50 



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



end of the spectrum-tube by a cement of sodium silicate 

 (liquid glass). The light from the tube was focused on the 





slit by a quartz lens, having an aperture of 1 inch (2*54 cen- 

 tim.) and a focal length of 6 inches (15*2 centim.). The air- 

 pump used was an ordinary Sprengel ; and the arrangement 

 for admitting gases into the tube was the one recommended 

 by Cornu*. In this there is, in connexion with the spectrum- 

 tube, a barometer-tube whose lower end is joined to a movable 

 cup by a rubber tube. When the spectrum-tube is exhausted, 

 the mercury which is poured into the movable cup rises in the 

 barometer-tube and forms a trap. A U-shaped tube in which 

 water can be electrolysed is in communication with this 

 barometer-tube near the bottom. So, if the cup is lowered, 

 a bubble of gas or air will be forced into the spectrum-tube 

 by the atmospheric pressure. Drying-tubes of caustic potash 

 were introduced ; . and on each side of the spectrum-tube 

 several inches of sulphur, held between plugs of glass-wool, 



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



