TO A THERMO-ELECTRIC DIAGRAM. 135 
of temperatures mentioned above; though it showed a remarkable peculiarity 
which must be more closely studied, as it appears to point to the cause of 
the above effects in a property of iron. It was therefore employed to give 
(very roughly) an indication of the actual temperatures in these experiments. 
But as for this purpose it is necessary to measure the simultaneous indications 
of two circuits whose hot and whose cold junctions are respectively at the 
same temperatures, I was obliged to employ a steadier source of heat than the 
naked flame. I therefore immersed the hot junctions in an iron crucible con- 
taining borax glass, subsequently exchanged for a mixture of fused carbonate 
of soda and carbonate of potash; but, to my surprise, the former of these 
substances at ared heat disintegrated both the platinum and the alloy, and 
thus broke both circuits without sensibly acting on the iron, while the mixture 
(evidently by the powerful currents discovered by ANDREws, Phil. Mag. 1837) 
interfered greatly with the indications of the thermo-electric circuit, as will be 
seen by the dotted curve in the wood-cut. [I may remark here that the devia- 
tions of this curve from its form when these currents are prevented are quite 
easily observed and plotted by the process next to be mentioned, so that the 
study of the Andrews effect may be carried out with great accuracy by my 
method.| Finally, determining to dispense altogether with fused salts, which 
conduct too well besides acting on the metals, I simply suspended a red-hot 
bombshell, vent downwards, in such a way that the hot junction was near its 
centre. This arrangement worked admirably, until a white heat was required, 
for this melted the shell. In its place a wrought-iron tube (an inch in bore, 
four inches long, half an inch thick, and closed at the upper end) has been 
substituted, and answers excellently. It does not cool too fast for accurate 
reading at the higher temperatures, and by elevating it by degrees from over 
the hot junction we can make the cooling fast enough at the lower ranges. 
In fact, I believe that if I do not succeed in getting a sufficient number of 
practically infusible metals to construct my proposed thermometric arrange- 
ment, I may be able to make a fair approximation to temperatures by simple 
time observations made with the hot tube, surrounded by some very bad 
conductor, such as sand, where the surface in contact with the air is always com- 
paratively cool, and where therefore we can accurately calculate the rate of 
cooling. 
“Curves I., II., IIT., in the wood-cut were drawn by means of this apparatus. 
The hot junction consisted of an iron wire, a palladium wire, and (for the 
several curves’ in order)—I. Hard platinum; II. Pt 85, Ir 15; III. The 
other alloy of Pt and Ir. The free ends of the palladium wire, and of the 
platinum or alloy, were joined to tron wires, and the junctions immersed in 
test-tubes filled with water resting side by side in a large vessel of cold water. 
The other ends of these three iron wires, and the wires of the galvanometer, 
