[ 3 £° ] 
of the electricities is truly plus, and which of them 
is minus. 
You muft remember, Lord Charles Cavendijh fir ft 
oblerved the luminous appearance in one continued 
dream throughout the whole vacuum of an exhauded 
tube. It is a fine experiment, and affords more in- 
formation than I at fil'd imagined, therefore I fhall 
beg leave to recite it from the Philofophical Tran- 
fabtions *, before I give you any farther particulars 
about it. 
“ This apparatus confided of a cylindrical glafs 
“ tube of about three tenths of an inch in diameter, 
“ and of feven feet and half in length, bent fome- 
u what like a parabola, in fuch a manner, that thirty 
“ inches of each of its extremities were nearly 
<c draight, and parallel to each other, from which 
an arch fprung, which was likewife of thirty in- 
<{ ches. This tube was carefully filled with mer- 
u cury : and each of its extremities being put into its 
“ baton of mercury, fo much of the mercury ran 
u out, until, as in common barometrical tubes, it 
“ was in equilibrio with the atmofphere. Each of 
<c the bafons containing the mercury was of wood, 
<c and was fupported by a cylindrical glafs of about 
<c four inches in diameter, and fix inches in length ; 
“ and thefe glades were fadened to the bottom of a 
“ fquare wooden frame, fo contrived, as that to its top 
“ was fufpended with filk lines the tube filled with 
“ mercury before mentioned ; fo that the whole of 
“ this apparatus without inconvenience might be 
* Vol. xlvii. 
a moved 
