Prof. Graham on the Law of the Diffusion of Gases. 1 83 



diffusion-instrument may be stated with sufficient accuracy as 



follows : — 



Air and vapour originally present 11*85 



Dry air which has entered 212*84- 



Vapour in ditto 5*11 



229-80 

 The conclusion is, that 823*83 measures dry hydrogen have 

 been replaced by 212*84 dry air. Now, 



C O Ci • <2 ?4 



1 — — = 3*87 = diffusion-volume of hydrogen. 

 212-84 J b 



The diffusion-volume of hydrogen comes out above the theo- 

 retical number in this experiment; but an addition of not 

 more than 2 per cent, to the quantity of return-air, would re- 

 duce it below the theoretical number. The quantity of va- 

 pour which was supported by the hydrogen at the commence- 

 ment of the experiment was 19*17 measures, but at the end 

 of the experiment we find only 5*11 measures vapour; the 

 difference has condensed, from the loss of a permanently 

 elastic fluid necessary to support it. 



As the quantity of hydrogen and of return-air is amplified 

 in the same proportion by vapour, provided the temperature 

 be the same at the beginning and end of the experiment, it is 

 unnecessary 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 diffu- 

 sion-volume of hydrogen. 



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

 instrument 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 

 filled with hydrogen gas. This was effected, by first filling 

 up with hydrogen, leaving a small quantity of air in the upper 

 part of the instrument as in the previous experiment, then 

 withdrawing this impure hydrogen by the air-syphon, and 

 filling up a second or third time with the same gas, where- 

 upon the proportion of air remaining ceased to be appreci- 

 able. The apertures of the plug were closed, by pressing the 

 finger upon its upper surface; and in this manner any diffu- 

 sion 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. 



