Hutchins — Absorption of Gases in a High Vacuum. 61 



Aet. IX. — Absorption of Gases in a High Vacuum / by 

 C. C. Hutchins. 



The term " absorption " is here used to cover the general 

 phenomenon of the disappearance of the residual gas in a high 

 vacuum under the electric discharge, whatever may be the 

 cause of the disappearance. 



It is well known to all users of X-ray apparatus that under 

 the discharge from an induction coil the tubes rapidly rise in 

 vacuum, and the current, after a longer or shorter time, ceases 

 to pass, and the usefulness of the tube is at an end. This rise 

 in vacuum constitutes the most serious defect in such tubes 

 and is a source of constant annoyance. 



It is probably safe to reject the theory that the electromo- 

 tive force drives the molecules of the residual gas through the 

 walls of the glass. It is possible to have an opening into a 

 highly exhausted tube without the entrance of air. 



This laboratory has for twenty years possessed a Crookes 

 radiometer that has during all that time been cracked entirely 

 around near the top of the bulb, but which, notwithstanding, con- 

 tinues to function perfectly. We also had for several months 

 an X-ray tube similarly cracked. By constant use this tube 

 became so high in vacuum that it was punctured by a spark; air 

 entered and the cracked end fell away of its own weight. 



The disappearance of the gas is therefore probably due to 

 mechanical and chemical combination with the metallic elec- 

 trodes of the tube, and it should be possible to find some gas 

 or vapor whose rate of absorption would be less in rate or 

 extent than that which takes place in the ordinary tubes. 



The first step in the inquiry is to learn what the ordinary 

 high-vacuum tube contains. 



If a Pliicker tube, made from carefully cleaned glass, be 

 very highly exhausted and then the current from an induction 

 coil turned into it, the vacuum falls at once, air and water 

 being driven from the walls of the tube. If now by heating 

 the tube, continuing the current and working the pump, the 

 vacuum be again raised to the point at which the fluorescence 

 of the glass appears, the spectrum of the tube will be found 

 to consist of the red and blue lines of hydrogen (C and F), 

 and three or four lines of oxygen ; that is, the tube contains 

 only water. The tenacity with which glass retains water upon 

 its surface is well known ; and it is necessary to heat the glass 

 nearly to its softening point before the last of the water comes 

 away. When the vacuum in a tube is so high that the current 

 will not pass, heating the tube will dislodge a little water and 

 the current will pass until the water has again condensed upon 

 the cooling of the tube. 



