-i-4 



NA TURE 



[Septkmukr 5, 1895 



mechanical i;4Ui\.ilcnt •.•t the hc.it dcvcloiied when a mqlccule 

 of hydrogen combines with one of chlorine to form two mole- 

 cules of hydrochloric acid would be equal to 2|li; + 2|Cl'. 

 Thus we see that if the enet^- changes in (2) preponderated 

 largely over those in (I) and (3), the heat produced when an 

 element A combined with another element B to form the com- 

 pound AB, could be expressed as the sum of two numbers JA} 

 and jBI, where {AI depends solely on the element A, IBl solely 

 on the element B. In some cases of chemical combination 

 between dilute solutions there seems evidence that the heat pro- 

 duced can be expressed in this way (see Lothar Meyer, " The 

 Evolution of the Doctrine of Affinity,"' Phil Mag., vol. .\xiii. 

 p. 504), but when we attempt to apply the same law to com- 

 bination between gases, it seems utterly to break down ; 

 indicating that in such cases the greater part of the changes in 

 energy occur in the splitting up of one set of molecules and 

 the subse<iuent formation of others. This view seems to be 

 supported l>y the phenomena attending the discharge of elec- 

 tricity through rarefied gases, for the smallest diti'erence of 

 potential which can send a discharge through an electrified gas 

 (which we have reason to believe involves the splitting up of 

 molecules into atoms), is very many times the electromotive force 

 required to liberate the ions from an electrolyte, though the 

 latter progress requires changes in the electrical charges on the 

 ions. These reasons seem to indicate that we can hardly expect 

 to get any clear indication of the charges carried by the atoms 

 in gaseous compounds from the study of the thermal changes 

 which <x:cur when gases enter into chemical combination. 



Vapours 0/ Organic Compounds. — These show very interesting 

 differences between the spectra on the two sides of the plate 

 when the discharge passes through them. Thus when the dis- 

 charge first passes through the vapour of ethyl alcohol, C.jlI„(J, 

 the spectrum on the positive side of the plate is the candle 

 spectrum, that on the negative side the carlxinic oxide spectrum. 

 For some little time after the discharge commenced I could not 

 detect any hydrogen lines on either side of the plate ; after a 

 time, however, they appeared on the negative side but not on 

 the positive. If the discharge was kept running for some time 

 without letting a fresh supply of alcohol into the tube the 

 " candle spectrum " on the positive side of the plate was replaced 

 by the CO spectrum, which now occurred on both sides of the 

 plate accompanied on the negative side by the hydrogen 

 spectrum. This is the appearance presented by all the 

 comixjunds of carl)on, oxygen, and hydrogen which I examined, 

 when the spark had been passing through them for a considerable 

 time, and it is what would <Kcur if the va|Tour were dccomiwsed 

 by the spark into carlwnic acid, water, and hytirogen. 



The appearance of the candle spectrum on the positive side of 

 the plate with the CO on the negative was observed in many 

 other cases. Thus on sparking through a tube filled with CO, I 

 could not detect any difference l)etween the spectra on the two 

 sides of the plate, but when a little hydrogen was let into the 

 tube the " candle .spectrum '' apjx'ared on the positive side of 

 the plate, the carfxmic oxide spectrum on the negative. The 

 same effect was observed in a tube filled with cy.inogen mixed 

 with a little hydrogen. When the lube was filled with the 

 vapour of methyl alcohol, ClIjOII, the candle spectrum was on 

 the positive side of the plate, the carlxjnic oxide and hydrogen 

 spectra on the negative ; with this vapour, unlike that of ethyl 

 alcohol, I could not detect any stage when the hydrogen 

 spectrum was al>sent. 



The first explanation which occurs to one of this phenomenon is 



that it is owing to the |x>lcntial gradient at the negative side of 



the plate iK-ing slee|)cr than that on the positive, so that we 



may imagine we h.ive a fierce spark on the negative side, 



a mild one on the (xjsitixe, and that the fierce spark gives 



th>- CO spectrum, the mild one the candle S|x:clrum. There 



■T, some phenomena which seem inconsistent with 



,ition : ill the first place, if the current is reversed 



i; in one direction, traces of the former spectra 



• me time at the sides of the plates, and, secondly, 



•tcrence is due to the greater <lecom|Misition at 



the ncgaiive siilc of the plate, how is it that in the case of the 



vnji-tiir <'f ethyl alcohol the hydrogen spectrum is not seen, at 



icement of the flischarge, on the ne^.itive side of the 



ly ap|>cars after the di.schirge has fiasscd through for 



' ' f has priiKably lieen set free by the 



r by the *lischarge. If the absence 



: - - , .1 the negative side of the plate is due 



10 the apark bcmg .ho mtcnsc that the hydro-carbon which is 



NO. 1349, VOL. 52] 



