Prof. Williamson on the Unit-volume of Gases. 191 



(CH 2 ) 74 . According to the formula 



(CH 2 ) tt + (0 3 ) n = (C0 9 )» + {H* 0)», 



we find that 14 parts by weight of paraffine require 48 of 

 oxygen for their combustion; so that 150 grammes require 

 514*3 grammes of oxygen, equal to 385*7 litres; 21 : 100 = 

 385*7 : x gives us 1830*6 litres of air. 



4. Given a room of 80 cubic metres capacity full of air at 

 15° C. and 760 millims. pressure, what weight of oxygen does 

 it contain? The proportion 105*4977 : 100 = 80; # gives us 

 75*8 cubic metres as the volume of our air reduced to 0° C; and 

 this multiplied by ^q gives us 15*918 as the volume of the 

 oxygen, whence 15*918 : #=11'2 : 16 gives us 22*740 grammes 

 as the weight required. 



5. Required the weight of a litre of ether- vapour, measured 

 at 100° C. 



The formula C 4 H 10 O= 2 vols., gives us 37 grammes as the 

 weight of 11*2 litres at 0° C, whence 3*305 grammes for one 

 litre at 0° C. 2*42 grammes is therefore the weight at 100° C, 



These examples will no doubt suffice to explain the use of this 

 constant, and the advantage derivable from it ; and I have found 

 that students of chemistry learn its use very easily, and, aided 

 by it, are enabled to compute rapidly the answers to numerical 

 questions involving a transition between measures of weight and 

 measures of volume. 



It appeared most natural to start from weight in fixing an 

 absolute volume, because our symbols are used to denote certain 

 relative weights of elements, and ought to be used as the basis 

 of every calculation. 



Equivalent calculations may easily be made by the aid of an 

 absolute volume defined in grains and cubic inches. One grain 

 of hydrogen at the normal temperature and pressure measures 

 nearly 44*5 cubic inches, and 44*5 cubic inches would be the 

 unit- volume of gases for those who use grains and cubic inches 

 instead of the metrical system. 



The merest beginner in science understands when he is told that 

 a volume means 11*2 litres, and can easily calculate problems 

 relating to gases, with the aid of this constant. After using the 

 absolute volume for some time, he learns to see the same rela- 

 tions between the volumes of gases in a more general and abstract 

 manner, but the only philosophical beginning for hira is an 

 absolute volume defined intelligibly in itself. 



