THE MECHANICAL EQUIVALENT OP HEAT. 
395 
although the same water was never used for two experiments, we found that it became 
acid—in some cases strongly so. 
This year the water remained unchanged for twenty consecutive experiments and 
we were unable to detect any acidity, although all water withdrawn from the calori¬ 
meter was invariably subjected to a careful examination. 
The insulation of the leads passing through the steel cover and the lid of the calori¬ 
meter was a troublesome matter, as there was great difHculty in making the insulating 
junctions absolutely air-tight. 
Fig. 3. 
A, B was a brass pin, which had a screw thread on'the end B and terminated in a 
nut Sit A. C was a ring of ebonite fitting tightly into the hole' in the metal plate 
and having a j^rojecting flange at the lower end. 
The surface of the ebonite was moistened with gutta-percha solution (made by 
dissolving gutta-percha in chloroform); the ring, C, was then forced into the hole and 
the brass pin passed through it. A ring of ebonite, D, was then threaded on to the 
pin and a brass nut, E, firmly screwed down—the ends of the wire being soldered on 
at A and B. 
This form of joint proved to be an excellent one, and when once adopted (after the 
failure of many other contrivances) gave no further trouble. The steel lid was pierced 
by such junctions in four places, and the calorimeter lid in two. As tap-water was 
continually flowing over the top of the steel lid it was necessary to insulate the 
exposed parts at B. A j^iece of wide india-rubber tubing enveloped the wire, the 
lower end fitted over the ebonite ring D, and was wired firmly on to it. The upper 
end of this tubing projected above the surface of the water in the tank. The tubes 
themselves were filled with distilled water to prevent any rise in temperature of the 
leads. The general arrangement is shown in Plate 2, fig. 2. 
Leads 1 and 3 (fig. 4, p. 396) were connected with the Clark cells, and thus no current 
passed along them. 2 and 4 carried the current. Any heat developed in the parts 
above AD passed into the water of the outside tank, and might be disregarded. A 
portion of the heat developed in the wires hM and clN would pass to the water in the 
calorimeter. By sufficiently diminishing the resistance of hM and clN the heat thus 
3 E 2 
