94 MESSRS. W. DE LA RUE AND H. W. MULLER ON THE 
Date. 
Cells. 
Copper point + 
Length of spark. 
Aluminium point + 
Length of spark. 
Eatio of spark with 
aluminium to that 
with copper = 
inch. 
inch. 
1876. March 21 and 22 
5640 
0-240 
0-320 
1-300 
1877. July 9 . . . 
8040 
0-340 
0-411 
1-209 
„ 10 ... 
” 
0-340 
0-414 
1-217 
Mean D242 
The length of the spark differs in various gases at ordinary atmospheric pressures, 
as we anticipated that it would do from our experience with vacuum tubes ; but the 
difference in the length of the spark does not bear any precise ratio either to the 
specific gravity of the gas or its viscosity in reference to mechanical impulse ; we 
therefore propose to ascribe it, at all events provisionally, to a difference of electric 
viscosity. In order to make experiments on the discharge in different gases, we placed 
the discharger, already described page 58, fig. 1, under a bell glass G G' (fig. 21), open at 
the top and covered with a glass plate, P. 
The glass plate has two screw-clamps, which are connected at its under surface 
with wires led from the screw-clamps c and c (fig. 21) of the discharger. In connexion 
with these, on the outside surface, are two other screw-clamps c c with which the 
terminals of the battery T T' are connected. Through a stuffing box in the glass cover a 
