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DK. T. E. EOBINSON AND ME. T. GEUBB’S DESCEIPTION OP 
favoured me, is so white* that, though in uncertainty as to its permanence, I did 
not venture to propose casting a large speculum of it, Mr. Grubb thought so well of 
it that he has tried it in a duplicate small speculum of 8 inches, which will test its 
endurance. 
From the great difference between the melting-points of copper and tin, and the great 
liability of the latter to oxidation, the alloy is generally formed by pouring the tin into 
the melted copper, stirring them, and rapidly casting into ingots. The alloy fuses at a 
far less heat than the copper, and the castings made from the second melting are sup- 
posed to be less porous. Pores, I believe, are never totally absent ; it seems that the 
alloy absorbs gases at a high temperature, as is notably the case with silver and copper, 
and as has lately been shown in some remarkable instances by Graham for some other 
metals. Copper when so charged with oxygen is brittle, and it is made marketable by 
“ poling,” stirring it while fluid with a pole of dry wood, the carbon and hydrogen of 
which combine with this oxygen as they bubble through it ; and the same process has 
been found to lessen considerably the porousness of speculum metal. All these pre- 
cautions were taken here. 
f[Plate III. fig. 1 is a plan of the casting- and annealing-room, 36 feet long by 16| 
wide. A is the hot hearth for heating the bed of hoops which (as described by Lord Posse) 
forms the bottom of the mould. It consists of four low pillars of brick with intermediate 
grates for coke fires. B is the melting-furnace (which replaced two smaller ones used in 
mixing the alloy) ; it is similar to the common brass-founder’s one. Its internal dimen- 
sions are 42 inches square by 69 from top to the fire bars, with a central pillar rising 15 
inches above them to support the crucible. C is the mould as laid for a casting ; and 
at D is a crane which commands A, B, and C. It also commands E, the open cradle, 
seen in section (Plate IV. fig. 8), in which is placed the crucible full of fused metal 
previous to pouring. F is an inclined causeway leading to the mouth of the annealing- 
* The experiments of James, already referred to, give the coefficients of perpendicular reflection for each ray 
from several metals. I give those for speculum metal and zinc. 
Speculum. Zinc. 
Eed 0-692 0-576 
Orange 0-654 0-594 
Yellow 0-632 0-602 
Green 0-625 0-616 
Blue 0-606 0-628 
Indigo 0-599 0-635 
Yiolet 0-593 0-636 
It appears from this that while the reflective power of the speculum metal decreases as the refrangibility 
increases, that of zinc increases. It is therefore possible that a proper mixture of the two might act equally on 
all the rays. 
t The matter enclosed in brackets is contributed by Mr. Gkubb ; and I am sure that the scientific world will 
duly appreciate the valuable information which he has so liberally communicated. 
