A History of Hindu Chemistry from the Earliest Times to the Middle of the Sixteenth Century A.D. v1.c18.s3.4

v1.c18.s3.4

An interesting and rather curious experiment clearly illustrates the above explanation. If a piece of gold containing a little copper be heated in air and care be taken not to rub the metal in any way so as to spread the black oxide of copper over the surface, it will be found that the metal, if free from external dirt or soot, will be still yellow and tolerably bright. The yellow of the more numerous gold particles overpowers the colour of the black oxide of copper. If, however, this black substance be spread over the surface of the metal, the colour instantly changes. The spreading of the oxide is easily brought about by hammering, and thus we can take an apparently bright piece of metal and, by hammering reduce it to a jet black mass.