ON THE STRUCTURE OP METALS. 
421 
Polishing. —Even the finest emery paper tears the metal and produces relatively deep 
scratches. These are effaced by the use of Brillant Beige, a commercial product of which 
the exact composition is unknown, though it is widely used by jewellers, as well as for 
domestic use in cleaning copper. This Brillant Beige is spread on a sheepskin covering 
a revolving plate; it, however, leaves scratches. The polishing is continued with 
English rouge of the best quality, prepared by the calcination of pure oxalate of iron, 
as prescribed by M. Henrivaux. This rouge is spread dry in small quantities on 
chamois leather covering, as in the case of the Brillant Beige , the surface of a 
revolving disc. In order to polish successfully, a certain lightness of hand is neces¬ 
sary, otherwise the rouge scratches and soils the surface of the gold ; with a little 
practice, however, a polish which appears very beautiful to the naked eye may be 
obtained, but it is nevertheless imperfect. This may be seen from photographs 15 
and 16, where the principal mass of the metal is not etched or hardly so. There are 
a few large, purely accidental scratches which give no trouble, while the surface 
remains finely striated. We have not succeeded in doing better by this treatment; 
gold is very soft and it spreads without receiving a polish when such powders are 
employed as do not actually scratch it. It is, moreover, very difficult to prepare 
powders in a sufficiently pure state to exclude all traces of gritty constituents, but 
this relative imperfection of the polishing is in some respects advantageous and affords 
a method of investigation which has been successfully employed in other circumstances. 
If the impurities in the little section under examination have become concentrated in 
compounds which have liquated from the mass, these are generally harder than the 
pure gold, and their presence will probably be revealed by a more mirror-like polish. 
Further, polishing on a soft substance like chamois leather, will cause these liquated 
portions to appear in relief. Finally their colour in certain cases differs more or less 
from that of gold, and enables them to be distinguished from the mass. 
We have insisted upon the indications furnished by polishing, as they are often 
very useful in metallography, by enabling a constituent to be defined. The sunk 
lines, which may be the result of a general attack by etching (or attacking the 
section with a reagent) are in themselves by no means characteristic of the presence 
of a free constituent in the mass. In default of the additional evidence afforded by 
polishing, they probably represent lines of minimum compactness, along which the 
etching is deeper, or even, according to Commander Hartmann, lines of tension.* 
In fact, polishing revealed some cavities which were relatively important in gold 
alloyed with bismuth, thallium, or potassium. In this last case, and especially in 
the alloy with indium, we observed grey filaments, which were possibly due to the 
incorporation of traces of rouge in a network of fissures, although they might also be 
attributed to the presence of a cement, and in all cases they deserved attention. 
Partial Attack .—It is known that certain alloys of gold, especially those with iron, 
* ‘ Distribution des deformations dans les metaux sounds a des efforts Paris, Berger-LevrAULT, 1896, 
