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XXVII. The Bakerian Lecture.— On Radiant Matter Spectroscopy : 
The Detection and wide Distribution of Yttrium. 
By William Crookes, F.R.S. 
Received May 24,—Read May 31, 1883. 
Introduction. 
1. In March, 1881, I sent to the Royal Society a preliminary notice of some results 
I had obtained when working on the molecular discharge in high vacua.'" When the 
spark from a good induction coil traverses a tube having a flat aluminium pole at each 
end, the appearance changes according to the degree of exhaustion. Supposing 
atmospheric air to be the gas under exhaustion, at a pressure of about 7 millims. a 
narrow black space is seen to separate the luminous glow and the aluminium pole 
connected with the negative pole of the induction coil. As the exhaustion proceeds 
this dark space increases in thickness, until, at a pressure of about 0'02 millim. 
(between 20 and 30 M.)t, the dark space has swollen out till it nearly fills the tube. 
The luminous cloud showing the presence of residual gas has almost disappeared, and 
the molecular discharge from the negative pole begins to excite phosphorescence on 
the glass where it strikes the side. There is great difference in the degree of 
exhaustion at which various substances begin to phosphoresce. Some refuse to glow 
until the exhaustion is so great that the vacuum is nearly non-conducting, whilst 
others begin to become luminous when the gauge is 5 or 10 millims. below' the 
barometric level. The majority of bodies, however, do not phosphoresce till they are 
well within the negative dark space. This phosphorogenic phenomenon is at its 
maximum at about 1 M., and, unless otherwise stated, the experiments now about to 
be described were all tried at this high degree of exhaustion. 
Under the influence of this discharge, which I have ventured to call radiant matter, 
a large number of substances emit phosphorescent light, some faintly and others with 
great intensity. On examining the emitted light in the spectroscope most bodies give 
a faint continuous spectrum, with a more or less decided concentration in one part of 
the spectrum, the superficial colour of the phosphorescing substance being governed 
by this preponderating emission in one or other part of the spectrum. 
Sometimes, but more rarely, the spectrum of the phosphorescent light is discon¬ 
tinuous, and it is to bodies manifesting this phenomenon that my attention has been 
* Proceedings of the Royal Society, No. 213, 1881. 
| M. = one-millionth of an atmosphere. 
