1878.] the Illumination of Lines of Molecular Pressure. 103 



I. " On the Illumination of Lines of Molecular Pressure, and 

 the Trajectory of Molecules." By William Crookes, 

 F.R.S., V.P.C.S. Received November 30, 1878. 



(Abstract.) 



Induction Spark through Rarefied Gases. Dark Space round the 

 Negative Pole. 



The author has examined the dark space which appears round 

 the negative pole of an ordinary vacuum tube when the spark from an 

 induction coil is passed through it. He describes many experiments 

 with different kinds of poles, a varying intensity of spark, and different 

 gases, and arrives at the following propositions. 



Illumination of Lines of Molecular Pressure. 



a. Setting up an intense molecular vibration in a disk of metal by 

 electrical means excites a molecular disturbance which affects the 

 surface of the disk and the surrounding gas. With a dense gas the 

 disturbance extends a short distance only from the metal ; but as 

 rarefaction continues the layer of molecular disturbance increases in 

 thickness. In air at a pressure of "078 millim. this molecular dis- 

 turbance extends for at least 8 millims. from the surface of the disk, 

 forming an oblate spheroid around it. 



b. The diameter of this dark space varies with the exhaustion ; 

 with the kind of gas in which it is produced ; with the temperature 

 of the negative pole ; and, in a slight degree, with the intensity 

 of the spark. For equal degrees of exhaustion it is greatest in 

 hydrogen and least in carbonic acid, as compared with air. 



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

 distance separating the poles ; nor, only very slightly, with altera- 

 tion of battery power, or 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 follows : — An ordinary radiometer 

 is made, with aluminium disks for vanes, each disk coated with a 



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