228 Mr GRAHAM on the Law of 



experiments made with the purpose of determining the propor- 

 tion between the gas diffused and the return-air, it was therefore 

 necessary to guard against any inequality of pressure, which was 

 managed much more easily when the tube was standing over 

 water than over mercury, 



The capacity of a mass of stucco to absorb and condense in 

 its pores the various gases, was made the subject of experiment, 

 as this property might interfere with the results of diffusion. 

 The mass was previously dried at 200 F. It absorbed at the 

 temperature of the atmosphere, which at the time was 78. 



6.5 volumes ammoniacal gas, 

 0.75 sulphurous acid gas, 

 0.5 cyanogen, 

 0.45 sulphureted hydrogen, 

 0.25 carbonic acid. 



Oxygen, hydrogen, nitrogen, carbonic oxide, olefiant gas, coal- 

 gas, were not absorbed in a sensible proportion, even when the 

 temperature was 58. It is evident, therefore, that the absorbent 

 power which stucco enjoys, as a porous substance, is inconsider- 

 able. Placed in humid air, the same mass of stucco absorbed 1^ 

 per cent, of hygrometric moisture. In setting, 100 parts of the 

 stucco had retained 26 parts water uncombined, which escaped 

 on drying at a moderate temperature, so as to avoid decompos- 

 ing the hydrated sulphate of lime. It can be shewn from this, 

 that the vacuities must have amounted to one-third of the vo- 

 lume of the mass. 



I shall treat in succession of the escape of the different gases 

 from a diffusion instrument into air. As the contained gas bears 

 no proportion in quantity to the external air, the gas escapes 

 entirely, and is wholly replaced by air. It is of the utmost im- 

 portance to determine the proportion between the volume of gas 

 diffused, and the replacing volume of air eventually found in the 



