180 Prof. Graham on the Law of the Diffusion of Gases. 



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° Fahr. 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 — sulphuretted hydrogen, 

 0*25 — carbonic acid. 

 Oxygen, hydrogen, nitrogen, carbonic oxide, defiant 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 sub- 

 stance, is inconsiderable. Placed in humid air, the same mass 

 of stucco absorbed 1 J 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 tem- 

 perature, so as to avoid decomposing the hydrated sulphate 

 of lime. It can be shown from this, that the vacuities must 

 have amounted to one third of the volume 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 importance to determine the proportion between the 

 volume of gas diffused, and the replacing volume of air even- 

 tually found in the instrument. We thus obtain the equiva- 

 lent diffusion-volume of the gas, which it will be convenient to 

 state in numbers, with reference to the replacing volume of air 

 as unity. I shall begin with hydrogen gas, although attended 

 with peculiar difficulties, as it introduces in a distinct manner 

 to our notice several circumstances which may slightly modify 

 the results of diffusion. 



1. Diffusion-volume of Hydrogen Gas. 



I shall in this paper adopt the specific gravities of the gases 

 generally received in this country. Of hydrogen the specific 

 gravity is 0*0694 (air = I), of which number the square root 

 is 0*2635. Now, according to our law, 1 volume hydrogen 

 should be replaced by 0*2635 air. But to have the replacing 

 volume of air = 1, ' 0*2635 : 1 :: 1 : 3*7947; 



or, = 3*7947; that is, 1 air should replace 3*7947 



hydrogen. With the specific gravity of hydrogen adopted by 

 Berzelius, namely, 0*06885, the equivalent diffusion-volume 

 of hydrogen is 3*8149. 



