41 8 AGRICULTURAL ANALYSIS 



measure exactly 100 cubic centimeters at o and 760 millimeters 

 pressure of mercury, is calculated by the formula 



_ IPO (273 + Q 760 



273 



where t equals observed temperature, b the barometric pressure 

 less the correction noted below, and f the tension of the vapor of 

 water under existing conditions. For example : 



Temperature ............................... 18 



Barometric reading ..... .................. 755 



Correction for b ............................ 2 



Corrected barometer ....................... 753 



Vapor of water tension ..................... 16 



Then V - I0 (27 , 3 + I8) = 109.9- 

 273 (75316) 



This indicates that 109.9 cubic centimeters of air would occupy 

 a volume of 100 cubic centimeters when subjected to standard 

 conditions. 



The tubes A, B, and C, are filled with mercury of which about 

 two and a half kilograms will be required. By means of the 

 leveling tube B, the stopper in C being opened, the mercury in 

 C is brought exactly to 109.9 cubic centimeters. The stopper 

 in C is then closed, mercury poured into D, which is then closed 

 with a rubber stopper, carrying a small glass tube, as indicated 

 in the figure. 



The leveling tube B serves to regulate the pressure on the 

 gas in A, and this is secured by depressing or elevating it, as the 

 case may require. 



The tube for reducing the volume to standard conditions of 

 temperature and pressure, viz., o and 760 millimeters of mer- 

 cury, is shown in C. In its narrow part, which has the same in- 

 ternal diameter as A, it is graduated into tenths of a cubic centi- 

 meter. The upper end of C is furnished with a heavy glass neck 

 D, surmounted by a glass cup. In the neck is placed a ground- 

 glass stopper, carrying a groove below, which corresponds to a 

 similar groove above in the side of the neck, whereby communica- 

 tion can be established at will between the interior of C and the 

 exterior. The joint is also sealed by pouring mercury into D, as 



