Mr. J. B. Hannay's Examination of Vacua. 235 



the treatment, as it was similar to that before employed. 

 The vacuum obtained by this method would not pass a spark. 

 The metal used in this experiment had extra lead added to it, 

 so that it could be frozen. 



The bulbs were detached in the following way. The metal 

 was passed up for the last time and frozen, and the tube 

 melted through the metal at a, leaving a good column of 

 frozen metal between the bulbs and the seal, so that any gas 

 given off could not get into the former. The metal was then 

 lowered out of the bulbs, and again frozen above b, where the 

 tube was again melted through, and the air given off kept 

 imprisoned by the solid metal above h. This vacuum was 

 tested by the radiometer, but gave no retardation beyond 

 that due to the glass ; it would not, however, conduct a 

 spark. To test whether the glass had been really freed from 

 gas by this process, a pair of bulbs were evacuated as before, 

 and the metal passed fully half up the upper one. The bulb 

 was then heated to softening, so that its sides were sucked in 

 in several places just below the surface of the metal; and it 

 was invariably found that a sweat of gas-bubbles was formed 

 between the glass and the metal, showing that the glass still 

 retained air. The only method by which a real vacuum 

 might be obtained (putting aside glass vapour), would be by 

 having the bulbs made of hard glass outside and soft glass 

 inside, so that the inside might be fused and its gas layer 

 driven off, without collapsing the bulbs ; but I am not suffici- 

 ently skilled in glass-blowing to make such bulbs. As I have 

 been unable to get them made, the experiment has not been 

 tried. 



The work, as far as it goes, shows that no conclusions can 

 be drawn as to the nature of properties of high vacua, after 

 such vacuous space has been cut off, by sealing one of the 

 tubes by fusion ; and it is doubtful whether the measurements 

 by the McLeod gauge represent the true density of the 

 residual atmosphere. The object of this investigation — the 

 testing of Euler's hypothesis — has not been fulfilled, although 

 nearly six months' work was spent upon it, the accidental 

 breakages making the testing of a method a matter of a 

 month's work. I must record my hearty thanks to my two 

 assistants, Messrs. Campell and McConechy, for the unwearied 

 aid thev have given me in this troublesome investigation, 



