432 Mr. Sjpottiswoode [May 21, 



althougli apparently no part of the discliarge proper, as Mr. Crookes' 

 experiments with a magnet, and others, seem clearly to show, are in- 

 variable accompaniments of it ; and by means of them we may hope 

 to learn something of the circumstances of the discharge in higher 

 vacua. Now, although these molecular streams become more pro- 

 minent as the exhaustion proceeds, and as the positive column 

 sinks into insignificance, it is important to show that, in one form or 

 another, they may be present in discharges at all pressures ; and for 

 this purpose it is necessary only to increase the violence of the dis- 

 charge, by increasing the length of the air- spark employed. The ejBfect 

 is at once shown by the appearance of phosphorescence in the neigh- 

 bourhood of the negative terminal, and by the relief discharge from 

 the finger with a positive air-spark. But not only so, the phenomena 

 of material streams issuing from the negative terminal are not confined 

 to the molecules of gas, but are also exhibited by particles of finely- 

 divided solid matter, such as lamp-black, when heaped over that 

 terminal. With these, relief and special effects, analogous to those 

 found in gaseous streams, may be shown. 



Having thus launched ourselves into the region of high vacua, it 

 is necessary to show that, notwithstanding the absence of the positive 

 column, positive and negative air-sparks give rise to positive and 

 negative charges on the tube, exactly as in lower vacua. 



For this purjDose, it will be best to make use of a second tube 

 carrying a current, as a test, or, as we have called it, a standard tube. 

 By connecting the outside of the tube to be tried with that of the 

 standard tube, and by observing the ejBfect of the one upon the other, 

 we can immediately determine the nature of the discharge passing 

 through the tube under examination. It is then found that, with a 

 positive air-spark a positive discharge, and with a negative air-spark 

 a negative discharge, passes from one end of the tube to the other, in 

 high exactly as in low vacua. 



In order to be quite clear as to the source of the molecular 

 streams which cause the phosphorescence in the relief effects with a 

 positive air-spark, the following should be mentioned : — If any solid 

 object, such as a piece of wire, should be present in the tube below 

 the point of contact, it will cast a shadow on the phosphorescence, 

 precisely as in Crookes' experiments with the streams from the nega- 

 tive terminal. If there be two points of relief contact, the same object 

 will throw two shadows, in directions conformable with radiations 

 from each. To these, other experiments might be added. 



A determination of the precise directions in which these molecular 

 streams issue from a relieving surface is not a very simple problem ; 

 and we must here content ourselves with showing that, in the case of 

 intermittent discharges at least, the streams do not issue normally. If 

 a strip of tinfoil placed along the tube be used as a relieving surface, 

 the phosphorescence takes the form of a sheet wrajjped round the 

 tube ; if the strip be wrapped round the tube, the phosphorescence 

 takes the form of a sheet laid along the tube. If contact be made 



