MEASUREMENTS 



319 



246. Calculations and Corrections. The experiments are made 

 at a temperature t and at an atmospheric pressure H. To com- 

 pare the quantities with each other, and to calculate the energetic 

 expenditure by the oxygen consumed ( 101) the volume V, thus 

 measured must be reduced to a temperature of C. and a baro- 

 metric pressure of 760 millimetres of mercury. The volume of a gas 

 increases with the temperature by oc = -^g- per degree C., and 

 diminishes as the pressure rises. The volume at C. and 760 

 mm. is : 



V = 



or, approximately, 



at X 760 



0-36 X V, X H 



273 +t 



In the case of Laulanie's eudiometer, H is a measure of the pres- 

 sure of the air, presumed to be dry, and the tension F of the 

 water vapour at the temperature t, since water is used The 

 tension F can be found from Regnault's tables ( 253). There 

 fore : 



V O' 36 X v ' ( H F ) 



273 +t 



H being the atmospheric pressure at the moment, F the tension 

 ofjthe water vapour at the temperature t. The volume of the 

 dry gases is therefore obtained at and 760 millimetres. H is 

 read on the mercury barometer, the only 

 really correct one (fig. 227). It comprises 

 a tube of 2 to 2-5 centimetres diameter, 

 closed at the top and open at the other end 

 and inverted, full of mercury, in a cup of 

 mercury, C. The tube is fixed by strong 

 bars on to an oak case, and is graduated 

 from to 800 millimetres, its total length 

 being about 90 centimetres. The level of 

 the mercury is adjusted to the point p, 

 corresponding to the zero of the scale 

 by means of a plunger K operated by 

 a micrometer screw, V. If the height of 

 mercury is KU at the temperature of t, it 

 can be reduced to 0, by taking into account 

 the co-efficients of expansion of mercury 

 and glass. Then : 



H = H, a, 



according tq- whether t is lower or higher 

 than 0. a can be found from tables. 

 Approximately 



a = 0-000168*H*. 



Fie. 111. 



Perpendicular 

 barometer. 



