Spectrum of Aqueous Vapor. 



225 



The operation of heating the spectrum tubes is highly 

 important ; moreover the discharge from a condenser should 

 be employed in the process of exhaustion and the operation 

 should be continued for several hours. In this subject of the 

 exhaustion of tubes I am much indebted to suggestions of Dr. 

 William Rollins and Mr. Heinze of Boston, who have had 

 long experience in the preparation of X-ray tubes. This 

 experience I believe is indispensable to one who essays to 

 investigate the spectra of gases in glass vessels. 



The vacuum pump was a modification of Rapp's pump. It 

 was automatic. There was but one valve and the number of 

 ground-glass joints with mercury seals was reduced to a 

 minimum. The hydrogen was produced by the electrolysis of 

 phosphoric acid and doubly distilled water. The gasholder 

 contained many liters of the gas. The nitrogen was prepared 

 by passing air through ammonia water, then through heated 

 copper gauze, then through tubes of caustic potash ; and 

 finally through tubes of phosphoric pentoxide. 



The spectroscope consisted of a Rowland grating with a 

 ruled surface five inches by two inches, and a radius of curva- 



