fluid conductors when transmitting the electric current. 167 
in a horizontal plane, thus continually urging the circumfe- 
rence farther and farther out, by diminishing the radius of 
curvature of the vertical section of its edge. 
10. That friction against the vessel is the principal cause 
of the apparent attraction of a globule of mercury to the ne- 
gative end, may be proved evidently by the substitution of a 
glass for a Wedgwood- ware basin. In this case the currents 
are produced as before ; but, though equally forcible, the 
globule shows little or no tendency to move bodily, but if 
placed on a plate of emeried glass, or on any other rough 
surface, it will move with great activity ; nay, so strong is 
its tendency to the negative pole, that globules of consider- 
able magnitude may thus be sustained without contact of 
either wire, on surfaces many degrees inclined to the horizon. 
11. It is essential to the production of the motions in ques- 
tion, that the mercury be in actual contact and free commu- 
nication with the acid, and so situated as to be within the 
influence of the electric current. It is not necessary, how- 
ever, that a continuity of the acid should subsist between the 
positive and negative wires ; they will appear in any inter- 
rupted circuit of mercury and the liquid medium. The expe- 
riment indeed is difficult to try in sulphuric acid, whose 
capillary attraction for mercury is such that the least drop, 
applied to any part of a clean surface of that metal, instantly 
spreads over the whole, but with other conducting media it 
may readily be made. We have only to drop a little of the 
liquid to be tried on two different spots of a large clean sur- 
face of mercury, and bring the poles in contact with them, 
taking care not to plunge them in the metal, when the same 
phasnomena will be observed to take place about each pole as 
