372 Prof. J. Trowbridge on the Spectra of 



were then exhausted and filled with hydrogen made bv the 

 electrolysis of distilled water and phosphoric pentoxide. " The 

 gas was sent through tubes filled with caustic potash and 

 many drying-tubes filled with phosphoric pentoxide. The 

 gas was kept in the drying-tubes many hours, and its flow 

 was delayed by partitions of glass-wool : more than a litre of 

 the gas was used in the process of flushing out the spectrum- 

 tubes, so that the entire pump and connecting-tubes were for 

 several hours presumably filled with hydrogen gas. 



When the tubes, having been exhausted to the most lumi- 

 nous stage, were excited by a condenser-discharge and were 

 examined by a straight-vision spectroscope, the ordinary four- 

 line spectrum of hydrogen alone seemed to be present. When, 

 however, the invisible portion in the violet was photographed, 

 the bands beginning approximately at wave-lengths 3900 

 and 4315 were invariably present, unless the tube had been 

 maintained, during the process of filling, at a temperature of 

 more than 350° C. After such a process of heating the 

 spectrum became that represented in fig. 2 (PI. VI.), while 

 before heating it was that shown in fig. 1. In both figures 

 the normal spectrum is above the gaseous spectra. Further 

 toward the ultra-violet under all conditions there were also 

 faint nitrogen bands. Long heating diminished the strength 

 of these bands. This process of experimentation shows that 

 mere eye-inspection of glass tubes filled with rarefied gases 

 is generally fallacious ; we might conclude from this eye- 

 study that the presence alone of the four-line spectrum of 

 hydrogen denoted that we had this gas in a pure state ; 

 whereas the photography of the invisible portion would show 

 that this was far from the truth. 



When the glass tubes filled with rarefied hydrogen were 

 submitted to the influence of a steady current of electricity, 

 it was found that perfectly pure copper was deposited in a 

 lustrous state on the glass walls of the tube which surrounded 

 the negative terminal, while an olive-green oxide of copper 

 covered the walls around the positive terminal. When the 

 same tube was excited by a RuhmkorfY coil, no difference could 

 be detected in the deposits around both terminals : they were 

 both rusty-green, with here and there it may be streaks of 

 pure copper. The mirrors produced by a strong, steady 

 current at the negative terminal were very lustrous, and 

 showed no trace of an oxide of copper. It was evident that 

 the current had dissociated water-vapour in the presence of 

 an excess of hydrogen, and had reduced the copper at the 

 negative pole, and had set free oxygen at the positive pole 

 which had, in turn, combined with copper. The rarefied gases 

 thus acted like a voltaic cell. 



