The alteration in colour produced by heating the metal may possibly be due not only to the removal of a coating of organic matter from its surface, but also to the following action. It is well known that an alloy of silver and gold, which contains so much of the former metal as to resemble it in colour, may be made to acquire the colour of gold by superficially removing the silver. This may be effected by various processes, some of which, there is reason to believe, were known to the ancients. When such alloys of silver and gold as those above mentioned are heated to redness for a certain time, after having acquired superficially the colour of gold by any of the processes in question, they resume their original silvery colour. The large oval medal-like coins of the Japanese furnish an excellent illustration of this fact. Such a coin has been found in the Laboratory of the Royal School of Mines to consist of about two parts by weight of silver, and one part of gold. On heating such an alloy sufficiently, it becomes almost silver-white; and on subsequently treating it with hot sulphuric acid the original golden colour is restored. * * * * * II.--SHEET GOLD. (No. 543.) [Illustration: No. 543. A piece of Sheet Gold. Sepulchre IV. Actual size.] The total weight of the specimen was 1·702 grain, and its thickness was about 1-100th of an inch. Its specific gravity at 60° Fahr. was 18·867. Annexed is a sketch of the specimen, of the actual size. It was yellow, soft, ductile, and marked or indented on the surface, which appeared as though slightly tarnished. After cleaning with warm water, dilute hydrochloric acid, and gentle rubbing, the metal weighed 1·698 grain, which is equal to a loss of 0·235 per cent. 1·4 grain of the cleaned metal was analysed with the following results:-- COMPOSITION PER CENT. Gold 89·36 Silver 8·55 Copper 0·57 Iron 0·20 ----- 98·68 ----- The absence of lead suggests that possibly the metal may have been native gold, or prepared with native gold, of which silver is always a constituent in varying proportions. * * * * * III.--PART OF A SILVER VASE. A sketch of this, of the same size as the original, is annexed; it was distinctly curved, having formed part of a hollow thin vessel. [Illustration: Fragments of a Silver Vase. Sepulchre IV. Actual size. No. 544. Convex Surface. No. 545. Concave Surface.]