II. On the sensitive state of vacuum discharges. Part II

This paper forms a sequel to that published under the same title in the “Phil. Trans.,” 1879, p. 165. It describes a continuation of the research into the nature and laws of the disruptive discharge, or electrie spark. The methods of the earlier paper have been extended, and others adapted to the new circumstances have been devised m order to carry the investigation into high vacua. In particular, independent sources of electricity have been used for affecting the discharge, whether in the sensitive or in the non-sensitive state; and the results have been confirmatory of the conclusions derived from the more limited means formerly described. Further, the effects of various tubes containing discharges in the sensitive state upon a tube containing a discharge in the non-sensitive state have been observed and compared; and the tube so used as a test has been called the standard tube, and the method of its use the standard tube method. By this means, principally, the laws of the discharge in comparatively moderate vacua have been extended to high vacua. In the higher vacua, the phenomena of molecular streams, and the phosphorescence consequent on them, that have been studied and described by Mr. Crookes, present themselves. These derive great importance for the purposes of the present paper from the fact that in high vacua the ordinary luminous discharge becomes so feeble m appearance that it is often difficult to observe. Under these circumstances the phosphorescence, which like the ordinary luminous effects may exist either in a sensitive or in a non-sensitive state, forms the best index of what is going on within the tube. Much information as to the nature and procedure of the discharge may be derived from the mode of interference of one molecular stream with another, from the direction and character of shadows cast by these streams, and by a form of interference which has here been called that of virtual shadows.