apt 
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287 
iron filings moistened with water. On this occasion chloride of 
calcium was used to dry the vacuum, but did not act as well as 
the sulphuric acid, for the gauge could not be got below 0'-25. 
The light was faint till the Leyden jar was connected, when 
it appeared as a violet spark, surrounded by a yellow atmo- 
sphere. The envelope of the negative ball was yellowish, 
not blue, and no bands were noticed; but the fluorescence 
was strong. As both watery vapour and hydrogen were pro- 
bably present, I repeated this experiment, obtaining the gas 
by a process for which I am indebted to Dr. Lyon Playfair, 
decomposing water of ammonia by bleaching powder. The 
ammonia must be diluted, the Wolfe’s bottle in which the 
decomposition takes place kept cool, and the gas well washed. 
The extrication of it, though very rapid, is manageable, 
and it seems to be quite pure. From the nature of the 
manipulation in filling the receiver, I am sure that it could 
not have contained more than ;4, of common air. The dis- 
charge began to pass at 0:30, a brilliant reddish violet; the 
light on the positive ball more pink than in air; that on the 
negative one, indigo; the bands obscure, but the fluores- 
cence intense. On continuing the exhaustion to 0-04, the 
colour of the stream became a tawny brown, and much 
fainter, the menisci became distinct, and the fluorescence con- 
tinued strong. 
‘7, Carbonic oxide was procured by heating crystals of 
oxalic acid with eight times their weight of sulphuric acid. 
As some of the carbonic acid might pass the potassa solution 
from the rapid way in which the extrication takes place, a 
vessel containing that solution was placed in the sulphuric 
acid vessel to complete the absorption. At 0*15 the stream is 
beautiful; bright green, yellowish at the positive end, bluish 
at the other. The latter has still the two envelopes separated 
by a dark interval, and green combustion sparks on it. The 
stream is not broad, but shows the menisci in all its length 
very plainly. At 0'-12 the negative envelopes became blue 
