4 2 4 



NA TURE 



[August 28, 1884 



existence of traces of a band spectrum demonstrating the fact that 

 in some parts of the discharge the tension of dissociation is 

 insufficient to prevent the reunion of the atoms to form the 

 molecule. 



To this instance of the light thrown on molecular relations by 

 changes in the spectra, others may be added. Thus the low- 

 temperature spectrum of channelled spaces, mapped by Schuster 

 and myself, in the case of potassium, corresponds to the molecule 

 of two atoms and to the vapour-density of seventy-nine, as 

 observed by Dewar and Dittmar. Again, both oxygen and 

 nitrogen exhibit two, if not three, distinct spectra : of these the 

 line spectrum seen at the highest temperatures corresponds to the 

 atom ; the band spectrum seen at intermediate temperatures 

 represents the molecule of two atoms ; whilst that observed at a 

 still lower point would, as in the case of sulphur, indicate the 

 existence of a more complicated molecule, known to us in one 

 instance as ozone. 



That this explanation of the cause of these different spectra of 

 an element is the true one, can be verified in a remarkable way. 

 Contrary to the general rule amongst those elements which can 

 readily be volatilised, and with which, therefore, low-tempera- 

 ture spectra can be studied, mercury exhibits but one spectrum, 

 and that one of bright lines, or, according to the preceding 

 theory, a spectrum of atoms. So that, judging from spectro- 

 scopic evidence, we infer that the atoms of mercury do not unite 

 to form a molecule, and we should predict that the vapour- 

 density of mercury is only half its atomic weight. Such we 

 know, from chemical evidence, is really the case, the molecule 

 of mercury being identical in weight with its atom. 



The cases of cadmium and iodine require further elucidation. 

 The molecule of gaseous cadmium, like that of mercury, consists 

 of one atom ; probably, therefore, the cadmium spectrum is also 

 distinguished by one set of lines. Again, the molecule of iodine 

 at 1200 separates, as we know from Victor Meyer's researches, 

 into single atoms. Here spectrum analysis may come again to 

 our aid ; but, as Schuster remarks, in his report on the spectra 

 of the non-metallic elements, a more extensive series of experi- 

 ments than those already made by Ciamician is required before 

 any definite opinion as to the connection of the different iodine 

 spectra with the molecular condition of the gas can be ex- 

 pressed. 



It is not to be wondered at that these relations are only ex- 

 hibited in the case of a few elements. For most of the metals 

 the vapour-density remains, and probably will remain, an un- 

 known quantity, and therefore the connection between any 

 observed changes in the spectra and the molecular weights must 

 also remain unknown. The remarkable changes which the 

 emission spectrum of a single element— iron, for instance — 

 exhibits have been the subject of much discussion, experimental 

 and otherwise. Of these, the phenomenon of long and short 

 . lines is one of the most striking, and the explanation that the 

 long lines are those of low temperature appears to meet the fact 

 satisfactorily, although the effect of dilution, that is, a reduction 

 of the quantity of material undergoing volatilisation, is, remark- 

 ably enough, the same as that of diminution of temperature. 

 Thus it is possible, by the examination of a spectrum by Lockyer's 

 method, to predict the changes which it will undergo, either on 

 alteration of temperature, or by an increase or decrease of quan- 

 tity. There appears to be no theoretical difficulty in assuming 

 that the relative intensity of the lines may vary when the tem- 

 perature is altered, and the molecular theory of gases furnishes 

 us with a plausible explanation of the corresponding change when 

 the relative quantities of the luminous elements in a mixture are 

 altered. Lockyer has proposed a different explanation of the 

 facts. According to him, every change of relative intensity 

 means a corresponding change of molecular complexity, and the 

 lines which we see strong near the poles would bear the same 

 relation to those which are visible throughout the field, as a line 

 spectrum bears to a band spectrum ; but then almost every line 

 must be due to a different molecular grouping, a conclusion 

 which is scarcely capable of being upheld without very cogent 

 proof. 



