column and other phenomena connected with the discharge. 307 



accidental impurity or whether they may be considered a normal 

 feature of the discharge. I have frequently been able to produce 

 them in hydrogen, the gas in which De la Rue and Miiller ob- 

 served them, but, with one exception, have never seen them in 

 any other gas. This exception was a tube which contained moist 

 air and which was showing the fine striae described above ; on 

 greatly decreasing the current it presented the appearance shown 

 in the accompanying photograph. In this case they may be 

 easily explained by supposing that water vapour and air give rise 

 to separate sets which are far enough apart to be distinguished. 

 In every other case the discharge through moist air gave rise to 

 striations which were perfectly regular in their spacing. 



If proper precautions are taken the double striae appear to be 

 a normal form of the discharge in hydrogen. Generally they are 

 more easily obtained by a coil than by a battery if the interrupter 

 works regularly; the tuning-fork mentioned earlier gave good 

 results here. As the appearance in hydrogen differs considerably 

 from that shown by the figure a description of the former is given 

 which is more detailed than that given by De la Rue and Miiller 

 in the paper already referred to. 



Bordering on the Faraday dark space is a very clearly defined 

 striation which is generally blue in colour, especially just on the 

 convex part. Next to this and nearer the anode is another 

 slightly more convex than the former; it is usually faint red in 

 colour. These two together make up one double striation ; the 

 distance they are separated increases slightly as the pressure 

 diminishes and is greater in wide tubes than in narrow ones. 

 The distance between the two parts is considerably less than that 

 between the red part of one striation and the blue part of the one 

 immediately behind it. The double appearance frequently, but 

 by no means always, becomes less and less distinct as the anode 

 is approached, due to the component parts of each double striae 

 becoming further separated, until near the anode they are equally 

 spaced, alternate striae being bluish and the others pink. It is 

 not often that this appearance persists for more than a few 

 seconds with the battery, and with either coil or battery it can 

 only be obtained within very narrow limits of pressure. These 

 limits depend on the diameter of the tube, the wider the tube 

 the narrower the limits and the lower the pressure at which the 

 double striae first appear. Wire electrodes seem to give them 

 rather more certainly than disc ; in a tube whose diameter exceeds 

 15 mm. they are difficult to obtain at all, but this may be due to 

 the difficulty of producing striae in a perfectly steady state in 

 these wide tubes. 



If a battery of voltage sufficient to maintain the discharge 

 when once it is started but not high enough to start it be 

 vol. x. pt. v. 22 



