Physical Properties of an Extensive Series of Alloys of Iron. 111 
As the specimens of the alloys with which we had to deal were in the form of 
rods 102 ems. long and only half a centimetre in diameter, the gradient of tem- 
perature could only be found by thermo-couples, or by Ingenhouz’s method of 
coating the rod with some easily fusible substance, such as wax, and measuring the 
length of wax melted along each rod when a permanent temperature was attained, 
one end of the rod being kept at.a high and constant temperature. Both these 
methods were employed, and fairly concordant results obtained. The results at 
present obtained do not, however, profess to be more than a preliminary investi- 
gation; to obtain exact measurements, specimens of more suitable size and of 
more uniform purity are required, and also greater precautions would be 
necessary. 
A rough experiment—made by fixing the rods vertically and placing their lower 
ends in a freezing mixture, which was renewed until a steady gradient of tempera- 
ture along each rod was attained—showed, by the varying length of the deposit 
of hoar-frost on each rod, that wide differences existed in the thermal conductivity 
of these alloys. 
A much better comparative series of experiments on the conductivity of heat 
through these alloys was made by carefully measuring the length of wax melted 
on each specimen, after one end of the rod had been kept for a long time at a high 
and steady temperature. When this permanent state is reached, then-~if the rods, 
fixed horizontally, have the same cross-section and the same emissivity, and the 
same external conditions are preserved—the thermal conductivities of any pair 
of rods are proportional to the squares of the lengths of wax melted on each rod. 
The rods used were very nearly of the same cross-section, and their emissivities 
were rendered as nearly similar as possible by the fact that a thin coating of 
wax remained on each rod. 
The method of experiment was as follows :—An iron vessel some 30 ems. long, 
7 ems. deep, and 7 cms. wide, had eight holes, about 3 cms. apart, drilled along one 
side. Hach hole was bushed with brass and asbestos, so as to fit closely the rods under 
experiment. One end of each of eight rods was fixed horizontally in these holes, 
and projected about 4 cms. inside the trough. The trough was filled with molten 
lead, and kept, as far as possible, at a uniform temperature by a suitable arrange- 
ment of Bunsen gas-burners, the temperature of the molten lead being taken by 
means of a special thermometer reading to 500° C. The rods were, of course, 
carefully screened from the heat radiated from the gas flames and trough, and 
from all surrounding air-currents. Before the experiment began, each rod was 
painted with a thin and uniform coating of paraffin wax. Molten lead was then 
poured into the trough, and its temperature kept fairly constant for a period of 
from one to two hours. The length of the wax melted on each rod was taken by 
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