Mr. W. Spottiswoode on Stratified Discharges. 145 



in view, viz. the character and behaviour of the striae; and of 

 these, together with some recent additions, I now propose to 

 • ; offer a short account to the Society. 



My general instrumental arrangements appear to have been 

 similar to those of Wullner ; in fact they could hardly have been 

 very different. The tubes were attached to the coil in the usual 

 way, and a contact-breaker of the ordinary form with its own elec- 

 tromagnet was in the first instance used. By suitably adjusting 

 the velocity of the mirror to the rapidity of the contact-breaker 

 the image could be kept tolerably steady in the field of view. In 

 order to obtain greater steadiness a special contact-breaker was 

 next devised. This was mechanically connected with the spindle 

 of the mirror, and so arranged as to break the current when the 

 image was in the centre of the field of view. The only point in 

 this part of the apparatus which requires special notice is the 

 fact that this contact-breaker, like all others, should be placed 

 in close proximity to the condenser of the coil, otherwise a great 

 loss of light is sustained. For the last-mentioned form there was 

 finally substituted a mercurial break (successfully arranged by 

 my assistant, Mr. Ward), the plunger of which works on a cam 

 attached to the axle of the mirror ; so that the action of the con- 

 tact-breaker is regulated by that of the mirror, instead of the 

 reverse as in the former arrangement. With the broader tubes a 

 slit was used ; with the narrower this adjunct was less necessary ; 

 while with capillary tubes, such as are used for spectrum-analy- 

 sis, it could be dispensed with altogether. 



In experiments for comparing the unstratified statical discharge 

 with the stratified at the same pressure of gas within the tube, 

 and for observing the transition from one to the other, a Leyden 

 jar and a spark of air, the length of which could be regulated at 

 pleasure, were introduced into the secondary circuit. 



Striae as observed by the eye have been divided into two classes, 

 viz. the flake-like and the flocculent or cloudy. Of the former 

 those produced in hydrogen-tubes may be taken as a type; of 

 the latter those produced in carbonic-acid tubes. But upon 

 examining some tubes especially selected for the purpose, it was 

 found that, while to this apparent difference a real difference cor- 

 responds, a fundamental feature of the striae underlying both was 

 brought out. 



The feature in question was this — that the striae, at whatever 

 points produced, appear to have generally during the period of 

 their existence a motion along the tube in a direction from the nega- 

 tive towards the positive terminal. This motion, which I have 

 called, for convenience, the proper motion of the striae, is for given 

 circumstances of tube and current generally uniform; and its 

 variations in velocity are at all times confined within very nar- 

 row limits. The proper motion in this sense appertains, strictly 

 speaking, to the flake4ike striae only. The apparent proper 

 motion of the flocculent striae is, on the contrary, variable not 

 only in velocity, but also in direction ; and on further examination 

 it turns out that the flocculent striae are themselves compounded 



Phil. Mag. S. 5. Vol. 3. No. 16. Feb. 1877. L 



