550 



NATURE 



\Oct. 6, 1881 



course intended that every detail that is obiervable in the former 

 caies can be distinguished in the latter ; for the very nature of 

 the ffa-;, its viscosity or other properties, may prevent this. But 

 all the characteristic features which prevail at high pressures are 

 found also at 1 jw pressures, 0.1 a larsjer scale and in more marked 

 delineation. From this consideration, as well as from others to 

 ba njticei below, we are led to the conclusion that rarefied 

 gases form a promising field for future research into the nature 

 of the electrical discharge. 



Proceeding on this basis, I now desire to present to you the 

 actual discharge in two or three tube> from which the air has 



Fig. 7. 



been exhausted in various degrees. In the first, where the pres- 

 sure is that of about 3 or 4 mm. of mercury, or '004, say one 

 twenty-fifth part, of that of the atmosphere, the discharge takes 

 the form of a column of light, slender in breadth and flexible in 

 shape, extending throughout the entire length of the tube. The 

 colour of the discharge depends on the nature of the residual 

 gas. In the present case that gas is air ; and the reddish hue is 

 due to its constituent, nitrogen. 



In the next tube the exhaustion has been carried further, viz., 

 to a pressure of about 2 mm., or "0026, say one-fortieth part, of 

 an atmosphere. In this case the luminous column has become 

 thicker ; there are traces of a dark interruption towards the 

 negative end ; while beyond this break, and about the negative 

 teroiinal, the light is no longer red, but of a deep blue colour. 

 This strong contrast of colour at first sight appears inexplicable, 

 and as a matter of fact the difficulties of explanation have not 

 yet been altogether surmounted. But these difficulties are much 

 diminished by a spectroscopic examination of the phenomena, 

 from which it appears that, notwithstanding the contrast between 

 the light near the negative terminal and that in other parts of 

 the tube, the spectrum of the former differs from that of the 

 latter, generally, not in its fundamental character, but mainly in 

 the addition of certain strong lines in the blue and violet. 

 Besides this, there is occasionally a weakening of the lines in 

 the less refrangible part of the spectrum seen in the light of the 

 positive column. The extension of a spectrum in the direction 



of the more refrangible end is known generally to depend upon 

 an increase of temperature ; and as there are other grounds for 

 attributing a higher temperature to the region near the negative 

 terminal than to the other parts of the tube, it would seem that 

 we must look to thermal conditions for an explanation of the 

 contrast in question. 



But, besides the contrast just described, some tubes show a 

 diversity of Colour in the same striated column. Or perhaps, 

 more strictly speaking, there coexist two, or even three, columns 

 (usually pink and blue, with an occasional intervening green) 

 blended together near the positive, but more separated towards 

 the negative end. At the negative end they are in some cases 

 completely separated ; in oth.rs they are united so as to give 

 the appearance of parti-coloured strise. In every case, how- 

 ever, the blue striae are found nearer to the negative end than 

 the green or pink. 



In the next tube the pressure is about half a mm., or "00065, 

 say one 1 60th part of an atmosphere ; and here we find the 

 dark space near the negative terminal, observable in the previous 

 case, greatly increased. But besides this, the whole column is 

 no longer continuous, but is broken up into strife w ith dark 

 intervening spaces. 



As the exhaustion proceeds the stri« become more and more 

 separated, as well as individually thicker. At first mere flakes 

 of light, they gradually increase in thickness, until they assume 

 the jiroportions of blocks of light sometimes of larger dimensions 

 in the direction of the axis of the tube than in that of the 

 diameter. At the same time the main dark space between the 

 head of the column and the solitary luminosity about the negative 

 terminil, as well as the dimensions of that luminosity itself, 

 increase in length. A dark space immediately .surrounding the 

 negative terminal, and limited by the solitaij stria, also begins 

 to show itself, and to increase with the exhaustion. This space 

 has been named after Mr. Crookes, who first made a study of it. 

 As we proceed yet further, the column retreats towards the 

 positive terminal ; and at the last stage the solitaiy luminosity 

 shares the same fate. The Crookes' space occupies the whole 

 tube, and no gaseous illumination whatever remains. To the 

 phenomena which arise in this condition of things I will make 

 allusion at a later stage. 



This dependence of the distance between the stris upon the 

 pressure of the gas may be well illustrated by using a tube fitted 

 at one end with a chamber containing potash. The potash has 

 the property of absorbing gases of almost every kind, of giving 

 them out when it is heated, and of re-absorbing them when it 

 is allowed to cool again. This process may now be seen in 

 actual operation. 



The nuaiber and disposition of the striae will naturally depend 

 also on the length of the tube. The effective length may be 

 altered without altering any other conditions of the experiment, 

 by having one terminal attached to the wire leadnig into the tube 

 by a flexible spiral wire ; so that the terminal itself may be 

 shifted. At first sight it might have been supposed that any 

 change due to an alteration in the length would have depended 

 very much upon whether the shifting terminal is the positive or 

 the negative. But whichever be the case, the striae are seen to 

 drop one after another into the positive terminal ; the solitai^ 

 stria and the adjacent dark space remain unaltered, and no 

 change is apparent beyond a reduction in the number of the 

 str-i£e. 



This is, however, not the only way in which the disposition of 

 the strise may be made to vary. In some gases at suitable pres- 

 sures an increase in the strength of the current used, or in the 

 quantity of electricity discharged through the tube, reduces the 

 number of the strise, and to some extent shortens the column by 

 draw ing or driving the stria: one by one into the positive terminal. 

 In such a ca=e it alo increases their mutual distances in the same 

 manner as if they were threaded on an elastic string. In other 

 gases the reverse is the case. Thus, in this sulphide of hydrogen 

 tube, the strise in the column are numeruus and crowded while 

 the machine is in rapid motion. As the speed is diminished the 

 stria: recede from one another, until only a few lingering speci- 

 mens are left, separated by the broad dark and mysterious spaces 

 wh'ch you now see. 



The long continuance of the discharges from the induction 

 coil afford an opportunity of examining the various phases of 

 strise during their existence. The details of the instrumental 

 arrangements, as well as other particulars of the ob.-ervations, 

 have been elsew^here described ; but the main features observed 

 may be apprehended by the illustrations subjoined. 



