Mr. W. Spottiswoode on Stratified Discharges. 151 



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 constriction 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 magnetic field are in accordance with the experiment of 

 Faraday, 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 (Grassiot's No. 454) I have succeeded in 

 starting with a very weak current, capable of producing only ele- 

 mentary striae, and thence passing to the production of floc- 

 culent striae, either by strengthening the battery-current, or by 

 inducing upon the existing current the action of a magnetic field. 

 The identification of the results of these two independent processes, 

 especially when combined with the comparison made above of the 

 effect of magnetism with that of narrowing the tube, can hardly 

 fail to have some important signification in the ultimate theory of 

 the striae. 



Besides the tubes above mentioned many others were tried ; 

 but these will probably suffice for the present purpose. 



The following are some of the general conclusions to which the 

 foregoing experiments seem to lead : — 



I. The thin flake-like striae, when sharp and distinct in their 

 appearance, either are short-lived or have very slow proper motion, 

 or both. 



II. The apparent irregularity in the distribution of such striae, 

 during even a single discharge of the coil, is due, not to any actual 

 irregularity in their arrangement, but to their unequal duration 

 and to the various periods at which they are renewed. These 

 striae are, in fact, arranged" at regular intervals throughout the 

 entire column. The fluttering appearance usually noticeable is 

 occasioned by slight variations in position of the elementary striae 

 at successive discharges of the coil. With a view to divesting 

 the coil-discharge of this irregular character, as well as for other 

 purposes, I devised two different forms of contact-breakers (one of 

 which is described in the Eoyal Society's ' Proceedings,' vol. xxiii. 

 p. 455); but I postpone a description of the second, as well as of 

 the experiments arising from its use, to another occasion. 



III. The proper motion of the elementary striae is that which 

 appertains to them during a single discharge of the coil. This 

 appears to be generally directed from the positive towards the 

 negative terminal. Its velocity varies generally within very narrow 

 limits. It is greater the greater the number of coils employed, 

 or the greater the electromotive force of the current. In some 

 tubes it may be seen to diminish towards the close of the discharge ; 

 and even in rare instances alternately to increase and to diminish 

 during a single discharge. 



