334 [Dec. 18, 



but luminous discharges are obtained, which are distinctly separated 

 by the revolving mirror, and are consequently intermittent. 



15. No. 319, with 2240 cells of the battery and a resistance intro- 

 duced in the circuit : the discharge can, by careful manipulations, be 

 modified so as to assume the appearance of a positive and negative 

 discharge, impinging on and intermingling with each other without any 

 dark space intervening (Plate I. fig. 3). Around the negative ter- 

 minal the luminosity extends to the sides of the tube ; from thence to 

 the positive wire the discharge, as represented in the figures, takes 

 place in a line of about 2 inches diameter, emitting a very faint light. 

 The slightest variation in the resistance obtained by raising or depress- 

 ing the connecting wire in the columns of distilled water (fig. 1) 

 alters the appearance of the discharge to that represented in fig. 4, 

 where the dark portion is clearly defined. As the resistance is reduced, 

 the dark space increases by the positive discharge receding towards the 

 wire, the negative becomes brighter and more clearly defined, at its 

 termination an approach to stratifications is observed, until, as the 

 resistance is further reduced, the discharge will suddenly assume the 

 form (as in fig. 5) of two bright clouds, the outer edges presenting 

 a greenish -blue colour, crescent-shaped, clearly and sharply defined, 

 of about gth of an inch diameter. The other portion of the clouds 

 remains of a bright reddish purple, gradually deepening in colour 

 in approaching the other edge, where it becomes less defined, leaving 

 towards the next cloud a dark space. The same gradations of colour 

 are observable in the inner cloud next the positive wire ; but not so 

 clearly or distinctly defined. The discharge is at this time continuous 

 at intervals ; but, without altering or apparently interfering with these 

 striee, sudden discharges take place : two or three similar cloud-like 

 striae will be observed at the positive wire ; and at the same instant 

 brilliant stratifications are visible, overlapping the negative (fig. 6)*. 



With the 2240 series distinct sounds were heard in the tube ; 

 with the whole battery of 3360 series, the overlapping striae would 

 remain for several seconds, but the sounds were not appreciable until 

 a magnet was presented near to, and in a line with, the overlapping 

 striae ; the action of the magnet causes these striae to spread along the 

 surface of the glass tube, and in this state of the discharge the sounds 

 were again audible. 



* White tongue discharge (Phil. Trans. 1859, p. 140). 



