EXPERIMENT STATION BULLETINS. 559 



In this equation, t = temperature of room, Centegrade ; B = pre- 

 vailing barometric pressure; W = tension of aqueous vapor at t° C; 

 V = volume under the prevailing conditions; and Vq = the reduced 

 volume at 0° and 760 m. m. mercury pressure. 



The values of "t" and "B" could be read from the thermometer and 

 barometer. The value of *'W" is given in tables of water vapor ten- 

 sion; and since ''Vo" was taken as 50 c. c, the equation could be solved 

 for "V." Suppose it was found that 50 c. c. (dry air, 0° C. and 760 

 m. m.) would occupy 58 c. c. under the prevailing laboratory room 

 conditions. The apparatus would then be prepared for use as follows: 

 The two-way cocks "m" and ''c" became alternately opened to the out- 

 side and then into the manometer arms until the air in the ''arms" 

 was at atmospheric pressure and the mercury in the two arms stood at 

 the same height as shown by the scale. Cock "m" was then closed and 

 ''c" opened to the outside. Cock "d" was opened and the mercury- 

 bottle, "B," manipulated until exactly 58 c. c. of air and a single drop 

 of water had been drawn into "C" through the outlet tube of cock, 

 "c." (The arm of the manometer connected with "C" contained O.S 

 c. c. of air and that was taken into account in making the measure- 

 ment.) This air was compressed to exactly 50 c. c. and "d" was 

 again closed and left closed. After that, wiiatever the change in tem- 

 perature or pressure the reduced volume of the air in ''C" always 

 equalled 50 cubic centimeters. Without going into further detail, it 

 may be seen by a study of the figure how after any gas (saturated with 

 moisture) was drawn into "M," the apparatus could be manipulated 

 to balance the compression of that gas against the compression of the 

 air in ''C." The mercury manometer made it possible to do that accu- 

 rately. If the temperatuire of the room in which the work was done 

 remained constant or changed but slowly, the gas in ''M" became meas- 

 ured under exactly the same conditions of temperature and pressure 

 as the gas in "C." The reduced volume of the gas in ''M" must there- 

 fore equal its volume at 0° C. and 760 m. m. mercury pressure, since 

 under the same conditions of temperature and pressure volumes of 

 gases are proportional. 



Care was necessary in manipulating the apparatus, at first, to pre- 

 vent the mercury in the manometer from being forced over into one of the 

 burettes. If that happened, the manometer could again be filled to the 

 Droper height by raising the mercury in one of the burettes until it poured 

 over through the narroAv manometer-arm. Then the required amount of 

 air for ''C" must be again calculated. 



Moreover, observation tests were necessary to make sure that the 

 connections remained gas tight. It was found necessary to keep the 

 manometer connections under mercury seal. The work was done in a 

 basement room where any change in temperature came very slowly. 

 If the hand was laid upon one burette or a gas-container for a mo- 

 ment, it was necessary to wait uintil the temperature again became 

 adjusted before taking any gas measurement. When the proper care 

 had been observed, burette '"C" would stand the test for two to three 

 weeks without the necessity of being refilled; and after one became 

 accustomed to the apparatus, accidents scarcely ever occurred. 



The gas pipettes used were the various forms of Hempel pipettes 



