288 



Drs. De La Rue and H. W. Muller. [Nov. 20, 



diagram, fig. 2, which only indicates the central bright portion of the 

 arc, this never quite reached the negative terminal, near which there 

 was always the well-known dark discharge. 



As the pressure was diminished the arc widened out until at last the 

 entire surface of the negative disk was covered with a luminous halo, 

 and the discharge took up a stratified appearance. 



The appearances presented by the arc in air, hydrogen, and carbonic 

 acid are copiously illustrated by copies, in mezzotint, of photographs 

 and drawings as in Part II in the case of stratified discharges in tubes. 

 During the course of these observations the increased pressure caused 

 by the current was recorded, and an arrangement was made to ascer- 

 tain whether the increase of pressure was greater near one or the 

 other terminal. For this purpose a divided bell-jar was constructed, 

 both ends of the two halves being accurately ground, and a glass disk 

 divided the chamber into two parts ; in this there is a hole half an inch 

 in diameter made with a raised rim, in order that plates of mica with 

 holes of different diameters might be cemented with Canada balsam 

 centrally to the diaphragm. The capacity of the upper half was 

 found to be 1,530 cub. centims., that of the lower half 1,755, total 

 3,285. 



That portion of the chamber which was in communication with the 

 gauge was connected alternately with the positive or negative terminal 

 of the battery, and the depression noted. When this chamber was 

 either — or + the ratio of increase to the normal pressure was — 





+ 



1-44 



1-39 



1-34 



1-41 



113 



1-20 



* 1-22 



1-34 



1-38 



1-40 



1-16 



1-11 



1-19 



1-16 



1-266 



1-287 



It would appear therefore .that the dilatation of the gas is the same 

 both in the positive and negative chambers. 



In order to prosecute their experiments in a vessel of still greater 

 capacity, the authors had constructed a larger jar with a neck at each 

 end, or more properly speaking perhaps, a tube supported horizontally 

 on ebonite crutches. It is 37 inches long and 5-^-f inches in diameter, 

 its cubical content was found to be 14,435 cub. centims., or 3"8 times 

 that of the bell-jar employed in the experiments on the electric arc. 



The experiments with this tube will necessarily occupy a con- 

 siderable period, partly on account of the longtime it takes to exhaust 



