Brass* 
m 
and is admirably fitted for the reflection of light for all op- 
tical purposes. It is then called speculum metal which 
however, for the extreme perfection required in modem 
astronomical instruments, is better mixed with a very 
small proportion of other metals particularly arsenic, brass, 
and silver. But the basis of these compositions is cop- 
per alloyed with nearly half its weight of tin® The use of 
this alloy for the same purpose is of great antiquity, and 
certainly was in frequent use in the time of Pliny. Klap- 
roth analyzed a portion of an ancient speculum in the fol- 
lowing manner. The fragment was compact, very hard,, 
and brittle, the fresh fracture greyish white, which by po- 
lishing acquired a beautiful lustre. A hundred grains 
were heated with nitric acid, whereby a blue solution was 
made, and a part remained undissolved. The solution 
being first tried without effect with muriat of soda for sil- 
ver, was mixed with a solution of sulphat of soda, which 
gave a white precipitate of sulphat of lead, equiva- 
lent to six grains of lead.* The copper was then separated 
by iron, and amounted to 62 grains. The undissolved 
portion slowly digested with muriatic acid, gave a straw- 
yellow solution, which, decomposed by zinc, gave 32 
grains of metallic tin. This speculum therefore consisted 
of 62 parts of copper, 32 of tin and 8 of lead, which last 
was probably an adulteration of the tin and not added de- 
signedly. 
When more tin is added than amounts to half the weight 
of the copper, the alloy begins to lose that splendid white- 
ness for which it is so valuable as a mirror, and becomes 
more of a blue-grey. As the tin increases, the texture be- 
comes rough- grained,, and, as it were, rotten, and totally 
unfit for manufacture. The speculum metal is therefore 
In the highest proportion of alloy of tin that copper will 
%dmit for any useful purpose. 
A perfect speculum-metal should be quite white with- 
