i'O 



NA TURE 



[January 3 1, 



1S95 



spec'.' ' ' " .graphed, since the dispersion will also be 



less - n.H yd l>een filled for ihe Bnice telescope, 



sinci. _ experienced in making charts in which the 



imager shall tje circular. Experiments are in progress in this 

 direction, .and it is hoped that the method described above as 

 applied to the ij-inch Boyden telescope will prove equally suc- 

 cessful with this instrument. 



A variety of exi>eriments have been made to deteniiine the 

 photogr.iphic magnitudes of the brighter stars on a uniform scale. 

 It is now expected that this can l>e done with the transit 

 photometer for stars brighter than the third m.agnitude, and that 

 the scale can be extended to stars from the third to the sixth 

 magnitude by a scries of photographs which are being taken with 

 a portrait lens having an aperture of 25 inches. The images 

 a'e thrown out of focus, and the inlensiiy of the circular discs 

 thus ohtaine<l can be accurately measured. 



ELECTRIC DISCHARGE THROUGH GASES} 



(^NE of the most important and interesting branches o*^ 

 physical science is that which deals with the connection 

 between electrical and chemical effects.. 



The investigations on electrolysis made within these walls 

 by D.ivy and Faraday proved that the imporlant class of elec- 

 trical phenomena associated with the passage of electricity 

 through liquids, are connected in the closest way with chemical 

 action. They proved that no electricity will pass through most 

 liquids unless chemical action occurs, and that for each unit of 

 electricity which passes through the liquid there is a definite 

 amount of chemical decomposition. 



This case, though it is one where the laws are most accu- 

 rately known, is but one among many electrical phenomena 

 which are inseparable from chemical action. 



So many instances of this kind have been discovered, that 

 we may perhaps venture to hope that we are not (ar from the 

 time when it will be universally recognised that many of the 

 most fundamental questions in chemistry and electricity are but 

 different aspects of one and the same phenomenon. 



.\nyihing which throws light on the connection between 

 electricity and matter, interesting as it is on its own account, 

 acquires a'Iditional interest when regarded as elucidating the 

 connection between chemical and electrical effects, and no phe- 

 nomena seem more suitable for this purpose than those which 

 are the subject of the discourse this evening — the discharge of 

 electricity through gases. For in gases we have matter in the 

 state in which its properties have been most carefully studied, 

 while the investigation of the electrical effects is facilitated by 

 the visibility of the discharge, affording us ocular, and not 

 merely circumstantial, evidence o( what is taking place. 



The points to which I wish to refer particularly this evening 

 are, firstly, some phenomena connected with the passage of 

 electricity from the gas to the electrode, or fr.>m the electrode 

 to the gas ; and secondly, some of the properties of the dis- 

 charge when its course lies entirely in the gas. 



By taking a long discharge tube, say, one fifty feet long, and 

 observing the luminous discharge through a rotating mirror, we 

 can trace the course of the luminosity due to a single discharge, 

 »ay, one due to once breaking the primary circuit of an induc- 

 tion c mI ; if we do so, we find that the luminosity follows the 

 direct iofi of the pisitive current through the tube. That is, the 

 luminosity begins at the positive electrode, it then rushes down 

 the tutie with enormous velocity, but when it gets to the nega- 

 tive electrode, it receives a check ; it docs not disappear at once 

 in thAt electrode like a rabbit going down a hole, but lingers 

 around the electrode some time belore entering it. In conse- 

 quence of this delay in the positive discharge in getting out of 

 the ga-. there it an accumulation of positive electricity in the 

 neighi.-jurh..od of the negative electrode until the potential fall 

 at this electrode increases to about 200 or 300 volts. 



The potiiive electricity which accompanies the discharge thus 

 find* considerable difficulty in getting from the gas to the metal, 

 (hough, as I hope to show you later on, a? long as it keeps in 

 the gas, it meets with what we may, in consideration of the 

 views sometimes enunciated on this subject, call a ridiculously 

 small amount of resistance, its real difficulty is to get out of the 



gas. 



Though this effect has long been known, it is so important 

 ire delivered :il ihe Roynl Inililulion by Prof. J. J. Thomson, 

 NO. 1318. VOL. 51] 



that I will venture to show one or two experiments which 

 illustrate it. The arrangement of the first experiment is shown 

 in Fig. I. The apparatus consists of a main discharge tube, 

 across which is fastened a diaphr.agni made of excessively thin 

 platinum leaf; there is a side passage from the tube, leading 

 from one side of the diaphragm to the other, this is connected 

 to a barometer tube, and by raising the cistern containing the 



^g 



mercury I can stop up the pass.ige by a pellet of mercury. We 

 will first observe the ilischarge when the side passage is open ; 

 you see that the discharge, instead of passini; acoss the thin 

 piece of platinum leal, takes the very much longer route round 

 the side tube, so as to avoid crossing the metal. We will now 

 raise the mercury cistern, and close the side lu'ie by a pellet o( 

 mercury ; the discharge now has no alternative but to cross the 



metal at some part of its course, and you see that the main 

 portion of the discharge goes back into the main tube. 



In the seond experiment the metal diaphrag'U is replaced by 

 a very thin plate of mica ; when the side passage is opened the 

 discharge goes lound, but when this is closed by a pellet of 

 mercury Ihe discharge prefers to go across the mica than through 

 the mercury. 



