Prof. Graham on the Laxv of the Diffusion of Gases. Ill 



absorbed with greatest difficulty, and in smallest quantity, by 

 charcoal and the other porous substances, tried by Saussure. 

 And we have no reason to suppose that the particles of hy- 

 drogen are smaller than those of the other gases. 



On repeating Dcebereiner's experiment, and varying the 

 circumstances, it appeared that hydrogen never escapes out- 

 wards by the fissure, without a certain proportion of air re- 

 turning inwards. In the experiment, however, as originally 

 performed, it is evident, that, as soon as the water rises in the 

 jar above its outer level, air will begin to be forced into the 

 jar mechanically through the fissure, by the pressure of the 

 atmosphere, independently of what we shall suppose enters 

 by diffusion. But if we press down the jar of hydrogen to a 

 certain depth in the water-trough, so that the level of the 

 water without is kept constantly higher than the level of the 

 water within the jar, then, on the contrary, a portion of 

 the hydrogen will be forced out mechanically, by the pressure 

 to which the gas is subject. In the last circumstances, how- 

 ever, no air can enter by the fissure, and mix with the hydro- 

 gen, except by diffusion, or in exchange for hydrogen. Now, 

 in a great number of experiments of this kind, the air which 

 entered by diffusion amounted to between one fifth and one 

 fourth of the hydrogen, which left the receiver at the same 

 time. But when the circumstances were reversed, and the 

 column of water allowed to rise in the jar above the level of 

 the water-trough, the quantity of air which entered by diffusion 

 was increased by a portion which entered mechanically ; and 

 varied from a third to a fourth part of the hydrogen, which 

 escaped at the same time. The results, therefore, oscillate, 

 as they should do, about our theoretical number. One volume 

 air should replace 3*794-7 volumes hydrogen ; or the whole 

 hydrogen, on escaping from the jar, should be replaced by 

 little more than one fourth of its bulk of air, and a very great 

 contraction ensue. 



But it is unnecessary to detail experiments made with the 

 jar with the fissure, as with every precaution they were not 

 precise, although at all times compatible with, and indeed illus- 

 trative of, the law. Thus a sensible contraction always took 

 place in the bulk of the gaseous contents of the jar when filled 

 with carburetted hydrogen of marshes, or with coal-gas, which, 

 like hydrogen, are lighter than air, and ought therefore to be 

 replaced by less than equal volumes of air. With olefiaut 

 gas and carbonic oxide, which approach closely to the den- 

 sity of air, no contraction was perceptible, not attributable to 

 other causes, although the gases as usual wholly escaped. In 

 the case of carbonic acid, which is heavier than air, a slight, 



Third Series. Vol. 2. No. 9. March 1833. 2 A 



