COMPLETE FREEZING-POINT CURVES OF BINARY ALLOYS. 
65 
matter separating out had the sarrie composition as the residual liquid. Little or no 
precipitate was noticed at the moment indicated by the thermometer as the com¬ 
mencement of freezing. Soon after 25 atoms we begin to have precipitate at the 
moment of freezing, and the freezing point gradually loses its resemblance to a 
eutectic point. 
A comparison with Kuster’s work on isomorphous mixtures strongly suggests 
that between 15 5 and 20 we are in the presence of one, and between 20 and 25 in 
presence of another, such isomorphous mixture. 
At 50 atoms of tin there is a point of inflexion, and, therefore, between 60 and 70, 
there must be what by a stretch of language might be called a summit. 
Before attempting to interpret these phenomena it will be well to consider briefly 
the conclusions of other workers in the same field. 
PtiCHE (‘ Ann. de Ch. et Phy.,’ 1873, xxx.), found that alloys of copper and tin 
undergo liquation, except SnCiu (20 atoms) and SnCug (25 atoms) ; but he does not 
appear to have examined alloys between these two values. 
Behrens, whose work, “ Das Mikroscopische Geflige der Metalle und Legierungen,” 
is a mine of valuable information, but a mine wherein one needs to dig diligently, 
notes a sharp change in the colour, the microscopic structure, and the chemical 
behaviour of bronzes at 25 per cent., by weight, of tin. This is the same as 15'3 atomic 
per cents., a point where our curve shows its first angle. His microscopic study of 
alloys with only a few per cents, of tin shows the crystals of copper embedded in a 
network of white or yellowish mother substance ; but at our first angle, at 15'3 atoms, 
all distinction in colour between crystals and mother liquor has disappeared. He says 
that the colour of this alloy is whitish-grey. It has a conchoidal fracture, and 
examined microscopically is seen to consist of bundles of parallel rods, scattered in all 
directions. He thinks that we have here to do with a compound SnCuf;. At 20 atoms 
of tin, our second angle, Behrens considers that there is another chemical compound. 
SnCiq. He says, “ that with this alloy we reach a maximum of hardness, brittleness, 
and resistance to chemical solvents.” Also, “ that if great care is taken to get an alloy 
of exactly this composition, we have a grey-white metal with a half-glassy, glimmering, 
minutely conchoidal fracture. There is no sign of crystallization ; and polishing and 
annealing show nothiim. The slower the coolino- the more brittle the mass.” He 
finds that at 25 atoms of tin the alloy is more easily attacked by reagents 
than at 20 atoms, and hence, and for other reasons, he refuses to accept the view 
that SnCug is a compound. He is disposed to regard it as a mixture of two alloys, 
which may, or may not, have separated in the liquid state. He finds that this 
alloy crumbles under the hammer, or even under a coarse file, into polyhedra with 
smooth faces. 
In contrast to Behrens, other writers have accumulated a mass of facts pointing, 
at least, to the unique character of the alloy with 25 atoms of tin. Laurie finds 
that the electromotive force of couples formed of copper and a copper-tin alloy 
MDCCCXCVHT.—A. K 
