Scientific amusements . with considerableeffervescence, giving off vapour very abundantly. Thisproperty has been put to good use in engraving with aquafortis. A copper plateis covered with a layerof varnish, and when itis dry some strokes aremade on it by means ofa graver ; if nitric acidis poured on the platewhen thus prepared, thecopper is only acted onin the parts that havebeen exposed by thepoint of steel. By afterwards removing the varnish, wehave an engraved plate, which will serve for printingpurposes. Among experiments that may be attempted with com-mon metals, we may mention that in w
Scientific amusements . with considerableeffervescence, giving off vapour very abundantly. Thisproperty has been put to good use in engraving with aquafortis. A copper plateis covered with a layerof varnish, and when itis dry some strokes aremade on it by means ofa graver ; if nitric acidis poured on the platewhen thus prepared, thecopper is only acted onin the parts that havebeen exposed by thepoint of steel. By afterwards removing the varnish, wehave an engraved plate, which will serve for printingpurposes. Among experiments that may be attempted with com-mon metals, we may mention that in which salts of tin areemployed. Tin has a great tendency to assume a crystal-line form, and it will be easy to show this property by aninteresting experiment A concentrated solution of proto-chloride of tin, prepared by dissolving some metallic tin inhydrochloric acid, is placed in a test glass ; then a rod oftin is introduced, as shown on page 28. Some water isnext slowly poured on the Iod, so that it gradually trickles. Tree of Saturn, 28 CHEMISTRY. down, and prevents the mingling of the proto-chloride oftin. The vessel is then left to stand, and we soon seebrilliant crystals starting out from the rod. This crystal-lization is not effected in the water ; it is explained by anelectric influence, into the details of which we cannot enter
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