READING THE BAROMETER 333 



cury and brass. For barometers with brass scales the following 

 formula is used for making the correction : 

 C = h ' ~ " 8 ^ . In this formula. / = temperature in 



1.113* -t- I0 978 c 



degrees Fahrenheit and /t observed reading of the barometer 

 in inches. 



Example: Temperature observed 72. 5 



Barometer reading observed, 29.415 inches, 



from which 0=0.1165, and this number, according to the con- 

 ditions of the formula, is to be subtracted from the observed 

 reading. The true reading in the case given is, therefore, 

 29.298 inches or 744-2 millimeters. 



The observed reading 747.1 

 And the correction 2.9 



Unless extremely accurate work be required, the correction for 

 temperature is of very little importance in nitrogen determina- 

 tions in fertilizers. Each instrument sent out by the Weather 

 Bureau is accompanied by a special card of corrections therefor, 

 but these are of small importance in fertilizer work. In order 

 then to get the correct weight of the gas from its volume, the 

 reading of the thermometer and barometer at the time of meas- 

 urement must be carefully noted. However, after the end of the 

 combustion, the azotometer should be carried into another 

 room which has not been affected by the combustion and allowed 

 to stand until it has reached the room temperature. 



Every true gas changes its volume under varying tempera- 

 tures at the same rate, and this rate is the coefficient of gaseous 

 expansion. For one degree of temperature it amounts to 0.003665 

 of its volume. Representing the coefficient of expansion by K 

 the volume of the gas as read by V, the volume desired at any 

 temperature by V, the temperature at which the volume is read 

 by t and the desired temperature by t', the change in volume 

 may be calculated by the following formula: 

 V'=V[i+K(*'-*)}. 



Example. Let the volume of nitrogen obtained by combus- 

 tion be 35 cubic centimeters, and the temperature of observa- 

 tion 22. What would be the volume of the gas at o ? 



