A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s5.17

v1.c18.s5.17

Accordingly, in spite of the fact that the amount of sulphur added regulates the whole operation of the last stage of colouring, we are constrained to search for the reddish violet colour that is developed in the formation of some other compound than a sulphide. The nature of the colour being the same as would be produced by a thin layer of a violet substance on a yellow surface, it follows that the colour of the compound formed must be violet. And the fact that when the colour is overdone, *i. e.* when a thick layer of the coloured substance is formed on the surface, the appearance of the metal is dark violet, abundantly confirms the hypothesis. The only compound of gold that possesses this characteristic colour being aurous oxide Au₂O, we are forced to the conclusion that the colour of gold is due to the formation of a thin layer of suboxide on the surface of the metal.