from an Arc and from Incandescent Metals. 249 



is greater than that of the platinum, and the conditions will 

 therefore be more favourable for having a greater mass travelling 

 with the carrier. 



Difference of velocity of the positive and negative carriers. 



(10) The method used to determine the velocity of the 

 carrier under an electric force, although it may not give a very 

 accurate result, can still be used to detect a difference in velocity 

 of the positive and negative, since all errors enter equally into 

 both determinations. The velocity, as we have seen, is a variable 

 quantity and depends on the nature of the arc, but keeping the 

 arc as steady as possible, and determining the velocity for the 

 carriers of both sign, we always find that in the same field of 

 electric force the negative carrier moves with a greater velocity 

 than the positive, and for air the excess is about 20 per cent. 



This difference is about the same as found for gas exposed to 

 Rontgen rays and in flame gas, although the absolute values of 

 the velocities are very different in these cases. Since the charges 

 on the positive and negative carriers must be the same, the 

 difference in their velocities must be due to a greater mass 

 travelling with the positive carrier, and the ratio of the masses 

 would seem to remain approximately the same in these different 

 cases, which is perhaps to be expected. 



The arc in oxygen. 



(11) Experiments similar to those described above were made 

 with oxygen. Two gas bags were used, and the gas passed from 

 one bag to the other through the apparatus, so that the same 

 supply of oxygen could be used for some time. 



As in air there is an excess of positive electricity in the con- 

 ducting gas drawn off from the arc, and this excess is greater 

 than in the case of air, but still small compared with the amount 

 of electricity of either sign. 



The velocity of the negative carrier under electric force is, as 

 in air, greater than that of the positive, and the excess was 

 estimated at 25 per cent., a rather greater difference than in air. 



The velocity of the carriers varied greatly with the nature of 

 the arc, as was found for air. To compare the velocity in oxygen 

 with that in air determinations were made for each, keeping the 

 arc as constant as possible, and the velocity in oxygen was found 

 to be about § of that in air. Of course even when the current 

 through the arc is the same for the air and the oxygen we do not 

 know that the temperatures of the arc and its terminals are equal 

 in the two cases, so that the comparison of the velocities of the 



