236 Mr GRAHAM on the Law of 



New hydrogen gas was made for each experiment by the mo- . 

 derate action of dilute sulphuric acid on zinc, and it was collected 

 in the diffusion-instrument from the beak of the retort. The 

 observations could not be made with so much accuracy as to en- 

 title us to place any reliance on more than two decimal places of 

 the calculated diffusion-volumes. A great variety of experiments 

 were performed on the diffusion of hydrogen with the diffusion- 

 bulbs employed above, and several others of similar construction, 

 principally with the view of discovering the cause of the slight 

 variations in the results, and why the quantity of return-air was 

 pretty uniformly somewhat less than the theoretical quantity, 

 which has the effect of increasing the proportion of the hydrogen 

 diffusion-volume. 



It appears, that when the stucco-plug is in a parched state, 

 the quantity of return air is uniformly greater than it should be. 

 Thus 3.65 and 3.69 were the diffusion-volumes of hydrogen de- 

 duced from an experiment, in the one case with a plug which had 

 been dried at 100, and subsequently exposed for several hours 

 to the air, and in the other case, with a plug merely dried in air, 

 temperature 68. The obvious cause of this is, that the air is 

 dried in passing through the plug, and is subsequently expanded 

 while in the diffusion-instrument by the ascent of vapour into it. 

 Hence, the first time a diffusion-bulb is tried, it generally gives 

 the diffusion-volume of hydrogen below the truth. 



On the other hand, I apprehend, that when the pores of the 

 stucco are saturated with hygrometric moisture, which, from the 

 circumstances of the experiments, must be almost always the case, 

 the hydrogen, in making its way through the plug, actually avails 

 itself to a small extent of this moisture, inducing it to vaporize, 

 and exchanging places with it instead of air. Hydrogen which 

 escapes in this way will not be represented by return-air, the 

 quantity of which is thus diminished. This process, however, is 

 extremely intricate, and has not yet been fully investigated. Its 



