THE PROPAGATION OF ELECTRICITY. 17.5 



determination is of little importance. Not so, however, when the discharge is 

 once transmitted in regard to the influence which the pressure of the gas it 

 traverses exerts on its intensity. The following are two comparative experi- 

 ments made with nitrogen and hydrogen. These two gases were successively 

 introduced into a tube 5 centimetres in diameter and 50 in length; the dis- 

 charge passed between two balls of platina one centimetre in diameter, placed 

 respectively near each extremity of the tube, so that the passage across the 

 gaseous medium was quite 50 centimetres in extent. 



Nitrogen. — The intensity of the discharge was measured by means of that 

 of the derived current received by the two points of platina phniged, at a fixed 

 distance of 120°'™, in the distilled water which was placed in the circuit : 



Pressure. Intensity of derived current. 

 9™™ Galvanometer almost insensible. 



"jiinm AO 



6"°^ 13° to 16 = 



4mm to 5mm 2G° to 30^ 



3mm to 4mm 350 



3™™ 38= to 40° 



2"™ 42° to 45° 



Hydrogen. — Proceeding at first as in the case of the nitrogen, the two points 

 of platina which received the derived current were left at a fixed distance from 

 one another : 



Pressure. Intensity of derived curi'ents. 



GO™™ to 30"""^ 1° to 4° 



26™™ 5° 



18™™ 6° 



15™™ 7° 



13™™ 13° 



10™™ 40° 



9™™ 50° 



For pressure of less than 9 millimetres the points of platina were in each 

 case brought nearer together, so as to have a constant current. At a distance 

 of 55™™, the derived current, which under a pressure of 9™™ had been 50°, Avas 

 reduced to 40°. Tbe following series was then obtained; and here, in order 

 to restore the derived current to 40°, it was requisite, in proportion as the press- 

 ure diminished, to bring the points closer together, so as to render the interval 

 of derivation smaller: 



Pressure. Distance of points. 



gmm ^ _ _ 65™™ 



Cmm 4^mm 



•7min 30™™ 



gmm 25™"" 



gmm 9Qmm 



4mm _ ji^mm 



3mm _ J 4mm 



omm 12™™ 



Thus, as for as 2™™ of pressure, the intensity of the derived current, and 

 consequently the conductibility of the gas, goes on increasing as well for the 

 hydrogen as for the nitrogen ; but we see how much more considerable is the 

 conducting power of tlife hydrogen than that of the nitrogen, since, under a 

 pressure of 9™™, all othei' circumstances remaining the same, the derived cur- 

 rent is, with the nitrogen, scarcely sensible, while it is 50° with tbe hydrogen. 



