Hutchins — Absorption of Gases in a High Vacuum. 63 



vanished in ten minutes, water alone remaining. As it 

 was possible in these two cases that the gases had under- 

 gone reabsorption upon the cooling of the substances from 

 which they had been expelled, a fresh portion of the gases 

 was driven off and the tubes allowed to stand twenty-hours. 

 No change could be observed in them. 



Tube containing iodine. — The iodine was shut off from the 

 main tube by a stopcock, fearing its vapor tension at ordinary 

 temperatures would be too great to admit of obtaining a suffi- 

 ciently high vacuum. The tube was exhausted, the stopcock 

 opened for a moment to fill the tube with iodine vapor, and 

 the exhaustion repeated. After sealing, the iodine disappeared 

 under the action of the current in less than a minute. The 

 tube was filled with iodine vapor again by opening the stop- 

 cock and warming the tube. Upon again starting the current 

 the platinum mirror upon the glass at the cathode was seen to 

 be attacked and converted into a dark bronze-colored substance 

 and the iodine again disappeared. 



Tube containing oxide of mercury, yielding oxygen when 

 heated, but no water vapor. Tube pumped and heated very 

 thoroughly, and when sealed showed the spectrum of oxygen 

 and the red hydrogen line faintly. The vacuum rose slowly ; 

 after fifteen minutes the hydrogen line disappeared ; at the 

 end of an hour the vacuum had become quite high, the spark 

 in parallel with the tube being two inches, but the current still 

 passing easily. The spectrum of the tube then consisted of 

 three lines of oxygen. 



Experiments were also made with carbon dioxide and some 

 other gases, the details of which need not be given here, inas- 

 much as it was found that oxygen was absorbed far more 

 slowly than any other substance tried. Mercury oxide is also 

 eminently adapted to maintaining a condition of constant 

 vacuum. It yields oxygen only at a high temperature, and the 

 flow of gas ceases immediately upon withdrawal of the source 

 of heat. 



To construct an X-ray tube in which the vacuum may be 

 maintained at the most efficient point, it is only necessary to 

 attach near the anode terminal a short tube having a sealed-in 

 platinum wire packed around with oxide of mercury. A plug 

 of asbestos may be used to hold the substance in place. When 

 by use the vacuum becomes too high, a portion of the current 

 is shunted from the cathode through the oxide of mercury for 

 a brief time ; a little gas is dislodged and the tube becomes as 

 new. The shunt may be left permanently in place with a 

 longer or shorter spark-gap at the mercury terminal. The 

 action will then be automatic, and vacuum remain indefinitely 

 at any desired point. 



Searles Physical Laboratory, 



Bowdoin College, November, 1898. 



