hundred and fifty leaves, "each leaf being four fingers square," or about three times thicker than our ordinary gold-leaf. On the coffins of the Theban mummies, gold-foil has been discovered of extraordinary thinness. The ancient Peruvians covered the walls of their temples with very thin sheets of gold, and the rude specimens of gilding on the palace of Tippo Saib, at Bungalore, prove that the art of gold-beating was practised in India. We also have Biblical authority for the antiquity of the art.[1]
Experiments made in modern times have shown that a single grain of gold may be beaten out so as to cover a space of seventy-five square inches; the thickness of the leaf is then only the three hundred and sixty-seven thousand six hundred and fiftieth (1367650) part of an inch, or about twelve hundred times thinner than an ordinary sheet of printing-paper.
Faraday states in his researches on "The Experimental Relations of Gold (and other Metals) to Light"[2] that a leaf of beaten gold occupies an average thickness of no more than 15 to 18 part of a single wave of light. He reduced the thickness of gold-leaves at pleasure, by spreading them upon glass plates and gradually dissolving the metal by means of a weak solution of cyanide of potassium. "By this means," he says, "I think fifty or even one hundred might be included in a single progressive undulation of light."[3]
Faraday's researches upon the nature of thin films of gold and other metals, and upon the size of finely-divided particles of gold diffused through various liquids, are of a most interesting and refined character. Availing himself of the well-known reducing power of phosphorus, he floated small particles of it upon the surface of weak solutions of chloride of gold. In the course of twenty-four hours he found that the surfaces of the liquids were covered with films of metallic gold, which were thicker near the pieces of phosphorus "possessing the full golden reflective power of the metal," but becoming so thin by gradations as to be scarcely perceptible. "They acted as thin plates upon light, producing the concentric rings of colors round the phosphorus at their first formation, though their thickness then could scarcely be the 1100, perhaps not the 1500, of a wave-undulation of light."
By treating very dilute solutions of gold with phosphorus he obtained the metal diffused through the liquid in extremely fine particles, producing a beautiful ruby-color. These particles, when in their finest state, often remain unchanged for months, and have all the appearance of solutions, but they never are such, containing, in fact, no dissolved but only diffused gold. The particles are easily rendered evident by gathering the rays of the sun (or a lamp) into a cone by a lens,
