332 AGRICULTURAL, ANALYSIS 



accurately calibrated, and the divisions so graduated as to per- 

 mit of the reading of the volume accurately to a tenth of a cubic 

 centimeter. For this purpose it is best that the internal diame- 

 ter of the measuring tube be rather small so that at least each 

 10 cubic centimeters occupies a space 10 centimeters long. 

 The volume occupied by any gas varies directly with the tem- 

 perature and inversely with the pressure to which it is subjected. 

 The quantity of aqueous vapor which a moist gas may contain 

 is also a factor to be considered. Inasmuch as the nitrogen in 

 the above process of analysis is collected over a strong solution 

 of potassium hydroxid capable of practically keeping the gas in 

 a dry state, the tension of the aqueous vapor may be neglected. 



293. Reading the Barometer. Nearly all of the barometers in 

 use in this country have the scale divided in inches and the 

 thermometers thereunto attached are graduated in Fahrenheit 

 degrees. This is especially true of the barometers of the Weather 

 Bureau, which are the most reliable and most easy of access to 

 analysts. It is not necessary to correct the reading of the 

 barometer for altitude, but it is important to take account of the 

 temperature at the time of observation. There is not space here 

 to give minute directions for using a barometer. Such direc- 

 tions have been prepared by the Weather Bureau and those 

 desiring it can get copies of the circular. 85 



The temperature of a barometer affects its accuracy in two 

 ways : First, the metal scale expands and contracts with chang- 

 ing temperatures ; Second, the mercury expands and contracts 

 also at a much greater rate than the scale. If a barometer tube 

 holds 30 cubic inches of mercury, the contents will be one 

 ounce lighter at 80 F. than at 32 F. The true pressure of the 

 air is, therefore, not shown by the observed height of the mercurial 

 column, unless the temperature of the scale and of the mercurial 

 column be considered. 



Tables of correction for temperature are computed by simple 

 formulas based on the known coefficients of expansion of mer- 



85 Barometers and the Measurement of Atmospheric Pressure, 2nd Edi- 

 tion, 1901. 



