58 Mr. W. Crookes on the Illumination of 



c. The shape and size of this dark space do not vary with the 

 distance separating the poles, nor (or only very slightly) with altera- 

 tion of battery-power, nor with intensity of spark. "When the 

 power is great the brilliancy of the unoccupied parts of the tube 

 overpowers the dark space, rendering it difficult of observation ; 

 but, on careful scrutiny, it may still be seen unchanged in size ; nor 

 does it alter even when, with a very faint spark, it is scarcely 

 visible. On still further reduction of the power it fades entirely 

 away, but without change of form. 



The author describes numerous experiments, devised to ascertain 

 if this visible layer of molecular disturbance is identical with the 

 invisible layer of molecular pressure or stress, the investigation of 

 which has occupied him for some years. 



The Electrical Radiometer. — One of these experiments is as fol- 

 lows : — An ordinary radiometer is made, with aluminium disks for 

 vanes, each disk coated with a film of mica. The fly is supported 

 by a hard steel cup instead of a glass cup ; aud the needle-point on 

 which it works is connected by means of a wire with a platinum 

 terminal sealed into the glass ; at the top of the radiometer-bulb 

 a second terminal is sealed in. The radiometer can therefore be 

 connected with an induction-coil, the movable fly beiug made the 

 negative pole. 



Passing over the phenomena observed at low exhaustions, the 

 author finds that, when connected with the coil, a halo of a velvety 

 violet light forms on the metallic side of the vanes, the mica side 

 remaining dark throughout these experiments. As the pressure 

 diminishes, a dark space is seen to separate the violet halo from the 

 metal. At a pressure of half a millim. this dark space extends to 

 the glass, and positive rotation commences. 



On continuing the exhaustion, the dark space further widens out 

 and appears to flatten itself against the glass, and the rotation be- 

 comes very rapid. 



"When aluminium cups are used for the vanes instead of disks 

 backed with mica, similar appearances are seen. The velvety violet 

 halo forms over each side of the cup. On increasing the exhaus- 

 tion the dark space widens out, retaining almost exactly the shape 

 of the cup. The bright margin of the dark space becomes concen- 

 trated at the concave side of the cup to a luminous focus, and widens 

 out at the convex side. On further exhaustion, the dark space on 

 the convex side touches the glass, when positive rotation commences, 

 becoming very rapid as the dark space further increases in size and 

 ultimately flattens against the glass. 



Convergence of Molecular Rays to a Focus. — The subject next in- 

 vestigated is the convergence of the lines of force to a focus, as 

 observed with the aluminium cup. As this could not be accom- 

 plished during rapid rotation, an instrument was made having the 

 cup-shaped negative pole fixed instead of movable. On exhaustion, 

 the convergence of the lines of force to a focus at the concave side 

 was well observed. "When the dark space is very much larger than 

 the cup, it forms an irregular ellipsoid, drawn in towards the focal 



