Discharge in Rarefied Gases. 381 



the heads of the stratifications, i. e. of the points from which 

 the separate discharges formed by the stratifications issue, de- 

 pend altogether upon the position and peculiarities of the 

 kathode, and not at all upon the conditions of the anode*. 



Let the discharges pass in a cylinder with terminal elec- 

 trodes which can be moved along the axis of the vessel towards 

 each other bj means of an arrangement which need not at pre- 

 sent be further described. 



If the anode in such a vessel is caused to approach the fixed 

 kathode, no displacement whatever of the stratifications in 

 front of the anode is observed ; they remain altogether im- 

 movable and unchanged, so far at least as their continued 

 existence is consistent with the new position of the anode. 

 Since the positive light in every case reaches only to the anode, 

 the layers which were situated in the portion of the tube 



* The experiments described in what follows are performed with stra- 

 tified discharges in pure dry air or in dry highly exhausted hydrogen, 

 produced from a sufficiently powerful induction-current by regular inter- 

 ruption of the primary current. Under these conditions we obtain thick 

 stratifications which do not vibrate to and fro like the so-called " saucer "' 

 stratifications, but, with a constant pressure of gas, preserve their position 

 unaltered. I think it necessary to mention this, because to many who are 

 still accustomed, upon mention of stratifications, to figure to themselves 

 only the " saucer " stratifications and their behaviour, the mention in the 

 following of motionless stratifications with constant intervals between 

 them may appear surprising. The thick stratifications, strange to say, 

 are always treated, sometimes as quasi-pathological developments, as phe- 

 nomena which are disturbances of the normal phenomena of stratification ; 

 sometimes as optical illusions, caused by rapid vibration to and fro of the 

 " saucer " stratifications assumed to exist alone. According to the evi- 

 dence I have given, the meaning and mutual relation of the different 

 forms of stratification are tolerably clear ; each separate layer in a cylinder 

 is qualitatively analogous to the discharge at a kathode which occupies 

 the whole section of the cylinder. "When the exhaustion is small this 

 kathode-light is only a thin layer, corresponding exactly to the thin 

 "saucer" stratification. If the exhaustion proceeds, the electric rays 

 which make up either structure lengthen and so increase its thickness ; 

 and just as the kathode-rays finally lengthen so much as to completely 

 fill the dark space and reach to the first positive layer, so the rays of stra- 

 tifications extend so much as to completely occupy the dark spaces between 

 them. As the density of the gas decreases the thickening proceeds further, 

 the intervals between the heads of the layers continually increasing, and 

 the rays issuing from the heads continually occupying the increasing 

 intervals. When the stratifications appear most plainly in a cylinder with 

 dry air, their thickness is very nearly equal to the diameter of the cylinder, 

 so that with wide vessels they are of considerable thickness. As little as 

 a kathode-discharge with extended rays is a monstrosity or disturbing 

 phenomenon, or consists entirely of a luminous layer vibrating to and fro, 

 so little can we entertain similar views about the thick layers ; and as the 

 laws of the kathode-light may be studied most easily in the longest pen- 

 cils of it which can be obtained, so also it is just the thickest layers which 

 represent the phenomenon most completely developed, which are in the 

 first instance "best suited for the study of the stratification. 



