Atmolysis. 415 



of a degree of thickness not exceeding one-half a millimetre, 

 about the thickness of an ordinary wafer : one of these plates 

 attached by cement to the end of a glass tube closes it and 

 forms a diffusiometer. During the process of filling oyer a 

 mercurial trough, the porous plate is covered with a thin sheet 

 of gutta-percha ; on removing which, diffusion immediately takes 

 place. 



If one of these tubes be filled with hydrogen, it is found 

 that the gas will totally diffuse itself into the air in the space 

 of about forty minutes ; about one-fourth as much air passing 1 

 through in the reverse direction, and the mercury rising 

 against the influence of gravity, to supply the space left by the 

 more rapid diffusion of the hydrogen. 



It is found that the pores of the compressed graphite are so 

 minute, that the gas in a mass does not pass through, but 

 that the molecules only pass by a sort of intestine motion. 

 According to the hypothesis advanced to account for this 

 circumstance, gases consist of perfectly elastic atoms which 

 move amongst each other with different degrees of velocity 

 in the different gases ; and when the gas is confined in a close 

 vessel, the atoms impinge against the sides and against 

 each other, but that this occurs without any loss of velocity 

 owing to their perfect elasticity. 



If, however, the sides of the vessel are porous, the atoms 

 moving against the open channels escape, and, in the same 

 manner, the air or any gas which may be external to the 

 vessel, enters. When the same gas is on both sides of a 

 porous vessel, the interchange takes place, but it is not at- 

 tended with any alteration of bulk of the contained gas. In 

 the case of some gases which diffuse at the same rate, as for 

 example, nitrogen and carbonic acid, the interchange takes 

 place without any change of volume. But with gases having 

 an unequal molecular velocity, as air and hydrogen, the diffu- 

 sion is unequal. 



The further investigations of Mr. Graham relate to the 

 laws affecting the passage of a gas, either by pressure or by 

 its own elastic force, through a graphite plate in one direction 

 only, a vacuum being maintained on the other side. A gas 

 may pass into a vacuum in three different modes : — 



1. When a gas passes into a vacuum through a minute 

 aperture, as a hole in a plate of platinum foil, its rate of 

 passage is regulated by its specific gravity. " A gas," says 

 Mr. Graham, " rushes into a vacuum with the velocity which a 

 heavy body would acquire by falling from the height of an 

 atmosphere composed of the gas in question, and supposed to 

 be of a uniform density throughout. The height of the 

 uniform atmosphere will be inversely as the specific gravity of 



