MR, W. CROOKES ON RADIANT MATTER SPECTROSCOPY. 
G93 
through the glass, was sealed to the pump and quickly exhausted. An induction 
current was immediately passed through, and.the light examined in the spectroscope. 
No mercury lines were detected. The tube was then heated strongly, the spectro¬ 
scopic examination being continued : no mercury lines were seen. The apparatus was 
then allowed to rest for a clay, and the experiments were repeated. In the cold no 
mercury lines could be seen, but on heating the tube they became distinctly visible. 
93. The tube was then exhausted to the highest possible point, and sealed off. In 
the cold it was impossible to get an induction-spark through the tube—it was 
absolutely non-conducting. However, on heating the tube with a Bunsen flame, 
keeping the coil going all the time, suddenly the current passed, lighting up the 
inside of the tube with a greenish-blue light, in which the spectroscope showed strong 
mercury lines. On allowing the tube to cool it again became non-conducting, and the 
experiment could be repeated. 
This shows that in a vacuum-tube which is so highly exhausted as to be non¬ 
conducting there is plenty of mercury present to carry au induction current, only it is 
not in the form of vapour, but is condensed on the metallic poles or on the sides of 
the glass. 
94. It is much more difficult than is generally supposed to keep mercury vapour 
from diffusing itself into the experimental tubes. Gold-leaf was packed loosely in a 
tube about 2 feet long, which was then interposed between the pump and the radiant 
matter tube. This answered for a short time, but whilst it certainly kept back some 
of the mercury, it let much through. 
For a long time I used a tube about 2 feet long, packed with coarsely powdered 
sulphur. This is effectual at first, but there is a liability of sulphur vapour getting 
into the tube, and this might be as disastrous as mercury. The sulphur was therefore 
kept out by interposing another tube containing finely divided copper. This, however, 
was not quite effectual: after a time mercury was found to have run the gauntlet of 
both sulphur and copper. It is of no use trusting to a solid reagent to absorb a 
gaseous body. Where the gas and solid meet there may be action, but some of the 
gaseous molecules are sure to get through without coming within the solid’s sphere of 
attraction. 
95. I finally adopted the following plan, which answers perfectly so far as my 
experiments have yet gone :—Sulphur is first prepared by keeping it fused at a high 
temperature till bubbles cease to come off, so as to get rid of water and hydrogen 
compounds. It is then allowed to cool, and is then pounded and sifted so as to get it 
in the form of granules averaging a millimetre in diameter. A glass tube, a 
centimetre in diameter and about 2 feet long, is lightly packed for half its length 
with this sulphur, and next about 2 inches of iodide of sulphur (LSo) is added, and 
the rest of the tube is then filled up with sulphur. Ignited asbestos is packed in at 
each end to keep the sulphur from blowing out whilst the vacuum is being made, or 
from being sucked through when air is suddenly let in. This contrivance entirely 
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