Septejmber 5, 1895] 



NA TURE 



453 



separation had occurred, and that the substance giving the A 

 lines was in excess on the positive side of the plate, that giving 

 the B lines on the negative. It is not safe to draw any conclu- 

 sions from the variations in intensity of one line or one group of 

 lines on the two sides of the plate, as the total quantity of light 

 coming from the neighbourhood of the cathode f>ften differs con- 

 siderably from that coming from the anode. When, however, 

 we get an increase in the brilliancy of one set of lines accoin- 

 panied by a diminution in the brightness of another set, when 

 we move across the plate we eliminate this source of error. The 

 differences in the spectra at the two sidesof the plate are most easily 

 observed at pressures where there is not any very great difference 

 between the luminosity of the cathode and the anode. As 

 was mentioned at the beginning of the paper, there is a range of 

 ]>ressure wiihin which the effects are irregular, and no decided 

 differences are observed between the spectra at the two sides of 

 the plate. It is desirable in these experiments to keep the tube 

 on to the pipe as long as the experiment lasts, for the discharge 

 always decomposes the compound gas, and unless the products 

 of decomposition are continually pumped off and replaced by 

 fresh supplies of the compound gas, the spectra of the discharge 

 keep changing. With organic compounds this is especially 

 necessary, as the character of the spectrum often changes en- 

 tirely very shortly after the commencement of the discharge 

 unless fresh gas is continually introduced. 



In (he following experiments the current was produced by a 

 large induction coil with a mercury slow break. 



When the tube was filled with hydrochloric acid gas at a low 

 pressure, the sej^aration of the hydrogen and chlorine was seen 

 very distinctly, the hydrogen line being much brighter on the 

 side of the plate which acted as the cathode (which we shall 

 call the negative side of the plate) than on the positive side, 

 while the chlorine, on the other hand, w'as brighter on the 

 [Kjsitive than on the negative side of the plate. 



When the tube was filled with ammonia gas, the hydrogen 

 lines were bright on the negative side of the plate, but were 

 aliseiit from the positive side, while on the positive side of the 

 plate there was the positive pole sjjectrum of nitrogen, and on 

 the negative side of the plate the negative pole spectrum of 

 nitrogen and the hydrogen spectrum. 



Sulphur Moiwdiloridc. — When the tube was filled with the 

 vapour of this substance at a low pressure, the chlorine lines 

 were brighter on the negalive side of the plate than at the 

 positive, while the sulphur lines were brighter at the positive 

 side than at the negative. Thus the chlorine in this substance 

 behaves in the opposite way to the chlorine in HCl ; in the 

 latter compound the chlorine iron has a charge of negative 

 electricity, while in the .sulphur monochloride it has a charge of 

 positive electricity. 



Influence of the Chemical Constitution of a Compound on the 

 Sign of the Charge of Electricity on one of its Constituent 

 Atoms. — In many organic compounds an atom ot the electro- 

 positive element hydrogen can be replaced by an atom of the 

 electro-negative element chlorine without altering the type of 

 the compound. Thus, for example, we can replace the four 

 hydrogen atoms in CHj by chlorine atoms, getting successively 

 the compound C1I|C1, CIL.Cl;, CIICl.,, and CCI4. It seemed 

 iif interest to investigate what was the sign of the change of elec- 

 tricity on the chlorine atom in these compounds. The point is 

 iif some historical interest, as the po.ssibility of substituting an 

 electro-negative element in a compound for an electro-positive 

 one was one of the chief objections assigned against the electro- 

 chemical theory of Berzelius. 



When the vapour of chloroform, CHCl,, was placed in the 

 tube, it was found that both the hydrogen and the chlorine lines 

 were bright on the negative side of ihe plate, while they were 

 absent from the positive side, and that any increase in the bright- 

 ness of the hydrogen lines was accompanied by an increase in 

 the brightness of those due to chlorine. The spectrum on the 

 |iositive side of the plate was that called the carbonic oxide spec- 

 irum; when first the discharge passed through the tube, the 

 spectrum on the positive side was the so-called candle spectrum, 

 l)Ut this very rapidly changed to the carbonic oxide spectrum. 

 The ajipearance of the hydrogen and chlorine spectra at the 

 same side of the plate was also observed in methylene chloride 

 and in ethylene chloride. Even when all the hydrogen in 

 CII4 was replaced by chlorine, as in carbon tetrachloride, CClj, 

 the chlorine spectra still clung to the negative side of the plate. 

