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V. The Bakerian Lecture. —On the Illumination of Lines of Molecular Pressure, 
and the Trajectory of Molecules. 
By William Crookes, F.R.S., V.P.C.S. 
Received December 5,—Read December 5, 1878. 
[Plate 14.] 
Contents. 
Par. 
The induction current through rarefied gases 486 
Dark space round the negative pole. 486 
Illumination of lines of molecular pressure . . 498 
Electrical radiometers. 502 
Convergence of molecular rays to a focus. . .. 508 
Green phosphorescent light of molecular 
impact . 510 
Focus of molecular energy. 514 
Nature of the phosphorescent light . 519 
Par. 
Projection of molecular shadows . 524 
Phosphorescence of thin films . 533 
Mechanical action of projected molecules .... 541 
Magnetic deflection of lines of molecular force 545 
The trajectory of molecules. 551 
Alteration of molecular velocity. 555 
Laws of magnetic deflection . 557 
Focus of molecular heat of impact. 581 
An ultra-gaseous state of matter . 585 
THE INDUCTION CURRENT THROUGH RAREFIED GASES. DARK SPACE 
ROUND THE NEGATIVE POLE. 
486. When the spark from a good induction coil traverses a glass tube containing a 
rarefied gas, certain phenomena are observed which vary greatly with the kind of gas 
and the degree of exhaustion. There is one appearance, however, which is constant in 
all the gases which I have examined, and within very wide limits of pressure, viz.: the 
well-known dark space round the negative pole. I have long been impressed with the 
idea that this dark space coating the pole was in some way related to the layer of 
molecular pressure causing movement in the radiometer, and the following experiments 
were instituted with the object of testing this hypothesis. 
A glass bulb (fig. 1) was furnished with platinum wire terminals sealed into the glass, 
