On the Illumination of Lines of Molecular Pressure. 57 



was now uncovered, and the other covered, and by repeated 

 rotation of 'N 1 the equality of intensity of o, and o u restored. 

 These two positions of Ni gave the relative intensity of the 

 illumination of o y in the first and in the second case. 



Now, as the equality of the kinetic energies for rectangular 

 apertures had, in a previous investigation*, been demonstrated 

 as actually existing, the present investigation needed only to 

 be directed to the proportionality of the illumination in o y 



There was found here, as after the above examination was 

 already highly probable, an exact agreement, within the limits 

 of errors of observation, between the theoretic conclusions and 

 the results of observation. 



On the ground of these investigations we are justified in 

 pronouncing it a proposition confirmed by experiment, that, 

 when the diffraction-angles are small, whatever the shape of the 

 aperture, the kinetic energy of the incident light is equal to the 

 kinetic energy of the diffracted light. 



Phys. Inst. Univ. Budapest, 

 June 15, 1878. 



VII. On the Illumination of Lines of Molecular Pressure, and 



the Trajectory of Molecules. By William Crookes, F.R.S., 



V.P.C.SA 

 TNDUCTION Spark through Rarefied Gases. — Bark 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 va- 

 rying 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 ex- 

 tends 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 0*078 milhm. this molecular disturbance ex- 

 tends 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. Eor equal degrees of exhaustion it is greatest in hy- 

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



* Wiedemann's Annalen, vol. iii. p. 568. 



t Abstract of a paper read before the Royal Society, Dec. 5, 1878. 



