Resistance of Vacuum. 3 



moreover, in experimenting, various circumstances present 

 themselves which act in a perturbing manner upon the expe- 

 riments, and of which the consequences are difficult to calcu- 

 late. It will therefore be worth while to attempt to obtain, 

 by a synoptic account and the discussion of the researches 

 already made, some bases for the correct appreciation of this 

 important question. 



§2. 



Walsh took a glass tube bent into an arc in the middle so 

 that the two branches were parallel; this he rilled with mer- 

 cury and placed it vertically with the arc uniting the two 

 branches above, each branch dipping in an insulated cistern 

 filled with mercury. The mercury in the tubes then sank, 

 forming a vacuum of several feet length between the vertical 

 columns of mercury. When one of the cisterns was then 

 charged with electricity, this passed with a violet light through 

 the arc-shaped vacuum, and the mercury in the other cistern 

 showed itself electric. On the other hand, when the mercury 

 was carefully boiled in the tube in order to expel all traces of 

 gas or humidity, the luminous appearance was not presented, 

 and the mercury in the second vessel showed no trace of elec- 

 tricity. Thus the vacuum freed from air and moisture showed 

 itself perfectly insulating. The experiment took place in the 

 presence of Franklin, Smeaton, de Luis, and several other 

 men of science*. 



Morgan made an experiment for the same purpose, although 

 he proceeded differently. He filled with mercury a glass tube, 

 which he placed vertically in a cistern filled with the same 

 metal. This cistern could be closed hermetically by means of 

 a metal plate, and the air above the mercury removed by an 

 air-pump. When this was done the mercury of the vertical 

 tube descended, leaving a vacuum behind it. The upper ex- 

 tremity of the tube was covered over outside by a piece of 

 tinfoil. When the foil received electricity from the conductor 

 of an electrical machine, the interior of the tube became also 

 electric by induction; and this electricity passed, with a green 

 and violet light, through the vacuum and the column of mer- 

 cury to the cistern : this took place when the tube had been 

 filled with mercury without taking special precautions ; but 

 when the latter had been carefully boiled, the vacuum obtained 

 appeared to be insulatingf . 



For the investigation of the resistance of a vacuum Paul 

 Ermann made use of an ordinary cistern-barometer in which 



* Gilbert's 'Annals,' vol. ii. p. 161. t PHI. Trans. 1785. 



B2 



