be
placed at the end of a rod (1571.) over the uninsulated mercury, it is soon
swept away in particles streaming downwards towards the mercury. If another
drop be placed on the mercury beneath the end of the rod, it is quickly
dispersed in all directions in the form of streaming particles, the
attractive forces drawing it into elongated portions, and the currents
carrying them away. If a drop of mercury be hung from a ball used to raise
a column of the fluid (1598.), then the shape of the drop seems to show
currents travelling in the fluid in the direction indicated by the arrows
(fig. 141.).

1600. A very remarkable effect is produced on these phenomena, connected
with positive and negative charge and discharge, namely, that a ball
charged positively raises a much higher and larger column of the oil of
turpentine than when charged negatively. There can be no doubt that this is
connected with the difference of positive and negative action already
spoken of (1480. 1525.), and tends much to strengthen the idea that such
difference is referable to the particles of the dielectric rather than to
the charged conductors, and is dependent upon the mode in which these
particles polarize (1503. 1523.).

1601. Whenever currents travel in insulating dielectrics they really effect
discharge; and it is important to observe, though a very natural result,
that it is indifferent which way the current or particles travel, as with
reversed direction their state is reversed. The change is easily made,
either in air or oil of turpentine, between two opposed rods, for an
insulated ball being placed in connexion with either rod and brought near
its extremity, will cause the current to set towards it from the opposite
end.

1602. The two currents often occur at once, as when both terminations
present brushes, and frequently when they exhibit the glow (1531.). In such
cases, the charged particles, or many of them, meet and mutually discharge
each other (1518. 1612.). If a smoking wax taper be held at the end of an
insulating rod towards a charged prime conductor, it will very often happen
that two currents will form, and be rendered visible by its vapour, one
passing as a fine filament of smoky particles directly to the charged
conductor, and the other passing as directly from the same taper wick
outwards, and from the conductor: the principles of inductric action and
charge, which were referred to in considering the relation of a carrier
b