544 
MR. T. J. BAKER ON THE THERMO-CHEMISTRY 
This assumes that the alloy possessing that composition consists entirely of that 
compound, and this is perhaps open to question, although its physical properties and 
microsco 2 )ic structure seem to point to its perfect homogeiuity. 
Since the alloy corresponding to CuZn does not show the simplicity of physical 
structure appertaining to the alloy CuZiio, it is not justifiable to say that the heat of 
formation of 1 gramme of the compound CuZn is 46 calories, although we may make 
that statement with respect to the alloy containing copper and zinc in those pro¬ 
portions. 
It does not appear to be kno^vn at what stage, in the making of brass, the evolu¬ 
tion of heat of formation occurs. 
Suppose that on mixing molten copper and zinc in the proportion of two parts of 
copper to one part of zinc (oixlinary yellow brass), the heat of formation, amounting 
to 36 calories per gramme, is then manifested. Taking the specific heat of the 
molten metals as '1 (?) we should obtain a rise in temperature of 360° C. as a result 
of chemical combination. 
Suppose the molten copper is originally at 1200° C., that the zinc added is at 
0° C., and that the latent heat of fusion of zinc is 28 (Person) ; then the temperature 
of the mixture, neglecting heat of formation, is given by 
2 (1200 - ^) T = -U -b 28 
whence t = 707°. 
Adding the rise in tem])eratnre due to heat of formation, we obtain a final tem¬ 
perature = (707° + 360°) = 1067° C. This is well above the boiling point of zinc 
(940° C.), and may possibly be the explanation of the violent action often observed in 
making brass, although, as mentioned on p. 530, the direct volatilisation of zinc when 
placed in a crucible much above 1000° C. may l)e competent to produce the effect. 
The previously mentioned work of Herschkowitsch was carried out by the disso¬ 
lution of copper-zinc alloys in a water solution of potassium bromide and bromine. 
His measiirements were confined to two alloys containing 43‘95 and 72‘67 per cent, 
of copper, and he gives 9 calories and 12 calories respectively as their heat of forma¬ 
tion per gramme. 
As these quantities were in each case only about 1 per cent, of the heat of dissolu¬ 
tion of the alloy, Herschkowitsch was inclined to the view that the latter possessed 
no heat of formation. 
On reference to Tables IV. and VHI. it is apparent in many cases that the heat of 
formation of an ahoy is about 10 per cent, of its heat of dissolution, and as the 
deterininatlons are not likely to be more than 1 per cent, in error, it is quite certain 
that Herschkowjtsch’s results are inconclusive. 
