168 POPULAR SCIENCE MONTHLY. 



where s parts of salt are dissolved in 100 of water, s varying in Tait's experi- 

 ments 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 atmos- 

 pheres 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 ther- 

 mometers, 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 cur- 

 rent) might, like thermal and electrical resistance, be directly propor- 

 tional to the absolute temperature, he verified his assumption by experi- 

 ment, 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* 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 heat- 

 energy developed in the given conductor, according to the Thomson 



* The ' thermo-electric power ' of a couple for a given temperature is the 

 e. m. f. between its junctions when they are kept respectively y s ° above and y 2 ° 

 below that temperature. 



