82 Mr, W, Spottiswoode on Stratified Discharges. [May 18^ 



is a strong outburst at first. This, although sometimes appearing as a 

 bright line, is always, I believe, really stratified. Immediately after this 

 there follows a very rapid shortening of the column. The extent of this 

 shortening varies with circumstances ; but when, as is often the case, it 

 reaches far down towards the positive terminal, a corresponding diminu- 

 tion of intensity is perceptible in the negative glow. The column of 

 strise, after rising again, is often subject to similar fluctuations. These, 

 which are sometimes four or five in number, are successively of less and 

 less extent, and reach only a short distance down the column or strias. 

 The rifts due to these fluctuations then disappear, and the striae either 

 continue without interruption, or follow broken at irregular intex'vals, 

 until the close of the discharge, 



VIII. The effect of the proper motion, taken by itself, is to shorten 

 the column of strise. But, as we have seen, the striae are in many cases 

 renewed from time to time. In regard to this point, the head of the 

 column presents the most instructive features. After the cessation of 

 these rifts, the general appearance of the field is that of a series of 

 diagonal lines commencing at successive points which form the bounding 

 limit of the column at successive instants of time. If the points are 

 situated in a horizontal line, the striae are renewed at regular intervals at 

 the same place ; and the length of the column is maintained by a periodic 

 renewal of striae, a new one appearing at the head of the column as 

 soon as its predecessor has passed over one dark interval. If the boun- 

 dary of the illuminated field rises, the length of the column increases ; if 

 it descends, the column shortens. In every case, however, the growth of 

 the column takes place by regular and successive steps, and not irregu- 

 larly. The intervals of the new striae from one another and from the old 

 ones are the same as those of the old ones from one another. 



IX. The principal influence of a change in the electromotive force 

 appears to consist in altering the velocity of proper motion. A change 

 in the amount of battery-surface exposed produces a corresponding 

 change in the duration of the entire discharge, as well as apparently in 

 the development of some of the minor details of the striae. 



X. When the proper motion of the elementary striae exceeds a certain 

 amount, the striae appear to the eye to be blended into one solid column 

 of light, and all trace of stratification is lost. "When this is the case 

 the mirror will often disentangle the individual striae. But there are, 

 as might Avell be expected, cases in which even the mirror is of no avail, 

 but in which we may still suppose that stratification exists. A variety 

 of experiments have led me to think that the separation of the discharge 

 into two parts, viz. the column of light extending from the positive 

 terminal, and the glow around the negative, with a dark space interven- 

 ing, may be a test of stratified discharge ; but I cannot afiirm any thing 

 certainly on this point. 



