Page:Popular Science Monthly Volume 11.djvu/88

78 sensitive to the twentieth of a milligramme, or the  of a grain.

A gold "blush" of sufficient thickness to produce a fine gold color was then deposited by the battery. The plates were washed in distilled water, dried, and reweighed without rubbing, and were found to have each gained in weight exactly one-tenth of a grain. It thus appears that one grain of gold may be distributed, by the galvanic deposit, over the surface of two hundred square inches, as contrasted with seventy-five square inches by beating. In other words, the metal is more than two and a half times thinner in the former case than in the latter, or compared with  inch.

A still thinner deposit of gold could, of course, be detected on the delicate assay-balance, but, owing to the transparency of the film, it would not possess the true gold-color. It seemed important to ascertain whether the gold was evenly distributed over the copper surface, or whether it was deposited in spots. The strips were accordingly examined under a microscope.

A careful examination showed that there were no exposed surfaces of copper, and the gold appeared to have a fine, bright, smooth surface. This, however, was not considered sufficient proof, and several expedients were tried to obtain the gold films free from the copper plate, in order that they might be examined by transmitted light. Owing to their extreme thinness this was difficult to accomplish. One method, which was partially successful, was to heat the copper plates to a cherry red in the annealing muffle of an assay-furnace. On cooling, the gold film peeled off in flakes with a thin backing of oxide of copper; these flakes were pressed between two plates of glass, and nitric acid allowed to flow in by capillary action. The acid dissolved the copper; leaving a film of free gold, The difficulty was, that the bubbles of gas formed perforated the film of gold. Another plan was then tried. The gold-plating was removed from one surface of the copper plates by means of fine emory-paper. Pieces about one inch square were immersed for several days in very weak nitric acid. The copper was entirely dissolved, and detached films of gold were found floating intact on the surface of the liquid; these were carefully collected on strips of glass, washed with distilled water, and dried; they then firmly adhered to the glass.

When examined by reflected light they retain their brilliant gold color and lustre, but when viewed by transmitted light they are bright green and very transparent; the color is an even shade, having none of the mottled appearance of gold-leaf when seen by transmitted light, caused by its very uneven thickness. This monotone appears to be a positive indication of uniform thickness, for, when two