242 Mr GRAHAM on the Law of 



the plug more slowly than air does, or is replaced by more than 

 an equal volume of air, as will presently appear. 



Those experiments, previously narrated, are perhaps sufficient 

 to establish the law in regard to hydrogen, particularly when we 

 find it hold in the case of other gases. 



As hydrogen is a very light gas, I was anxious to establish 

 the law also in regard to a heavy gas, such as carbonic acid. 



2. Diffusion of Carbonic Acid Gas. 



The most satisfactory experiments with carbonic acid gas 

 were performed by confining it over a solution of common salt, 

 saturated in the cold, which absorbs this gas very slowly, and, in- 

 stead of the diffusion-instrument with bulb, a long diffusion-tube 

 was found most suitable. 



Experiment 1. Thermometer 64 ; dew-point 53. Barome- 

 ter 30.13. Left in diffusion-tube 17 air, and filled up over brine 

 to 197 with carbonic acid gas, which gives 180 carbonic acid. 

 As brine boils at 222 or 224, that is 11 or 12 above the boiling 

 point of water, we may suppose it to be proportionally less va- 

 porous at low temperatures, and take the tension of its vapour at 

 64 to be that of water at 53, which was also the dew point. 

 This was confirmed by confining 847 volumes of atmospheric air 

 over brine at the time ; the air was not expanded by vapour ris- 

 ing into it from the brine, nor did it contract. 



The initial contents of the diffusion are therefore, 



Air and vapour, 

 Carbonic acid gas, 

 Vapour, 



197.0 



