems impossible to fill spectrum-tubes with 

 perfectly dry and pure hydrogen ; traces of air must enter 

 from the purifying and drying apparatus, and the impurities 

 may be brought to light by powerful discharges. I have 

 shown in a previous paper that the electrical decompositions 

 in a tube apparently filled with pure hydrogen can produce 

 various spectra, among them that of argon. The most pro- 

 mising method of obtaining pure dry hydrogen appears to be 

 by the use of liquid hydrogen. 



In my paper in this Journal, Feb. 1903, I spoke of a re- 

 markable reversal of lines in the ultra-violet which were 

 obtained by the use of quartz tubes. Fig. 4 (PL XIII.) 

 shows these lines with a companion spectrum of magnesium. 

 These reversed lines apparently coincide with spark-lines 

 of silicon in air ; and one might conclude that the lines come 

 from a volatilization of the walls of the quartz capillary. 

 There is, however, absolutely no corrosion of the walls of the 

 quartz vessel. The surface of the quartz remains limpid and 

 clear. I have concluded that, just as in the case of the 

 supposed calcium lines discussed above, these reversed lines 

 are also due to a gas. In order to discover whether these 

 lines can be obtained from some gaseous constituent of the 

 air, I have studied the spectra obtained from powerful sparks 

 in air taken from a great variety of metallic terminals. The 

 spectra from terminals of pure platinum, electrolytic silver, 

 and iridium show strong lines which coincide, with the dis- 

 persion I have employed, with the great H.H. lines of the 



