456 Intelligence and Miscellaneous Articles. 



observed gas obtained at once by a single arithmetical operation. 

 The volume of any gas at the temperature 0° C. and pressure 760 

 millims. may be deduced from its volume at the temperature / and 

 pressure^ by the familiar expression 



V=V l h-h'-h" . (1) 



* 1 + 0-00367*' 760 ' ' ' ' w 



in which h is the observed height of the barometer (reduced to 0° C), 

 h' the tension of the vapour of water at t° when the gas is moist, and 

 h" the height of the column of mercury in the collecting-tube above 

 the level of the mercury in the cistern. For any other gas under 

 precisely the same circumstances of temperature and pressure, we 

 have the equation 



V =V l h-h'-h" 



' 1+0-00367*' 76t> ' w 



Whence, dividing the first equation by the second, we have 



( ^ 



V 



o 



or, as a proportion, 



V r .V\::V :V' ; (4) 



from which it appears that the reduced volume (volume at 0° and 

 760 millims.) of the second gas may be found without observations 

 of temperature and pressure, provided that the unreduced volume be 

 observed under the same circumstances of temperature and pressure 

 as the volume of the first gas, the reduced volume of which has been 

 previously determined. Let the first or standard gas be air ; then if 

 the weight of one cubic centimetre of dry air at 0° and 760 millims. 

 be w, the whole weight will be wV . In like manner we shall have 

 for the weight of the gas to be measured w^V 1 Q ; and since the 

 weights do not change with the temperature and pressure, we have, 



finall y' wV 1 : Wl V : : wV i m\V' . 



If now we suppose that the gas in the first tube, or standard gas, 

 is, for example, nitrogen, the volume remaining the same, and that 

 the gas to be measured is also nitrogen, we have 



or simply 



V, i V^Vo :•,¥<„ (5) 



The application of this formula in practice is as follows : — A gra- 

 duated tube holding about 150 cubic centims. is filled with mercury 

 and inverted into a mercury- trough. Two-thirds or three-fourths of 

 the mercury are then displaced by air, care being taken to allow the 

 walls of the tube to be slightly moist so as to saturate the air. This 

 tube may be called the companion-tube ; the volume of air which it 

 contains must be carefully determined in the usual manner by five or 

 six separate observations, taking into account, of course, all the cir- 

 cumstances of temperature and pressure. The mean of the reduced 

 volumes is then to be found, and forms the constant quantity V , 

 The gas to be measured is transferred from the receiver in which it 

 is collected into a (moist) eudiometer-tube, which is then suspended 



