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l 4 4 FRAGMEN TS OF SCIENCE.

Capital specimens of such glass are to be found at Salviati’s
in St. James’s Street. What artists call “chill” is no
doubt an effect of this description. Through the action of
minute particles, the browns of a picture often present the
appearance of the bloom of a plum. By rubbing the var-
nish with a silk handkerchief optical continuity is estab-
lished, and the chill disappears. Some years ago I wit-
nessed Mr. Hirst experimenting at Zermatt on the turbid
water of the Visp, which was charged with the finely-divided
matter ground down by the glaciers. When kept still for
a day or so, the grosser matter sank, but the finer matter
remained suspended, and gave a distinctly blue tinge to
the water. The blueness of certain Alpine lakes has been
shown to be in part due to this cause. Professor Roscoe has
noticed several striking cases of a similar kind. In a very
remarkable paper the late Principal Forbes showed that
steam issuing from the safety-valve of a locomotive, when
favorably observed, exhibits at a certain stage of its con-
densation the colors of the sky. It is blue by reflected
light, and orange or red by transmitted light. The same
effect, as pointed out by Goethe, is to some extent ex-
hibited by peat-smoke. More than ten years ago I amused
myself at Killarney by observing on a calm day the straight
smoke-columns rising from the cabin chimneys. It was
easy to project the lower portion of a column against a
dark pine, and its upper portion against a bright cloud.
The smoke in the former case was blue, being seen mainly
by reflectedlight; in the latter case it was reddish, being
seen mainly by transmitted light. Such smoke was not in
exactly the condition to give us the glow of the Alps,
but it was a step in this direction. Brilcke’s fine pre-
cipitate above referred to looks yellowish by transmitted
von der Rosskastanie, man stecke denselben in ein Glas Wasser, und in

der kiirzeSten Zeit werden wir. das vollkommenste Himmelblau entstelen
sehen.”—Goethe’s Wérke, b. xxix., p. 24.

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