14 PROF. OSBORNE REYNOLDS ON THE 



to maintain itself for more tlian twenty or thirty seconds, 

 whicli failure was clearly due to the presence of air ; for 

 after the mercury had fallen a small quantity of air was 

 always found to collect above it. Sometimes, when on 

 inclining the tube the liquid again reached the top, the 

 bubble which remained was so small as to be scarcely 

 visible, although subject to no pressure other than the 

 surface-tension ; but its presence always became apparent 

 instantly on erecting the tube. In no case was it possible, 

 after the mercury had once fallen, to get it to remain up to 

 any considerable height above that due to the pressure of 

 the atmosphere until the bubble of air collected had been 

 allowed to pass out. 



The tube was then again emptied, washed, and filled 

 with glycerine. This behaved much in the same manner 

 as the water ; but the diflSculty of getting rid of the air 

 was greater. 



Similar results were obtained when very dilute ammonia- 

 liquid was tried. 



The tube was then again carefully washed, first with 

 water, and then several times with concentrated sulphuric 

 acid. The mercury was subjected to nitric acid, washed 

 and dried, and then filtered into a bottle of sulphuric acid, 

 from which it was poured into the tube, some acid passing 

 in with the mercury. When first introduced into the 

 tube a few small bubbles could be seen rising between the 

 mercury and the tube and passing up through the sulphu- 

 ric acid into the vacuum above ; but after it had stood for 

 five or six hours no bubbles were perceived, the surface of 

 the mercury against the tube being perfectly clear ; never- 

 theless, on erecting the tube, the mercury would not rise 

 above the height of the barometer, and air was always found 

 to have collected above the mercury. Water was then 

 introduced so as to dilute the acid ; then the mercury was 



