66 Mr. Wo Makower on the Molecular 



was performed by treating a known volume of it with a con- 

 centrated solution of caustic potash and measuring the 

 diminution in volume. 



Sulphur dioxide. — This gas was obtained directly from a 

 siphon containing the liquid, care being of course taken to 

 remove the air from the connecting tubes. The gas left in 

 the diffusion-vessel at the end of an experiment was analysed 

 by treating a known volume with potassium bichromate 

 solution and measuring the diminution of volume produced. 



As already stated, the composition of the gases left at the 

 end of the diffusion experiments was determined volume- 

 trically. The estimation was carried out by means of a gas 

 burette of 60 c.c. capacity of the type described by Travers 

 (' Study of Gases,' p. 66), and graduated downwards in fifths 

 of a cubic centimetre, so that the volume of the gas in the 

 burette could be estimated to "1 c.c. with certainty. To avoid 

 errors due to variations of temperature, the burette was en- 

 closed in a water-jacket and the temperature read off on a 

 thermometer immersed in the water. The burette was care- 

 fully calibrated by filling with water and running out by 

 successive stages of 2 c.c. and weighing. About 20 c.c. of 

 the gas to be analysed was usually transferred from the 

 diffusion-vessel to the burette, so that its composition could 

 be determined to one per cent. 



6. Observations and Results. 



In calculating the values of X from the observations with 

 oxygen, account has to be taken of the fact that the upper 

 surface of the plug is in contact with air which contains 

 21 per cent, of oxygen, so that oxygen is diffusing into 

 as well as out of the diffusion- vessel throughout the experi- 

 ment. To allow for this it is necessary to slightly modify 

 equation (3). 



Since the concentration of the oxygen in the air in contact 

 with the upper surface of the plug remains constant, the 

 quantity dy of oxygen which, on the whole, escapes from the 

 diffusion- vessel is given by the equation 



whence 



1 v 01 | 100 



X= r ; ^ 7 -^ log. 



f 1 at *H* \ t °"\ 21 



L ^ + IT ~""'J \y-T7Tn& 



100" 



