the Diffusion of Gases. 233 



cessary to know the absolute quantity of vapour in either case, 

 in determining the diffusion-volume of hydrogen. We may 

 simply divide the gross amount of hydrogen gas diffused, by the 

 gross amount of return air, the quotient is the diffusion-volume 

 of hydrogen. 



Experiment 2. The thickness of the stucco-plug in the in- 

 strument used above, was reduced from six-tenths to two-tenths 

 of an inch, by cutting away the upper portion. The instrument, 

 of the same capacity as before, was now entirely rilled with hy- 

 drogen gas. This was effected, by first filling up with hydrogen, 

 leaving a small quantity of air in the upper part of the instru- 

 ment as in the previous experiment, then withdrawing this im- 

 pure hydrogen by the air-syphon, and filling up a second or third 

 time with the same gas, whereupon the proportion of air remain- 

 ing ceased to be appreciable. The apertures of the plug were 

 closed, by pressing the finger upon its upper surface ; and in 

 this manner any diffusion of the hydrogen was carefully guarded 

 against, till the process of filling was completed. The diffusion 

 was so rapid in the case of the thin plug, that this additional 

 precaution was absolutely required. Care was taken to have 

 the return-air saturated with moisture in this and every other 

 experiment of the same kind, and inequality of pressure was 

 avoided. 



At the beginning of the experiment, the instrument con- 

 tained 855 measures hydrogen, saturated with vapour at 62 ; in 

 three minutes a contraction of 95 measures took place, and in 

 the course of an hour the diffusion was sensibly at an end. The 

 instrument, however, was exposed for two hours longer, that the 

 diffusion might certainly be complete. During intervals so short 

 uniformity of temperature might be counted upon, with certain 

 precautions; and the variations in atmospheric pressure were 



