130. The cause which has now been assigned for the rotation in Arago's
experiment, namely, the production of electrical currents, seems abundantly
sufficient in all cases where the metals, or perhaps even other conductors,
are concerned; but with regard to such bodies as glass, resins, and, above
all, gases, it seems impossible that currents of electricity, capable of
producing these effects, should be generated in them. Yet Arago found that
the effects in question were produced by these and by all bodies tried
(81.). Messrs. Babbage and Herschel, it is true, did not observe them with
any substance not metallic, except carbon, in a highly conducting state
(82.). Mr. Harris has ascertained their occurrence with wood, marble,
freestone and annealed glass, but obtained no effect with sulphuric acid
and saturated solution of sulphate of iron, although these are better
conductors of electricity than the former substances.

131. Future investigations will no doubt explain these difficulties, and
decide the point whether the retarding or dragging action spoken of is
always simultaneous with electric currents.[A] The existence of the action
in metals, only whilst the currents exist, i.e. whilst motion is given (82.
88.), and the explication of the repulsive action observed by M. Arago (82.
125.), are powerful reasons for referring it to this cause; but it may be
combined with others which occasionally act alone.

[A] Experiments which I have since made convince me that this
particular action is always due to the electrical currents formed; and
they supply a test by which it may be distinguished from the action of
ordinary magnetism, or any other cause, including those which are
mechanical or irregular, producing similar effects (254.)

132. Copper, iron, tin, zinc, lead, mercury, and all the metals tried,
produced electrical currents when passed between the magnetic poles: the