432 Mr. T. Graham on the Molecular Mobility of Gases. 



burns in it as in pure oxygen. A mixture of oxygen and hydrogen 

 is not explosive till the hydrogen rises to 11 per cent. 



To illustrate the analogy of diffusion into a vacuum with dif- 

 fusion into air, the outer glass tube of the diffuser was now with- 

 drawn, and the porous tube of the instrument was exposed directly 

 to the air of the atmosphere. A mixture of equal volumes of 

 oxygen and hydrogen was again transmitted at the same rate of 

 velocity as in experiment 1. 



The gas atmolysed and collected was found to consist of 



Oxygen 51*75 



Hydrogen 5*47 



Nitrogen 42-78 



Too-oo 



And may be represented as containing 



Oxygen 40-38 



Hydrogen 5*47 



Air 54-15 



100-00 

 A nearly similar concentration of the oxygen of the mixed gas is 

 here observed as appeared in experiment 1 ; but the gas collected 

 is now diluted with air which has entered by diffusion. The 

 external air manifestly discharges the same function in the latter 

 experiment which the air-pump vacuum discharged in the former 

 experiment. 



Interdiffusion of Gases — double diffusion. 



The diffusiometer was much improved in construction by Pro- 

 fessor Bunsen, from the application of a lever arrangement to 

 raise and depress the tube in the mercurial trough; but the 

 mass of stucco forming the porous plate in his instrument ap- 

 pears too voluminous, and, from being dried by heat, is liable 

 to detach itself from the walls of the glass tube. The result 

 obtained of 3'4 for hydrogen, which diverges so far from the 

 theoretical number, is, however, no longer insisted upon by that 

 illustrious physicist. It is indeed curious that my old experi- 

 ments generally rather exceeded than fell short of the theoretical 

 number for hydrogen, <\/0-06926 = 3-7994. With stucco as the 

 material, the cavities existing in the porous plate form about one- 

 fourth of its whole bulk, and affect sensibly the ratio in question 

 according as they are or are not included in the capacity of the 

 instrument. Beginning the diffusion always with these cavities, 

 as well as the tube, filled with hydrogen, the numbers now 

 obtained with a stucco plate of 12 millims. in thickness and 

 dried without heat, were 3*783, 3*8, and 3*739 when the volume 



