52 og THE ALLOYS OF COPPER AND ZINC. 
together 50 parts copper with 50 of zinc, as being of a deep yellow color; while 
Karsten * states that it is considerably darker and redder than ordinary brass.j 
They have evidently all been misled by the superficial coating just described, the 
color of. which is completely analogous to that which occurs upon the crystals of — 
metallic bismuth, now so often to be found in chemical cabinets; or by the color 
of an almost equally superficial layer of yellow alloy, which forms upon those parts 
of the ingot which are suddenly cooled. To these changes produced by “tempering” 
I shall presently again allude. That the surface color is most probably mainly due 
to oxidation, however, would appear from the fact that not only has the brilliant 
yellow coating of my specimens of braziers' solder, and of other iridescent alloys, 
not been dimmed by exposure during several months in a damp cellar, where 
ordinary polished brass soon became tarnished, but in several instances in which 
it had been removed by filing, a somewhat similar yellow coat has reappeared after 
exposure to moist air. Although this iridescence is very strongly marked in the 
aloy known as braziers solder, yet an instance perhaps still more striking pre- 
sented itself in the alloy containing 41.48 per cent of copper. Being very brittle, 
a specimen of this alloy was accidentally broken into several fragments before it 
had become thoroughly cold, when a beautiful, glittering iridescent film of a light 
yellow color at once formed upon its surface. As the alloy was very compact and 
fine-grained, it could at first sight hardly be believed that the coloration was only 
superficial; but on breaking the mass after it had become cold, its true color was 
found to be a very light gray, — indeed, almost white, — as was also proved by filing 
off a portion of the yellow film. The ingot obtained from the liquid portion of 
this alloy, having been suddenly cooled, was nearly white, and presented no appear- 
ance of iridescence. Upon its under surface, however, which had been instantly 
solidified by contact with the cold stone upon which it was poured, a thin layer 
of yellow, malleable, tempered alloy was found. In another instance, the ingot 
obtained from a white alloy containing about 30 per cent of copper having been 
broken while still warm, the fractured surface instantly assumed a permanent bright 
* Loc. cit., S. 394. : thes 
1 When a piece of solder is heated prec it assumes at first a light : id color, which subsequently 
passes to violet, or even to blue violet; a beautiful play of colors, alternating between green, yellow, and . 
violet, succeeds, as the temperature becomes more elevated, until at length, after the lapse of some time, the 
specimen becomes tarnished with a thick, dull coating of oxide. Since in the founderies where it is prepared 
this alloy is often taken directly from the ingot-moulds and pounded to powder without having first been 
allowed to cool, the observation of Karsten admits of ready explanation. | 
