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Page:Popular Science Monthly Volume 61.djvu/174

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where s parts of salt are dissolved in 100 of water, s varying in Tait's experiments from to 21.4.

Six miles of sea water, at 10°C. throughout, are reduced in depth 620 feet by compression. Hence the pressure at a depth of six miles is nearly 1,000 atmospheres.

The maximum density-point of water is lowered about 3°C. by 150 atmospheres of additional pressure.

The maximum density-point coincides with the freezing-point at—2°.4 C, under a pressure of 2.14 tons.

As to the proper correction to apply to the Challenger thermometers, Tait showed that that previously given by Davis, viz.: .5°F. per ton per square inch, was greatly too large, and that of five sources of error which entered into the test experiments, only one held for the circumstances under which the Challenger thermometers were actually used, that the other four were proper for the experiments in the laboratory, but not for sea-soundings. The only cause of error active in the case of sea-soundings was the direct effect of pressure on the glass and mercury of the thermometer, and the correction due to this was but 0°.14 C. for every mile of depth.

Next to his work on the compressibility of water and the allied investigations, come Tait's experiments in thermo-electricity. He made two contributions in this field.

1. Having supposed that the Thomson effect (the absorption or liberation of heat-energy in a conductor whose temperature varies from point to point when traversed by a current of electricity, the effect being reversible, in any given conductor, with the direction of the current) might, like thermal and electrical resistance, be directly proportional to the absolute temperature, he verified his assumption by experiment, finding that the curves for the e. m. f. in terms of absolute temperature for junctions of any two of iron, cadmium, zinc, copper, silver, gold, lead and some others are parabolas with their axes vertical, if the e. m. f. be taken as ordinates, the apex corresponding to the neutral point, or point of reversal. This amounts to showing that the curve representing the thermo-electric power[1] of any couple in terms of the mean temperature of its junctions is a straight line. We need only draw the diagrams of the thermo-electric powers of all the metals taken separately with one of their number in order to learn the values of the thermo-electric powers of all the metals taken in pairs in any combination. Lead was adopted as the metal of comparison, because as Le Eoux had shown, its specific heat of electricity is zero.

By the 'specific-heat of electricity' is meant the amount of heatenergy developed in the given conductor, according to the Thomson

  1. The 'thermo-electric power' of a couple for a given temperature is the e. m. f. between its junctions when they are kept respectively 12° above and 12° below that temperature.