supposed to be the origin of this spectrum cannot exist, then we 

 ought to see the spectra of the substances which result from the 

 decomposition of the hydrocarbon, i.e. we ought to see the 

 hydrogen sjjectrum at the negative electrode. The view which 

 seems most in accordance with the results of observations on the 

 discharge through these vapours is that the " candle spectrum" 

 is the spectrum of carbon when the atom is charged with 

 negative electricity, or of some compound of carlx)n in which its 

 atom is negatively chained, while the "carbonic oxide" 

 spectrum is the spectrum of carlwn when the atom is charged 

 with positive electricity, or of some compound in which the 

 carbon atom is positively charged. 



Visiliarge through an Eletncntary Gas. — It has long been 

 known that when the discharge passes through some elementary 

 gases, the spectra at the two electrodes are diflerent. This was 

 first shown to be the case for nitrogen, then Dr. Schuster showed 

 that the same thing occurred with oxygen, and recently Mr. 

 Crfmkes has show n that it is also true in the case of argon. I 

 have observed a very striking change in the relative brilliancy of 

 the red and green hydrogen lines at the two electrodes. When 

 the tube with the plate across it was filled with hydrogen at a 

 low pressure, then on the positive side of the plate the red line 

 tends to be brighter than the green, while on the negative side 

 the green line lends to be brighter than the red ; in some tubes 

 this was so marked that on the positive side of the plate the red 

 line was bright, and the green invisible, while on the negative 

 side of the plate the green line was bright, and the red invisible. 

 The spectroscope I was using weakened the red rays much more 

 than the green, so that I cannot be sure that the red rays were 

 really altogether obliterated on the negative side of the plate ; 

 the above experiment is, however, sufficient to show that on the 

 positive side of the plate the red rays are more easily excited 

 than the green, while on the negative side the green line is more 

 easily excited than the red. On the negative side of the plate 

 we have an excess of positively charged hydrogen atoms, while 

 on the positive side of the plate there is an excess of negatively 

 charged hydrogen atoms, and I am inclined to attribute the 

 difference in the spectra partly at any rate to the diflerence in 

 properties between a [wsitively and a negatively chargeil 

 hyilrogen atom. The reason I do not attribute it wholly to the 

 diflerence in the potential gradient on the two sides of the plate 

 is that the cfTect is not reversed immediately, but only gradually 

 on reversing the coil, the former spectra clinging for some time 

 to the sides of the plate. 



Chlorine. — I have made a great many experiments to see if 

 there is any difference between the spectra given by chlorine on 

 the two sides of the plate, but with negative results. Chlorine 

 seems a gas in which we might ex|>ect to find this effect, for as 

 Dr. Schuster, in his Report on Spectrum Analysis, says, the 

 behaviour of its spectrum indicates that we have several spectra 

 superposed. I have not, however, been able to affect a separation 

 of its s|>cctra, the differences I observed between the spectra on 

 the two sides of the plate were irregular, and due, I think, to 

 impurities producing effects like those observed when the dis- 

 charge passes through a compound gas. I lowever, as has been 

 mentioned before, there is even in the case of gases where 

 distinct evidence of separation can be obtained, a region of 

 pressure within which the efTects are irregular, and I ascribe my 

 failure to observe separation in the case of chlorine to my having 

 failed to get the relation between the intensity of the discharge 

 and the pressure so adjusted .is to get outside this irregular 

 region. The cases, however, in which distinct dilTerences 

 between the spectra of a single gas occur at the two electrodes, 

 seem to indicate that the spectrum given hy' an element is 

 influenced by the sign of the electrical charge carried by its 

 atoms. 



I have made some experiments to determine whether there 

 wa'i any separation produced in a mixture of equal volumes of 

 hydrogen ami chlorine kept in the dark, when a considerable 

 dillercnce of poleiiiiil though not sufficient to produce discharge 

 w.is maintaineil between the two electrodes. The parts t)l the 

 tube adjacent to the two electrixles could be shut ofl' from each 

 other by a tap, and thi- amount of chlorine in the two siiles was 

 determined by absorbing it by caustic [wt^sh. The mixture was 

 at atmospheric pressure, and the electrodes were maintained at 

 a potential difference of about 1200 volts by connecting them to 

 a large battery of small storage cells. The potential dilference 

 between the terminals was maintained for about sixteen hours on 

 three seimrate occ-Lsions, but on analysing the vessels surrounding 

 the two electrodes, the amount of chlorine in the vessel adjacent 



