ON THE CHEMICAL NATURE OF ALLOYS. 4] 
alloyed with one another, or with one of those belonging to Class A, do not 
conduct electricity in the ratio of their relative volumes, but always in a lower 
degree than that calculated from the mean of their volumes. In the above 
statement I have assumed the theoretical conducting-power of an alloy equal 
to that of the components, under the supposition that each of them is a separate 
wire, lying side by side, and soldered together at the ends. 
If we now look at the curves representing the conducting-power of the 
different series of alloys, we shall find that (see Plate V.)— 
I. Those belonging to the alloys made of the metals of Class A with one 
another are almost straight lines. As type the lead-tin curve is given. 
II. The curves of those made of the metals of Class A with those of Class B 
show a rapid decrement on the side beginning with the metal belonging to 
Class B, and then turning and going in a straight line to the other side, 
namely to the Class A metal. As type the tin-copper curve is given. 
III. The curves of those made of the metals of Class B with one another 
show a rapid decrement on both sides of the curve, and the turning-points 
connected with each other by nearly straight lines. As type the gold-silver 
curve is given. 
The curves, then, representing the conducting-powers of the alloys having, 
according to the class of metals of which they are made, almost always the 
same form, we may, if we know to which class the metals composing them 
belong, draw a curve which will approximately represent the conducting- 
power of a series of alloys. Of the exceptions to this rule I shall presently 
speak. 
Let us now examine the first group of alloys, and see what grounds there 
are for supposing that the solid alloys belonging to it are only solidified 
solutions of the one metal in the other. In order to do this, I shall show that 
they are neither mechanical mixtures (with exception of the lead-zinc alloys) 
nor chemical combinations. 
First. If they were mechanical mixtures, the metals composing them, if 
their specific gravities were not the same, would separate into two layers 
when fused and cooled slowly, as in the cases of the lead-zine alloys*; for 
when these two metals (say equal parts) are fused together and allowed to 
cool slowly, they separate into two layers, the upper one (zinc) containing 
1-2 per cent. lead, and the lower one (lead) with 1:6 per cent. zinc. Now if, 
instead of cooling the mixture slowly, it had been cooled rapidly, such an 
alloy might be regarded as a mechanical mixture of a solidified solution of 
lead in zine and zinc in lead, or if well mixed in a liquid state, such a 
mixture would be analogous to one of ether and water when shaken up well 
together, and not as one of lead and zine analogous to oil and water. 
A true mechanical mixture of two metals, either in the liquid or solid state, 
is I believe not known. 
That the alloys of lead and tin, for instance, are not mechanical mix- 
tures, is proved by their not separating into two layers on being slowly 
cooled after fusion ; for, if they were, they would behave like lead and zine, 
as tin and zinc have nearly the same specific gravities, that of tin being 7-293 
and that of zinc 7-148. 
2ndly. If these alloys were mechanical mixtures of the metals composing 
them, we should not be able to press homogeneous wires; now it has been 
proved that wires of the same alloy have the same conducting-power, whether 
tested at the end coming first or last out of the press, or whether pressed at 
different times. 
. * Matthiessen and y. Bose, Proc. Royal Soc. yol. xi. p. 430. 
