ON ALLOYS AND THEIR USES. 
555 
2 . Those which in some cases are, and in others are not imparted to the alloy in the 
ratio in which they are possessed by the component metals. 
To the first belong specific gravity, specific heat, and expansion due to heat. It is 
easy to show this experimentally ; the specific gravity of an alloy may be shown to be 
equal to the mean of those of its component metals, by hanging on the one side of a 
balance the alloy and on the other side the metals composing it unalloyed, and then 
placing them both in water. 
The specific heat of an alloy may be proved equal to that of its components by placing 
the alloy and its components in boiling water, and then in equal volumes of cold water; 
when the rise of temperature in the two cases will be found the same, as may be shown 
by a differential-air thermometer. 
A brass bar placed in any apparatus for showing expansion by heat is seen to expand 
exactly as much as a composite bar, of which one portion is of copper, the other of zinc, 
—the length of the zinc portion being proportional to the amount of zinc in brass. 
To the second class of physical properties belong, conduction for heat and electricity, 
hardness, tenacity, etc. 
As a basis for the conclusion which will be drawn, the electric conducting power 
for alloys may be taken. Researches into this subject have shown that w r hen tin, lead, 
zinc, or cadmium are alloyed together, such alloys conduct electricity in the ratio of the 
relative volumes of the component metals, whilst in all other cases no such simple rela¬ 
tion exists between the conducting power of the metals and their alloys. If, for instance, 
gold be alloyed with silver, say in equal volumes, the conducting power of an alloy will 
be 15, that of silver being 100, and that of gold 80. 
If curves be drawn to represent the conducting power of different series of alloys, 
three typical forms will be observed: the first represented by nearly a straight line, the 
second by the letter L , and the third by the letter U. 
Wiedemann and Franz have proved experimentally that the values obtained for the 
conducting power of metals and alloys, for heat and electricity, are identically the same ; 
and the truth of this statement may be shown by the following experiment: —If bars of 
gold and silver and some gold-silver alloys be fixed, so that one end of all of them is in 
a hot-w r ater box and the other end in the bulb of a small air-thermometer, the depres¬ 
sion in the columns of the liquid in the tubes of the air-thermometers will indicate the 
relative conducting powers (approximately) of the several bars; and if through the tops 
of the columns of liquid a line be drawn, such line will form a curve similar to that 
referred to as obtained for the electric conducting power. 
That this is true is thus shown:— 
By the side of this apparatus is placed another of this construction:—Into the bulbs 
of several air-thermometers are fixed wires of the same size and length, and of the same 
materials as were used in the heat-conducting experiment. One end of each wire is 
soldered to one thick copper-wire, and the other end to another similar wire. These 
two wires are connected to the poles of a battery. The current will then divide itself, 
and a portion will pass through every wire proportional to the conducting power of 
that wire. This current will heat the wire and cause the liquid in the tubes connected 
with the air-thermometers to descend, and the line drawm through the top of the 
columns will be nearly similar to the curve already mentioned, which is formed by the 
bulbs in which the heat-conducting bars are fixed. 
The analogy between the relation existing in this case and in some others may be 
shown experimentally as follow's:— 
Sonority. When bars of alloys and their component metals are struck, a great differ¬ 
ence will be found in the note produced; and in almost every case where the experiment 
has been made, the most sonorous alloy was found to correspond in composition approxi¬ 
mately with that at the turning point of the electric conducting power curve. 
Tenacity. When wires of the same diameter of metals and alloys are broken by trac¬ 
tion, those of the alloys will require a much greater force than their component metals; 
and it may be deduced from what is known, that those alloys the composition of which 
corresponds to the turning point of the conducting power curve are more tenacious than 
any other alloy composed of the same metals. 
Elasticity. When spirals of wires of metals and their alloys are weighted to an equal 
extent, the alloys will be found on removing the weights to possess the property of re¬ 
suming their original form in a much higher degree than their oompoueuc metu.s. Here 
