1880.] On the Sensitive State of Vacuum Discharges. 303 



trie spark. The methods of the earlier paper have been extended, and 

 others adapted to the new cirenmstances have been devised, in 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 con- 

 taining 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 de- 

 scribed by Mr. Crookes, present themselves. These derive great im- 

 portance for the purposes of the present paper from the fact that in 

 high vacua the ordinary luminous discharge becomes so feeble in 

 appearance that it is often difficult to observe. Under these circum- 

 stances 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. 



The conditions of pressure and of electrical violence, under which 

 phosphorescence is produced, have been carefully studied ; and it has 

 been found that, with a suitable adjustment of the discharge, the 

 phenomena are not confined to high vacua, but can be obtained under 

 pressures much exceeding those of ordinary vacuum tubes. The phe- 

 nomena of these molecular streams have also been compared with 

 those exhibited by the projection of finely divided solid conducting 

 matter when heaped up over the negative terminal, with the view of 

 ascertaining the nature of the phenomenon and its position hi the 

 discharge. 



At the close of the paper the authors have discussed some of the 

 general conclusions which they think may be fairly drawn from their 

 present researches. First, as to the relative order of magnitude of 

 the time- quantities entering into the discharge ; e.g., the times occupied 

 by the discharge of positive or negative electricity, or of molecular 

 streams, in leaving a terminal ; the time occupied by the same ele- 

 ments in passing along the tube, &c. Secondly, as to the durational 

 character of the negative as compared with the positive discharge, 

 which appears to increase with the degree of exhaustion. Thirdly, as 



