80 Mr. W. Spottiswoode on Stratified Discharges. [May 18, 



intervals through its length. When viewed in the mirror the striaa 

 were seen to spread themselves out with slight, but irregular, proper 

 motion. With an increased battery-surface, or with a greater num- 

 ber of cells, but more notably with the latter, not only were the striae 

 lengthened, but from several of the long elementary strise shorter ones 

 were thrown out nearly at right angles to the former. These were of 

 short duration, and had great proper motion. The general appearance 

 of these compound striae was that of branches of fir trees, the twigs of 

 which represented the permanent striae, and the leaves the secondary. 



Beside these, a large (Teissier's " hydrocarbon ") tube was examined 

 with a magnet the pole of which was placed near the head of the 

 column ; and in order to trace more in detail the e:ffect of the magnet, its 

 strength was varied by raising or lowering the battery-plates. The 

 general character of the discharge without the magnet was very similar 

 to that represented in fig. 1. On slightly lowering the plates of the 

 magnet-battery the discharge spread itself over a greater breadth than 

 before. At the same time the elementary striae, which had for the most 

 part been continuous, were now broken up into short lengths, presenting 

 the first features of flocculent striae. On further lowering the plates 

 these flocculent striae became more and more developed until the whole 

 field in the neighbourhood of the magnetic pole became filled with such 

 striae. It is well known that one effect of the magnetic field is to bring 

 out striae in portions of tubes where no striae were visible before, and 

 also that the striae so brought out present a flocculent appearance ; but 

 the revolving mirror shows this fact in a more decisive manner. Another 

 effect of the magnetic field is to drive the discharge to one side of the 

 tube in accordance with Ampere's law — in other words, to constrict the 

 discharge. In narrower tubes than the one here described, the constric- 

 tion goes so far as to imitate the appearance of a capillary tube ; and this 

 effect is borne out by the revolving mirror. The intensification of the 

 discharge and its concomitant phenomena within the range of the mag- 

 netic field are in accordance with, the experiment of Earaday, wherein he 

 showed the increased loudness of the report perceptible on breaking a 

 current in between the poles of a magnet. 



In a carbonic-acid tube (G-assiot's No. 454) I have succeeded in start- 

 ing with a very weak current, capable of producing only elementary striae, 

 and thence passing to the production of flocculent striae, either by 

 strengthening the battery-current, or by inducing upon the existing cur- 

 rent the action of a magnetic field. The identification of the results of 

 these two independent processes, especially when combined with the com- 

 parison made above of the effect of magnetism with that of narrowiug 

 the tube, can hardly fail to have some important signification in the ulti- 

 mate theory of the striae. 



Besides the tubes above mentioned many others were tried ; but these 

 will probably suffice for the present purpose. 



