cut into two equal parts, of which one was placed under the blue glass, and the other under a white glass, as nearly at the same time as possible; but the one did not become black sooner than the other; nor on comparing them could any difference be perceived in intensity of colour, both having been equally exposed to the chemical rays. The experiment was repeated with the same result.
On the 26th of August, the thermometer at noon being 66°, two neutral pieces of clock spring were exposed to the sun, one under a thicker piece of the same blue glass, as in the former, experiment, and the other under green glass; both acquired polarity.
31st of August, the thermometer at noon 68°. Having thus succeeded in producing magnetism under the circumstances described, I next tried the effect of exposing neutral pieces of clock spring to the sun, enveloped in violet and green silk. The half of each was covered with paper as before, and the pieces of spring then wrapped, one in green, and the other in violet-coloured ribbon, were fixed to the inside of a pane of glass in a window, where they were left exposed to the sun all day; in the evening both had become magnetic, although they were two of the pieces of spring already said to have acquired polarity more slowly from having been heated; and as before, the parts exposed to the sun under the ribbon were north poles.
To learn if heat had any share in producing magnetism in this case, I exposed three pieces of the same steel to a bright
sunshine, on the 1st of September, the thermometer at noon being at 70°: one half of each was covered with paper, but
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