The Constitution of the Electric Spark. 



333 



generally equidistant points on the photographs, and may take the 

 average of all these figures as the mean velocity of the particle. In 

 the tables given in the paper, V T always refers to the mean velocity 

 between the pole and a point 2 mm. away from it, while V 2 refers to the 

 average velocity taken for different distances, as just explained. 

 The influence of change of capacity and change in the length of the 

 spark was investigated in the case of zinc, and the following table 

 exhibits the results. As the zinc lines are sharp near the pole, the 

 first of the above methods of measurement could be applied. 



Table I. Average Velocity (Vj) in metres/second of Zinc 

 Molecules. 



The first striking result to be deduced from the table is the uni- 

 formly higher velocity deduced from the double line 4925, as com- 

 pared with that found when one of the lines of the triplet is measured ; 

 for we have ascertained that the two first lines of the triplet are always 

 displaced by the same amount, and the third is so much mixed up 

 with the air lines in its neighbourhood that it cannot be measured. It 

 was one of the objects of the investigation to detect, if possible, 

 differences of this kind, which might be accounted for by the fact that 

 the molecules producing different lines of the same spectrum have not 

 necessarily the same mass. We nevertheless hesitate to ascribe the 

 smaller apparent velocity derived from A. = 4925 to this reason. This 

 line, as has been mentioned, is one component of a double line, and 

 the doublet is not resolved on the photographs taken with the moving 

 film. Near the pole where the light is strong, the edge of the least 

 refrangible component of the doublet would be considered to be the 

 least refrangible edge of the doublet ; but near the centre of the spark 

 the light is weaker, and the lines, owing to the motion of the wheel, 

 are drawn out towards the violet. The most intense portion of the 

 image will here be that part where the two lines are superposed, and 

 in wishing to set the cross wire on the edge of the line, we should be 



