232 Mr Ghree, On some experiments on [June 1, 



plate of aluminium. The gas originally in the tubes was always 

 air, and a mercury pump was used. 



In all three cases the following phenomena were observed 

 in the Hg. tube — i.e. the branch containing a mercury electrode. 

 At fairly high pressures when the spark first passed freely the 

 positive discharge was bright red, as is usual in air. As the 

 pressure diminished the colour became whiter, and there ap- 

 peared numerous striae, a distinct Faraday space, and a whiteish 

 blue negative glow. Before the striae became conspicuous the 

 discharge left an Hg. anode from its extreme summit, but as 

 the exhaustion proceeded it left from an increasing area, till 

 finally the whole surface became luminous. 



As the pressure was diminished there appeared what seemed 

 to be a Crookes' space over an Hg. cathode. It was not so clearly 

 defined as that space usually is, but the surface separating it from 

 the negative glow was at first tolerably distinct. This surface 

 was convex like the Hg. surface itself and had a very similar 

 curvature. This Crookes' space was, however, by no means very 

 dark, being in general of a distinctly red aspect, and on occasions 

 almost fiery in appearance. The introduction of a minute air 

 bubble into the tube made the boundary of this space appear 

 much more distinct. As the pressure was further reduced the 

 Crookes' space increased in length and the top of the Faraday 

 space moved up the tube. This went on until the Faraday 

 space extended to 80 or 90 mm. from an Hg. cathode. At this 

 stage the stratification in the tube containing an Hg. anode 

 was fairly distinct, the length of a stria being about 1 cm. and 

 its luminous portion being nearly pure white. After this stage 

 there appeared a change in the character of the phenomena most 

 conveniently dealt with in the separate discussion of the different 

 sets of electrodes. 



Electrodes both Hg. surfaces. 



When the stage just referred to was reached in this case, 

 the variety in the appearances at different parts of the discharge 

 tended to disappear, the whole assuming a more or less uniform 

 white colour. The Faraday space seemed on some occasions 

 certainly absent, and the Crookes' space became, to say the 

 least of it, exceedingly indistinct. The difficulty of coming to 

 a decision as to the existence of these spaces was much in- 

 creased by the deposition of matter on the walls of the tube 

 containing the cathode. The nature of this deposit was the 

 same as that described in the next case. All I can say with 

 certainty is, that the red-black space — assumed to represent 

 Crookes' space — did not continue to expand continually as the 



