Miscellaneous papers . ; the manometer tube is shown in perspectiveat C. The magnifying power of the cathetometer telescopeused was such that it could be set withcertainty to within 0*02 mm. The pres-sure, the difference of level betweenthe two surfaces, was measured by amicrometer eye-piece with two threads :the absolute height of the surface, rate of evaporation, was read offon the scale of the instrument. Thetemperature was varied by altering thegas supply. The apparatus was atfirst quite free from air: by admittingvarying, but always small, quantities ofair different pressures
Miscellaneous papers . ; the manometer tube is shown in perspectiveat C. The magnifying power of the cathetometer telescopeused was such that it could be set withcertainty to within 0*02 mm. The pres-sure, the difference of level betweenthe two surfaces, was measured by amicrometer eye-piece with two threads :the absolute height of the surface, rate of evaporation, was read offon the scale of the instrument. Thetemperature was varied by altering thegas supply. The apparatus was atfirst quite free from air: by admittingvarying, but always small, quantities ofair different pressures could be obtainedat the same temperature. If the pressure of the airintroduced amounted, say, to 1 mm. no evaporation in thesense here considered could take place so long as the pressureof the saturated vapour above the surface did not exceed 1mm., so long as the temperature of the surface did notexceed 120°; but when this temperature was exceeded the airretreated into the condensing tube, and evaporation began;. Fig. 21. vin EVAPORATION OF LIQUIDS 191 but of course it now took place under greater pressure than itdid at the same temperature before the air was quantities, h, P, and t, had to be measured. In deter-mining the first there was no difficulty. The determinationof P was not simply a case of measuring accurately thedifference of level; large corrections on account of the ex-pansion of the mercury, etc. had to be applied, and some ofthese were much larger than the quantity whose value wassou \tit. But by a careful application of theory and by specialexperiments these corrections could be so far determined thatthe final measurement could be relied upon to about 0*1 outstanding error was so small that the greater part ofthe observations would not be injuriously affected by it. Themost uncertain element was the determination of t. I thoughtit was safe to assume that the true mean temperature of thesurface could not differ by more than a few d
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysics, bookyear1896
