8 PROF. OSBORNE REYNOLDS ON THE 



The experiment which I had conceived would have been 

 equivalent to a vertical glass tube more than 32 feet long, 

 closed at the upper end and open at the lower, so that when 

 the tube was full of water the column would be higher than 

 the pressure of the atmosphere would maintain, and hence 

 could only be maintained by the cohesion of the water. 

 The difficulty of such an experiment, however, appeared to 

 be great. It was clear that if mercury could be substituted 

 for water this difficulty would be much reduced ; but then 

 mercury does not readily adhere to glass, and the ordinary 

 method of making barometers seemed to disprove the pos- 

 sibility of making it adhere. 



It was only on the 2nd of this month that an accidental 

 phenomenon at once aflPorded me the experimental proof 

 for which I had been looking. 



First Experiments. 



The phenomenon was observed in a mercurial vacuum- 

 gauge (a siphon gauge which admitted a column of mer- 

 cury 3 1 inches long) . Before the mercury was introduced 

 the tube had been wetted with sulphuric acid, a few drops 

 of which covered the mercury on both ends of the column. 



The gauge had been in constant use as a vacuum-gauge 

 for three weeks ; and, probably owing to the action of the 

 acid on the mercury, a little gas had been generated between 

 the mercury and the closed end of the tube, sufficient to 

 cause the column to sink to 27! when the barometer stood 

 at 29. To get rid of this air, the tube was removed from its 

 situation and placed in such a position that the bubble of 

 air passed along the tube and escaped, the open end of the 

 tube being entirely ^free. Before the tube was tilted in this 

 way, the unbalanced column was 27! inches long. When 

 tilted, the mercury ran back right up to the end of the tube 

 as the bubble of air passed out. On erecting the tube 



