41 6 Atmolysis. 



the gas, the atmosphere of hydrogen being, for instance, sixteen 

 times higher than that of oxygen; but as the velocity acquired 

 by a heavy body falling is not as directly the height, but as the 

 square root of the height, the rate of flow of different gases into 

 a vacuum will be inversely as the square root of their densities. 

 The velocity of oxygen being 1, that of hydrogen will be 4, 

 the square root of 16. This law has been experimentally 

 verified/' and the mode of passage is termed Effusion. 



2. If the aperture be in a plate of increased thickness, the 

 law of effusion no longer holds good. When the length of the 

 tube exceeds the diameter by 4000 times, a new ratio is 

 established, that of the Capillary Transpiration of Gases. 

 The rates of transpiration are not governed by specific gravity, 

 and are singularly unlike those of effusion, the transpiration 

 velocity of oxygen being 1, that of chlorine is 1'5, and that of 

 hydrogen 2*26. These ratios appear in constant relation to no 

 other known property of the gases, and they form a class of 

 phenomena remarkably isolated. Exceedingly minute capil- 

 lary tubes, however numerous, offer practically a perfect impedi- 

 ment to the passage of gas by transpiration. 



3. The diffusive, or molecular movement of gases, enables 

 them readily to pass through plates of graphite which are, 

 practically, impenetrable to gas by either of the two preced- 

 ing modes ; this penetration appears to be due entirely to their 

 own proper molecular motion, entirely unaided by transpiration, 

 and uninfluenced by pressure. The graphite appears, as it 

 were, to become a molecular sieve, allowing molecules only to 

 pass through, and, consequently, hydrogen is found experimen- 

 tally to pass through a graphite plate with precisely the same 

 velocity, whether it is passing into a vacuum or into air. 



These abstract investigations of Professor Graham have 

 already been pressed into the service of practical science. An 

 instrument termed an Atmolyser has been designed, by means 

 of which mixed gases of different diffusibility can be separated. 

 The most remarkable effects of separation are produced by 

 the tube atmolyser ; this is simply a narrow tube of ungiazed 

 earthenware, similar to a tobacco pipe stem, two feet in length, 

 which is placed in a shorter tube of glass, and secured in its 

 position by corks, so as to resemble in appearance a Liebig's 

 condenser ; the glass tube is then exhausted of air by being 

 connected with an air-pump, and a mixed gas is allowed 

 to flow through the earthenware tube, when the moro diffusive 

 gas is rapidly abstracted. Thus, when an explosive mixture 

 of two volumes of hydrogen and one of oxygen is passed, 

 tho gas issuing from the tube consisted of oxygen, with less 

 than 10 per cent, of hydrogen — a mixture which could not 

 be ignited. 



