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relative to each other, not the slightest effect, attractive or 
repulsive, or of any kind, can be observed. This is the 
phenomenon discovered by M. Arago, and he states that the 
effect takes place not only with all metals, but with solids, 
liquids, and even gases, i. e., with all substances." 
We first find that by the aid of magnetism a ray of light 
can be made to rotate. We then find that a copper plate 
when revolved causes a magnet to rotate, and vice versa, and 
this takes place on earth in spite of all the surrounding 
subtle disturbing causes, and with such power that magnets 
and plates of great weight are carried round by the influence 
thus exerted ; solids, liquids, and even gases are thus affected. 
Can we now draw the limit to this effect, can we say plates 
of a weight x may be caused to revolve, but those of a weight 
x and 1 lb. cannot be moved ? Is not this effect dependent 
upon the insulation in space of various bodies, and upon the 
amount of magnetic force in the relative bodies ? May not 
the distance of the two bodies be increased and the same 
effects result, if their insulation could be made perfect, and 
the magnetic force increased tenfold? Suppose this force 
could be increased ten thousand times, at what distance would 
the rotation of the one body induce the tendency in the other 
to follow its motion, and can we decide upon the variety of 
effects which would result, if the motion were continued 
during several years or centuries ? 
We have a certain fact with which to deal, viz., that the 
rotation in one body will produce motion in another, and that 
this is not satisfactorily accounted for may be seen from the 
following : — * 
" The cause which has now been assigned for the rotation 
in Arago's experiment, viz., the production of electrical 
currents, seems abundantly sufficient in all cases where the 
metals, or even other conductors are concerned ; but with 
• Experimental Researches, Vol. I. p. 38. 
P 
