275 



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 strise ; 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 

 through all the well-known phases of the striae, the mercury again 

 becoming quite level. 



At first almost the slightest heat applied to the potash alters the 

 form of the stratifications ; as the heating is repeated, longer appli- 

 cation is necessary ; but it shows how sensibly the electrical discharge 

 denotes the perfection of a vacuum, which cannot be detected by the 

 ordinary method of mercurial siphon-gauge. 



January 26, I860. 



Sir BENJAMIN C. BRODIE, Bart., President, in the Chair. 

 The following communications were read : 



