[ 223 ] 



XXVIII. Proceedings of Learned Societies. 



ROYAL SOCIETY. 



January 19, 1860. 



[Continued from p. 164.] 

 ■Sir Benjamin C. Brodie, Bart., President, in 

 the Chair. 



and 



HPHE following communication was read : — 

 -*■ ■ On Vacua as indicated by the Mercurial Siphon-gauge 

 the Electrical Discharge." By J. P. Gassiot, Esq., F.R.S. 



That the varied condition of the stratified electrical discharge is due 

 to the relative but always imperfect condition of the vacuum through 

 which it is passed, is exemplified by the changes which take place 

 in the form of the striae while the potash is heated in a carbonic 

 acid vacuum-tube. In order, if possible, to measure the pressure 

 of the vapour, I had a carefully prepared siphon mercurial gauge 

 sealed into a tube fifteen inches long, at an equal distance between 

 the two wires A, B. 



yt 



u 



V 



This tube was charged with carbonic acid in the manner described 

 by me in a former communication. When exhausted by the air-pump 

 and sealed, it showed a pressure indicated by about 0*5 inch difference 

 in the level of the mercury; the potash was then heated ; the mercury 

 gradually fell, until it became perfectly level. 



Dr. Andrews (Phil. Mag. February 1852) has shown, that with 

 a concentrated solution of caustic potassa, he obtained with carbonic 

 acid a vacuum with the air-pump so perfect as to exercise no ap- 

 preciable tension, as no difference in the level of the mercury in the 

 siphon-gauge could be detected. 



On trying the discharge in the vacuum-tube after the potash had 

 cooled, I found it gave the cloud-like stratifications, with a slight 

 reddish tinge ; consequently not only was the vacuum not perfect, 

 as denoted by the form of stratification, but in this tube the colour 

 denotes that even a trace of air remains, — probably that portion in the 

 narrow part of the siphon-gauge, which, from its position, was not 

 displaced by the carbonic acid. 



The potash was subsequently heated until the discharge was 

 reduced to a wave-line, with very narrow striae ; in this state moisture 

 is seen adhering to the sides of the tube ; but even in this state the 

 difference in the level of the mercury in the gauge did not ever vary 

 more than "05 inch. As the potash cooled, the discharge altered 



