Page:Experimental researches in chemistry and.djvu/409

394 Employing polarized light and an arrangement of sulphate of lime plates, it was found that other rays than the green could be transmitted by the gold-leaf The yellow rays appeared to be those which were first stopped 0.1 thrown back. Latterly I have obtained some pure gold-leaf beaten by Marshall, of which 2000 leaves weighed 4-08 grains, or 0.2 of a grain per leaf: its redected colour is orange-yellow, and its transmitted colour a warm green. Gold alloy containing 25 per cent. of silver produces pale gold-leaf, which transmits a blue purple light, and extinguishes much more than the ordinary gold-leaf.

So a leaf of beaten gold occupies in average thickness no more than from $1⁄5$th to $1⁄8$th part of a single wave of light. By chemical means, the film may be attenuated to such a degree as to transmit a ray so luminous as to approach to white, and that in parts which have every appearance of being continuous in the microscope, when viewed with a power of 700. For this purpose it may be laid upon a solution of chlorine, or, better still, of the cyanide of potassium. If a clean plate of glass be breathed upon and then brought carefully upon a leaf of gold, the latter will adhere to it; if distilled water be immediately applied at the edge of the leaf, it will pass between the glass and gold, and the latter will be perfectly stretched; if the water be then drained out, the gold-leaf will be left well extended, smooth, and adhering to the glass. If, after the water is poured off, a weak solution of cyanide be introduced beneath the gold, the latter will gradually become thinner and thinner; but at any moment the process may be stopped, the cyanide washed away by water, and the attenuated gold film left on the glass. If towards the end a washing he made with alcohol, and then with alcohol containing a little varnish, the gold film will be left cemented to the glass