RUDBERG ON THE EXPANSION OF DRY AIR. SIS 



well when the air in the receiver A B is cooled, down to 0"^, 

 as when it is heated up to the boiling point of water. In order 

 to measure Avith accuracy the altitude of the mercury in the 

 tube, a brass scale H I, divided into millimetres, is attached to 

 the tubes. The line which marks the commencement of the 

 divisions at C is so long, that it passes behind both tubes, and 

 thus the altitude of the extremity of the column of mercury in 

 the tube H I, above the mark on D E at C, is easily deter- 

 mined. 



The air in the receiver A B was dried before the tubes were 

 cemented into the box, in the following manner. The lower end 

 of the tube F was drawn out to a capillary point, and connected 

 with a very wide tube filled with chloi'ide of calcium, which 

 communicated with an air-pump. After the air had been fifty 

 times exhausted and re-admitted, the capillary point was sealed 

 and the tube cemented into the box G, which had been pre- 

 viously filled with dried mercury, and lastly, the sealed end 

 broken off under the surface of the mercury. 



The capillary depression of mercury at C was determined by 

 experiment before the narrow tube D E was joined to the re- 

 ceiver, and found equal to 1*85 centimetres. 



The tube F was taken of large diameter, in order to receive the 

 air as it expanded on being heated from 0° to 100°, and so ob- 

 viate the necessity of continually screwing up the mercury. 



The calculation and the method of observing are both equally 

 simple. When the air in the receiver A B is cooled down to 

 0°, and the mercury is screwed up to C in the tube D E, let the 

 mercury stand at M in the tube H I. At the same instant let 

 h be the altitude of the mercuiy in the barometer. Let the al- 

 titude of the mercury in H I, above the mercuiy in D E, or 

 C M = k, and let / be the capillary depression of the mercury 

 in D E ; then the pressure of the air in the receiver A B will 

 be h + k — I. When afterwards the air is heated up to the 

 boiling point of water, and the mercury is screwed up to C in 

 the tube D E, let the mercury stand at P in the tube HI. At 

 the same instant let h' be the height of the mercury in the baro- 

 meter, and the difference of altitude of the mercury C P = ^' ; 

 then the pressure of the air in the receiver will be 7i' + k' — I. 

 Let T be the temperature of steam corresponding to the baro- 

 metric height h', 100 A. the expansion of air from 0° to 100°, 

 and 100 G. the expansion of glass in volume from 0*^ to 100°; 



