OF ORGANIC SUBSTANCES ON THE RAYS OF THE SPECTRUM. 
259 
time. The coil, which could give a 6 or 7-inch spark in air, was excited by a battery 
of five Grove’s cells. Other batteries were tried, but none gave such satisfactory 
results when working for hours together. The Leyden jar was of such a size that each 
surface of tinfoil measured 72 square inches. 
The carriers for the metallic points are so fixed to the collimator tube that by the 
motion of two screws they may be moved from right to left across the slit, and at the 
same time as close as desirable under different circumstances. The proper position of 
the carrier is, of course, a vertical one, but it could be turned over in a horizontal 
direction so as to afford convenience for fixing the points. The electrodes are more 
conveniently held by screw forceps than by spring clips. Broad electrodes, as former 
experimenters have pointed out, are usually the best to work with (Miller and Stokes, 
loc. cit.), but we have in certain cases obtained better results with points, as for 
instance, with nickel wire. The spark apparatus was enclosed in a wooden box along 
with the end of the collimator tube to prevent the light emitted by the electrodes 
from escaping into the dark room, and to muffle the irritating noise of the electric 
discharge. We have had no difficulty in obtaining an uninterrupted stream of sparks 
yielding a perfectly steady light for three quarters of an hour. Professor Stokes used 
a condensing lens of 2\ inches focal length to concentrate the rays of the spark upon the 
slit, but since we sometimes employed an amalgam containing several metals as the 
lower electrode, we could not use the same, for the reason that mercury vapour would 
condense upon the lens. Instead of condensing the rays with a lens, we diminished 
the loss of light very greatly by fixing the electrodes as near as possible to the slit, and 
placing the cell holding the liquid to be examined in a little wooden box behind the 
slit, into which the collimator tube passed. This box was about 4 inches long, with 
a hinged lid, and it was possible to place a train of three or four cells full of liquid 
within it, through which the rays were transmitted. 
• The cells themselves were cut from wide glass tube by slicing it longitudinally in 
two pieces and cutting oft’ lengths of three-quarters of an inch. These were fitted in 
metal frames, and pieces of quartz were affixed to each end by means of screws 
pressing upon a rim of metal. The situation of the liquid behind the slit, and therefore 
at a distance from the spark, obviated a difficulty caused by the ignition of the vapour 
of volatile liquids. The tube of the collimator, however, became filled with vapour, 
which of itself’ would exert a strong absorption in many cases, and in order to remove 
this vapour a nozzle was fixed at right angles to the tube at a point as close to the 
lens as possible, and this was placed in communication with a concertina aspirator, two 
or three strokes of which mil draw five or six litres of air through the tube. 
The lens behind the prism was capable of being raised and lowered by means of a 
sliding front as on an ordinary landscape camera. This enabled us, when occasion 
required, to take three photographs on a plate 2 inches in width. According as was 
found most convenient we have made use of electrodes of silver, cadmium, zinc, 
aluminium, nickel (containing as we find by its spectrum a trace of copper), indium, 
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