MR. T. ROYDS ON THE CONSTITUTION OF THE ELECTRIC SPARK. 33!) 
streamer throughout its length shows the velocity of the vapour. Before the first 
streamer has completely died away, having reached only a short distance from the 
electrodes, it is met by a following streamer. If new vapour has been produced by 
the oscillation causing this streamer, it will, like the metal first vaporised, indicate, so 
long as it remains luminous, the velocity with which it is travelling from the electrode 
where it is produced. Also, unless the metallic vapour has been projected from the 
electrode, there will be vapour in the space between the electrode and the point to 
which the material first vaporised has travelled, and it is possible that a pulse of 
luminosity is propagated through this, i.e., that the particles are successively excited 
to luminescence without the vapour itself, as a whole, being in motion. A more 
complete discussion of this phenomenon is postponed to p. 343, but what here concerns 
us is that if this luminosity is propagated sufficiently rapidly it will reach the vapour 
produced at the commencement of the spark, which is still leading the way to the 
centre of the gap. Hence we should expect that when this occurs the streamer will 
suddenly bend over into line with the previous one and will then indicate the velocity 
with which the vapour originally produced is travelling. Then, after a further whole 
period, another streamer, excited by the third oscillation, begins at the electrode, and 
the same kind of procedure repeated, until when the streamer bends over into a line 
continuous with the second, the velocity of the vapour produced at the beginning of 
the spark is again indicated. 
Thus we see that the velocity of the vapour in the electric spark is indicated by a 
kind of envelope consisting of the whole of the first streamer and of the latter 
portions of the following streamers until the middle of the gap is reached. What the 
photographs reproduced in this paper show to consist of an envelope was the line 
measured by Schuster and Hemsalech. Schenck* obtained such an envelope, but 
did not interpret it in this way. 
If the metallic vapour remains luminous much longer than the time for which 
the oscillation which renders it luminous lasts, the streamers will not be separated in 
the moving photograph. This occurs apparently in the case of the calcium lines 
XX 3969 (H), 3934 (K), 3737, 3706, when the spark is produced between poles of 
calcium metal. This causes the appearance of these lines in the moving photograph 
to be strikingly different from that of the lead and bismuth lines. Their edge is a 
sharp, bent line from which the intensity gradually shades off, the streamers being 
entirely obliterated. In the lead spectrum in which the calcium lines show strongly 
as an impurity, the streamers are separated and their structure is seen to be similar to 
that of the lead lines. 
The magnesium line X 4481 is shown in Plate 29, fig. 8, and the mercury lines 
XX 4358, 4047, 3984, in Plate 29, fig. 9. The mercury was contained in a small cup 
and the upper electrode was, in this case, lead. The first streamer of the mercury 
lines is long, and in the cases of the lines XX 4358 and 4047 the following streamers 
* Schenck, ‘ Astropliysical Journal,’ 14, p. 116 (1901). 
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