1876.] Mr. W. Spottiswoode on Stratified Discharges. 75 



of air, the length of which could be regulated at pleasure, were intro- 

 duced into the secondary circuit. 



StrisD as observed by the eye have been divided into two classes, viz. 

 the flake-like and the flocculent or cloudy. Of the former those pro- 

 duced 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 appa- 

 rent difference a real difference corresponds, a fundamental feature of 

 the striae underlying both was brought out. 



The feature in question was this : that the striae, at whatever points pro- 

 duced, appear to have generally during the period of their existence a 

 motion along the tube in a direction from the negative 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 gene- 

 rally uniform ; and its variations in velocity are at all times confined 

 within very narrow limits. The proper motion in this sense appertains, 

 strictly speaking, to the flake-like 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 of the flake-like, 

 which latter I have on that account called elementary striae. 



Elementary striae are in general produced at regular intervals along 

 the tube. The series extends from the positive terminal in the direction 

 of the negative to a distance depending upon the actual circumstances 

 of the tube and current. The length of the column, and consequently 

 the number of the striae, depends mainly upon the resistance of the tube, 

 the duration of the entire current, and, to a certain extent, upon the 

 amount of the battery-surface exposed; and in that sense upon the 

 strength of the current. The velocity of the proper motion, other cir- 

 cumstances being the same, depends upon the number of cells employed; 

 in other words, upon the electromotive force. 



The appearances of the striae, however, their essential features, and 

 the conclusions which may be drawn from them will be better appre- 

 hended by means of sketches, even though imperfect, than by mere 

 description ; and I therefore subjoin a few examples. 



Y\g. 1 represents the appearance of (in the mirror) a carbonic-acid 

 tube with the slit attached. This tube, viewed by the eye, shows flake- 

 like fluttering striae, with a slight tendency to flocculency near the head 

 of the column. The commencement of the discharge is at the right 

 hand, and the negative terminal at the top. The drawing fairly repre- 

 sents the appearance of the upper part or head of the column of striae 

 during one complete coil-discharge. When the battery- surface exposed 

 is small, the whole consists of, first, three or four columns of striae of 

 decreasing length, and afterwards of an almost unbroken field of striae. 

 Each of the initial columns is perfectly stratified; and the same disposi- 