 To test the point still further, I tried the analogous compound 

 silicon tetrachloride, inserting a small jar in the circuit to brighten 



the spectrum. The chlorine sped rum was again brightest at the 

 negative side of the plate, while the silicon spectrum was 

 brightest at the positive. This is a very favourable case for the 

 ap])lication of this method, as there are two silicon lines (wave- 

 lengths 5058, 5043) quite close to two chlorine ones (wave- 

 lengths 5102, 5078), so that their relative brightness can easily 

 be compared. The experiment with the silicon tetrachloride 

 is more conclusive than those with the carbon compounds, as 

 with the latter the spectrum on the positive side of the plate is a 

 band spectrum, and since the potential gradient when the dis- 

 charge is ])assing is very much steeper on the negative side of 

 the plate than on the ]jositive, the effects observed might be sup- 

 posed to be due to the circumstances on the negative side being 

 better adapted for the production of line spectra than those on 

 the positive. This explanation is not, however, applicable to the 

 case of silicon tetrachloride, where the spectra on both sides 

 of the plate are line spectra. 



From these experiments it would appear that the chlorine 

 atoms in the chlorine derivatives of methane are charged with 

 electricity of the same sign as the hydrogen atoms they displace. 



When we can determine the signs of the electrical charges 

 carried by the atoms in a molecule of a compound, we can 

 ascertain whether any given chemical reaction does or does not 

 imply interchange between the electric charges on the atoms 

 taking part in the reaction. Thus take the reaction 



CHj + CU = CH3CI + IICl. 

 If we represent the sign of the charge of electricity carried by an 

 atom by -f or - placed below the symbol representing that 

 atom, we may write the last reaction as 



CH4 -I- ClCl - CH3CI -1- HCl, 



--I- + - -+ + + - 

 so that this reaction could be produced by a rearrangement of 

 the atoms without any alterations of their electrical charges. 

 If, however, we take the reaction — 



HH + ClCl = 2HCI, 



+- +- + - 



we see that in addition to a rearrangement of the atoms there 

 must in this case be an interchange of electric charges between 

 the atoms ; for before combination half the hydrogen atoms had 

 a negative charge, and half the chlorine atoms a positive one, 

 whereas after combination no hydrogen atom has a negative 

 charge, and no chlorine atom a positive one. We may thus 

 distinguish between two classes of chemical reactions, (l) those 

 which do not necessarily require any interchange of the elec- 

 trical charges carried by the atom, and (2) those which do. It 

 might, perhaps, repay investigation to see whether the occurrence 

 of chemical change is affected by the presence of a third sub- 

 stance in the same way in these classes of chemical combination. 

 Another point to be considered is the effect of this difference 

 between the chemical actions on the amount of heat developed 

 during chemical combination. W"hen hydrogen and chlorine 

 combine the heat produced may be regarded as the joint effect 

 of three jirocesses : — 



(1) The .splitting up of the molecules (H H) and (CI CI) into 



+ - + - 



the atoms H, II, CI, CI. 

 -f- - -I- - 



(2) A transference of electricity by which the negative charge 

 on one atom of hydrogen is replaced by an eepial positive charge, 

 while the positive charge on an atom of chlorine is replaced by 

 an equal negative charge. 



(3) The combination of the positively electrified hydrogen 

 atoms with the negatively electrified chlorine ones to form 

 hydrochloric acid. 



In that class of chemical action where the atoms retain their 

 charge (2) is absent, so that if the change in energy occurring in 

 the process (2) were considerable compared with the changes 

 occurring in processes (l) and (3), the thermal effects of the two 

 types of chemical combination ought to differ considerably. If 

 the changes in energy occurring in the process (2) had a great 

 preponderance over those occurring in (i) and (3), the thermal 

 effects produced by the combination of two elements ought to 

 follow very simple laws. For if 2 JIIJ is the excess of the 

 energy of an atom of hydrogen charged with the negative 

 electron over the energ)- of the atom charged with the positive 

 electron, 2 {Ci; the excess of the energy of an atom of chlorine 

 charged with the positive electron over the energy of the atom 

 charged with the negative electron, then if we could neglect the 

 energy changes in (i) and (3) compared with those in (2), the 



NO. 1349, VOL, 52] 



