Spectrum of Aqueous Vapor. 227 



not exceed a certain strength ; for a powerful one drives off 

 also rarified air from the walls of the tubes and there is a 

 process of occlusion set up if large metallic terminals are 

 employed. Such large terminals are necessary if very strong 

 currents are used. With aluminum electrodes under the 

 action of strong currents, spectra of the combinations of nitro- 

 gen with oxygen and hydrogen are produced in tubes presuma- 

 bly filled with pure dry hydrogen. When the tubes have been 

 subjected to very strong currents it seems impossible to refill 

 them with pure hydrogen. In one case I filled a tube with pure 

 dry nitrogen and jDassed a current through it sufficiently power- 

 ful to melt down one of the aluminum hollow cylinders which 

 formed an electrode. The aluminum surface was filled with 

 cavities as if gases had bubbled from the interior. When this 

 tube was filled with dry hydrogen it gave again spectra of com- 

 pounds of nitrogen and no trace of the hydrogen spectrum. 

 A powerful condenser discharge, however, produced the 

 aqueous vapor spectrum. Hydrogen, thus, is extremely elu- 

 sive when subjected to electrical dissociation in the presence of 

 other gases in glass vessels with metallic electrodes, and indeed, 

 a steady battery current can be employed at a certain stage of 

 exhaustion to occlude hydrogen to such a degree that the 

 vacuum is apparently raised and no discharge can be forced 

 through the tube until it is subjected to external heat. This 

 occlusion or destructive electrical dissociation is very much in 

 evidence when the terminals of a battery of ten or twenty 

 thousand storage cells are connected to an X-ray tube through 

 a resistance of several megohms. If this tube is properly 

 exhausted, that is by repeated heating and by the use of electri- 

 cal discharges, no current can be forced through it unless the 

 tube is repeatedly flashed with a powerful Bunsen burner. 

 Suddenly, however, under the action of the heat a blue cloud 

 rises in the tube. The cathode beam appears, the anticathode 

 grows intensely hot and the exterior resistance must be quickly 

 increased to save the tube. The pressure has apparently been 

 greatly reduced ; but the experiment of Dr. William Rollins 

 with two connecting X-ray tubes, one of which is heated and 

 the other not, the heated one carrying a current and the other 

 resisting an eight-inch spark, shows that this is not the case and 

 that we have to deal with electrical dissociation and not with 

 change of pressure. Let us follow this experiment further. 

 Presently the blue cloud-in the tube grows smaller, the cathode 

 beam disappears and the cioud slowly sinks into the positive 

 electrode. At that moment the X-rays flash out with great 

 brilliancy ; but in a few seconds the light in the tube is totally 

 extinguished, and a further heating from an external source 

 is necessary before the phenomena can be repeated. It can be 