The examination of the absorption-spectra of salts, saline and 

 organic liquids, first by Gladstone, and afterwards by Bunsen, 

 and by Russell, as well as by Hartley for the ultra-violet, and by 

 Abney and Festing for the infra-red region, have led to interest- 

 ing results in relation to molecular chemistry. Thus Hartley 

 finds that, in some of the more complicated aromatic compounds, 

 definite absorption-bands in the more refrangible region are only 

 produced by substances in which three pairs of carbon atoms are 



doubly linked, as in the benzene ring, and thus the means of 

 ascertaining this double linkage is given. The most remarkable 

 results obtained by Abney and Festing show that the radical of 

 an organic body is always represented by certain well-marked 

 absorption-bands, differing, however, in position, according as 

 it is linked with hydrogen, a halogen, or with carbon, oxygen, 

 or nitrogen. Indeed, these experimenters go so far as to say 

 that it is highly probable that by this delicate mode of analysis the 

 hypothetical position of any hydrogen which is replaced may be 

 identified, thus pointing out a method of physical orientation of 

 which, if confirmed by other observers, chemists will not be slow 

 to avail themselves. This result, it is interesting to learn, has 

 been rendered more than probable by the recent important 

 researches of Perkin on the connection between the constitution 

 and the optical properties of chemical compound. 



One of the noteworthy features of chemical progress is the 

 interest taken by physicists in fundamental questions of our 

 science. We all remember, in the first place, Sir William 

 Thomson's interesting speculations, founded upon physical 

 phenomena, respecting the probable size of the atom, viz. "that 

 if a drop of water were magnified to the size of the earth, the 

 constituent atoms would be larger than small shot, but smaller 

 than cricket balls." Again, Helmholtz, in the Faraday Lecture, 

 delivered in 1SS1, discusses the relation of electricity and che- 

 mical energy, and points out that Faraday's law of electrolysis, 

 and the modern theory of valency, are both expressions of the 

 fact that, when the same quantity of electricity passes through 

 an electrolyte, it always either sets free, or transfers to other 

 combinations, the same number of units of affinity at both 

 electrodes. Helmholtz further argues that, if we accept the 

 Daltonian atomic hypothesis, we cannot avoid the conclusion that 

 electricity, both positive and negative, is divided into elementary 

 portions which behave like atoms of electricity. He also shows 

 that these charges of atomic electricity are enormously large as 

 compared, for example, with the attraction of gravitation between 

 the same atoms ; in the case of oxygen and hydrogen, 71,000 

 billion times larger. 



A further subject of interest to chemists is the theory of the 

 vortex-ring constitution of matter thrown out by Sir William 

 Thomson, and lately worked out from a chemical point of view 

 by J. J. Thomson, of Cambridge. He finds that if one such 

 ring be supposed to constitute the most simple form of matter, 

 say the monad hydrogen atom, then two such rings must, on 

 coming into contact with nearly the same velocity, remain en- 

 chained together, constituting what we know as the molecule of 

 free hydrogen. So, in like manner, systems containing two, 

 three, and four such rings constitute the dyad, trjad, and tetrad 

 atoms. How far this mathematical expression of chemical theory 

 may prove consistent with fact remains to be seen. 



Another branch of our science which has recently attracted 

 much experimental attention is that of thermo-chemistry, a subject 

 upon which in the future the foundation of dynamical chemistry 

 must rest, and one which already proclaims the truth of the great 

 principle of the conservation of energy in all cases of chemical 

 as well as of physical change. But here, although the materials 

 hitherto collected are of very considerable amount and value, 

 the time has not yet arrived for expressing these results in general 

 terms, and we must, therefore, be content to note progress in 

 special lines and wait for the expansion into wider areas. 

 Reference may, however, be properly made to one interesting 

 observation of general significance. It is well known that, while, 

 in most instances, the act of combination is accompanied by 

 evolution of heat— that is, whilst the potential energy of most 

 combining bodies is greater than that of most compounds — cases 

 occur in which the reverse of this is true, and heat is absorbed 

 in combination. In such cases the compound readily undergoes 

 decomposition, frequently suddenly and with explosion. Acety- 

 lene and cyanogen seem to be exceptions to this rule, inasmuch 

 as, whilst their component elements require to have energy added 

 to them in order to enable them to combine, the compounds 

 appear to be very stable bodies. Berthelot has explained this 

 enigma by showing that, just as we may ignite a mass of dynamite 

 without danger, whilst explosion takes place if we agitate the 

 molecules by a detonator, so acetylene and cyanogen burn, as we 

 know, quietly when ignited, but when their molecules are shaken 

 by the detonation of even a minute quantity of fulminate, the 

 constituents fly apart with explosive violence, carbon and 

 hydrogen, or carbon and nitrogen, being set free, and the quantity 

 of heat absorbed in the act of combination being suddenly 

 liberated. 



