Mr. J. S. Ames on some Gaseous Spectra, 51 



were placed so as to stop the mercury- vapour. In my entire 

 apparatus there were only three joints where the glass tubes 

 were not fused together. In each of these the following plan 

 was adopted to render it air-tight. One tube was slightly 

 coned out, the other inserted for about an inch, and the entire 

 joint was filled and covered with a preparation of Venice- 

 turpentine and beeswax, known in the laboratory as " uni- 

 versal." This " universal " is easy to mould when slightly 

 warmed, is quite tough, and sticks closely to glass which has 

 been thoroughly heated. These joints proved to be absolutely 

 air-tight, and no gases of any kind were given off after a few 

 days'' use. In some cases, for greater security, I put " uni- 

 versal " over the sodium-silicate joint between the spectrum- 

 tube and the quartz-end. The apparatus was cleaned and 

 filled by first pumping dry air through, then washing the 

 interior with ozone as proposed by Cornu, and finally admit- 

 ting the gas desired. There were, however, certain impurities 

 always present. Mercury and sulphur, and sometimes traces 

 of air, could be detected ; but in no case was I troubled with 

 any bands of carbon or its compounds. 



The induction-coil was driven by a storage-battery of twenty 

 volts, and was most regular and satisfactory. 



The photographs were taken on dry plates made specially 

 for this research by the M. A. Seed Dry-Plate Company 

 of St. Louis, Mo. Lantern-slide glass was used, as the 

 plates must be held bent to a radius of 6' 75 feet (2*06 metres). 

 I cannot speak too highly of the quality of the plates, or of 

 their fitness for photographing gaseous spectra. Ten seconds' 

 exposure was sufficient for the solar spectrum at w.-l. 3700 ; 

 and an exposure of an hour was more than sufficient for the 

 weakest gaseous spectrum. The plates were 1 inch wide and 

 11*5 inches (29 centim.) long. The dispersion of the instru- 

 ment was such that, using the second spectrum of the grating, 

 one plate included wave-lengths differing by 400 units (40//,,a). 

 The definition was perfect over the entire length. 



Hydrogen. 



The spectrum of hydrogen is of unusual importance owing 

 to its supposed presence in the light emitted by the white 

 stars, and also because of Balmer's formula for the distribution 

 of its lines which form a series. 



Huggins * has measured the lines in the stellar spectra, and 

 Cornu f has investigated the spectrum of hydrogen with a 

 view to their identification. The so-called " secondary " 



* Phil. Trans, clxxi. part ii. (1880). 

 f Journ. de Phys. [10] v. (1886). 

 E 2 



