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(*31) Common experiments in Natural Philosophy. If two red rays from two
luminous points be admitted into a dark chamber so as to fall on a
white surface, and differ in their length by 0.0000258 of an inch,
their intensity is doubled. So also if the difference in length be any
whole-number multiple of that fraction. A multiple by 2 1/4, 3 1/4, &c.,
gives an intensity equal to one ray only; but a multiple by 2 1/2, 3
1/2, &c., gives the result of total darkness. In violet rays similar
effects arise when the difference in length is 0.000157 of an inch; and
with all other rays the results are the same--the difference varying
with a uniform increase from the violet to the red.
(*32) Place a platina crucible over a spirit lamp, and keep it a red
heat; pour in some sulphuric acid, which, though the most volatile of
bodies at a common temperature, will be found to become completely fixed
in a hot crucible, and not a drop evaporates--being surrounded by an
atmosphere of its own, it does not, in fact, touch the sides. A few
drops of water are now introduced, when the acid, immediately coming in
contact with the heated sides of the crucible, flies off in sulphurous
acid vapor, and so rapid is its progress, that the caloric of the water
passes off with it, which falls a lump of ice to the bottom; by taking
advantage of the moment before it is allowed to remelt, it may be turned
out a lump of ice from a red-hot vessel.
(*33) The Daguerreotype.
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