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of solidification withaut appreciable inertia. Having at my disposal 
instruments that fully met all imposed requirements *), I could venture 
to make an attempt to solve the problem proposed with a fair 
chance of success. 
The method of observation was as follows. Two small holes were 
bored in a glass tube at points which were diametrically opposed 
to each other. The thermo-element was put through these apertures. 
It consisted of termo-sheet tin of Dr. Morr, which had been rolled 
out for this research to a thickness as small as possible. The thermo- 
elements that were used in the final observations, had a thickness 
of 1.4 u. Strips of this material were cut about 0.1 mm. wide, one 
half of which consisted of manganin, the other of constantan. Such 
a strip was put through the openings made in the wall of the glass 
tube. Before cementing the thermo-element care should be taken 
that it was spread ont flat, normal to the axis of the tube, and 
further that the place where the metals were soldered together, was 
exactly in the centre of the tube. The cementing substance had to 
be proof to the substance with which the tube was filled (salol), 
and had to adhere to glass and metal. A mixture of water-glass 
and asbestos powder appeared to satisfy these requirements ®). 
Copper wires, which led to the galvanometer, were soldered to 
the extremities of the thermo-element, which projected outside the 
wall of the tube. The coil of the galvanometer had a very small 
moment of inertia, so that the instrument indicated quickly. In 
order to enhance the rapidity, the suspension-wires of the coil were 
chosen fairly thick. This, indeed, decreased the sensitivity, but it 
was nevertheless sufficient to enable us to measure accurately the 
rises of temperature that had to be observed. The time of adjustment 
of the galvanometer was about 0.07 sec. 
The glass tube was now filled with the melted substance, for which 
| have chosen salol. This substance offers some advantages, viz.: 
1. The rate of crystallisation is small, at most 3.68 mm. 
per minute. Consequently the conditions imposed on the method of 
measurement are not so great as with substances that crystallize 
1) It is with great pleasure that I express my indebtedness to Dr. W.J. H. Mou 
for the use of the unequalled combination of instruments of his own invention, 
without which the measurement of the rapid local temperature changes would 
certainly have been impossible. 
2) Between two observations the tube of salol had to be placed in hot water 
to melt the salol again. In order to protect the solidified mixture of asbestos and 
waterglass against the action of the hot water, it was covered on the outside 
with Canada balsam. 
