Mr. T. Graham on the Molecular Mobility of Gases. 41 7 



whole is filled, and mer- 

 cury enters the quill tube 

 of exit, h. The caoutchouc 

 extension of this tube is 

 then closed by a pinch. 

 The diffusiometer is now 

 elevated 30 or 40 inches, 

 when the mercury sinks 

 in the glass tube till it 

 comes to stand at the baro- 

 metric height for the time, 

 leaving the upper chamber 

 entirely vacuous. The gas 

 to be tried has in the mean 

 time been made to stream 

 over the upper surface of 

 the graphite plate, exactly 

 as in the experiment with 

 the former diffusiometer. 

 The graphite is permeated 

 by the gas, and the mer- 

 cury in the diffusiometer- 

 tube begins to fall, but it 

 now falls slowly, owing to 

 the considerable vacuous 

 space to be filled. It is 

 allowed to fall about half 

 an inch, and the exact time 

 is then noted, by a watch, 

 when the mercury passes 

 a certain point in the 

 graduation of the tube, 

 and again when the mer- 

 cury descends to another 

 fixed point an inch or two 

 below the former. The 

 time of permeation of a 

 certain volume of gas is 

 thus ascertained iu seconds. 

 The experiment is imme- 

 diately repeated with two or 

 more gases in succession,in 

 similar circumstances as to 

 pressure and with great care 

 taken to ensure uniformity 

 of temperature during the 

 whole period. 



Fig. 7 



Illi 



