33 2 



NATURE 



[August 3, 1882 



parallel 26°'4 ; the third and fourth comets of 1881 and Encke's 

 comet were also observed for position. The physical observa- 

 toiy at Potsdam was in full activity, and in addition to the more 

 special subjects of observation undertaken by this important 

 establishment, an extensive series of observations of varieble 

 stars was secured in 1881. From Stockholm Dr. Hugo Gylden 

 notifies his determination of the parallax of the star Bradley 

 3077, or No. 240 in Argelander's Catalogue of 250 stars, form- 

 ing part of the seventh volume of the Bonn observations: the 

 resulting value is o"'2S3 ± o"'0468 ; this star has considerable 

 proper motion. Prof. K. Wolf communicates, from Zurich, he 

 monthly numbers of days with and without sun-spots, and the 

 relative numbers : in the whole year's observing-days, the sun 

 was free from spots on five days, and exhibited spots on 297. 



A TO MIC A TTRA CTION 

 'PME theory of universal gravitation, as I understand it, asserts 

 that the mutual attraction exerted by any two bodies, A and 

 I!, is dependent only on their respective masses and on the dis- 

 tance between them, being entirely uninfluenced by the presence 

 of other bodies even in the immediate neighbourhood of A or 

 B, Thus at a given moment the Earth and Venus, being in 

 certain definite positions, exert upon each other a certain force 

 of attraction ; the attraction thus taking place between the 

 masses of the two planets would be unaltered by the removal 

 of the Moon from the sphere of action ; the gravitation of the 

 Earth and the Moon does not therefore tie up any portion of the 

 attractive energy of the Earth, and so diminish the force with 

 which other bodies gravitate towards it. 



A totally different assumption is usually made with regard to 

 that form of attraction which gives rise to chemical phenomena. 

 1 Iere it is supposed that two or more atoms, having combined 

 together, have thereby become incapable, at any rate in the 

 majority of cases, of attracting others to any appreciable extent. 

 Thus 1 imagine that most chemists hold the view that when 

 hydrogen and oxygen combine together to form water they 

 thereby exhaust, or nearly exhaust, their combining power, that 

 the power of attraction residing in the oxygen atoms is all 

 concentrated upon the hydrogen atoms, just as we might con- 

 ceive all the attractive power of the Eirth concentrated on 

 the moon, thus leaving all other bodies in its neighbourhood 

 free from the influence of gravity. We thus invest matter with 

 two separate forms of attraction differing entirely in their mode 

 of action, and having indeed nothing in common. It is however 

 possible to a certain extent to assimilate chemical attraction and 

 gravitation, and I propose here to discuss some of the results 

 which ensue from the elaboration of this idea Let us suppose 

 then that the act of chemical combination in no wise alters the 

 power of attraction which the combining atoms exert upon sur- 

 rounding bodies, and let us see what effect this hypothesis has 

 upon the explanation of various phenomena. In order to do this 

 we must first render as precise as possible our notions of the 

 construction of chemical compounds. 



It is now known with certainty that the atomic and molecular 

 volumes of substances are but slightly altered by combination, 

 that is to say, that under comparable conditions the atom of any 

 substance generally occupies about the same space with whatever 

 atoms, similar or dissimilar, it may be combined. This fact 

 seems to me to point to the conclusion that the atoms which make 

 up a molecule are as close together as their periodic motions will 

 permit, and are not merely held in certain positions of equili- 

 brium by various opposing forces ; for if the latter supposition 

 were true, I fail to see how it would be possible for the same 

 atom, together with its surrounding proportion of space, to have 

 always the same volume. The immediate proximity of the 

 several molecules in the liquid and solid states must also be 

 assumed, in order to account for the invariability of molecular 

 volumes. 



The innumerable facts which have been brought to light by 

 the efforts of those who have investigated the chemistry of the 

 carbon compounds all lead one to suppose that there is some 

 foundation lor the ideas propounded by chemists concerning the 

 position of the atoms, and that the constitutional formula: ascribed 

 to organic substances really represent the construction of the 

 molecule. If this be so it certainly furnishes a further argument 

 in support of the proximity of the atoms. 



The assumptions contained in the preceding paragraphs are in 

 no way opposed to the views generally held concerning molecular 

 and atomic motion which we owe to the development of the 



science of heat. Tbey merely state that there is no force of repul- 

 sion exerted between contiguous atoms, and that the vibratory or 

 other movements are small compared with the size of the moving 

 masses. 



The ohject of the following remarks is to show that the hypo- 

 thesis concerning chemical attraction mentioned above enables 

 us to offer some explanation of the relative volatility of bodies. 

 We all, I presume, look upon the maximum vapour tension of a 

 substance at a given temperature as affording to a certain extent 

 a means of estimating the attraction which its molecules exert 

 among themselves ; if there is considerable attraction there will 

 be a low vapour tension, and with little attraction there will be 

 a low boiling point. It f Hows from this that the attraction 

 between the molecules of hydrogen is relatively extremely small ; 

 that in the case of oxygen and nitrogen it is also very small, 

 though probably much larger than in the former case ; the attrac- 

 tion mutually exerted by molecules of chlorine will be more con- 

 siderable ; while with bromine, iodine, and other liquid and solid 

 elements it will be greater still. We must not however confound 

 the attraction exerted between atoms of a substance with that 

 between the molecules, for each atom attracts separately those of 

 the contiguous molecule, so that the attraction between two 

 molecules of bromine, for example, will be four times as great 

 as between two atoms, and generally when the molecule of a 

 substance contains « atoms the attraction between two molecules 

 will be approximately ri- times that between two atoms. This 

 is of course even approximately true only when the distance 

 between the two molecules i~ great relatively to their si^e ; when 

 the two molecules are close together the several interatomic 

 attractions will be exercised over very different distances, and 

 will therefore be very unequal in amount. Nevertheless, (he 

 above remark enables us to see that in some cases the apparent 

 attraciion, as estimated by the boiling-point, may be very mis- 

 leading. In sulphur, for example, of which the molecule in the 

 solid and liquid states is probably somewhat complex, we have a 

 substance of high boiling-point, though the mutual attraction of 

 the atoms may be comparatively small. The same is the case 

 with carbon and many ciher substances. 



Applying now the above considerations to a few actual cases, 

 we shall see that the relative volatility of different substances is 

 generally satisfactorily explained. Let us designate by (////) the 

 attraction at unit distance between two atoms of hydrogen, by 

 (00] the attraction between two atoms of oxygen, and generally 

 by (rs) the attraction at unit distance between any two atoms, 

 R and S. Then in the case of water the molecular attraction 

 will be represeated by — 



4A(///;) + 4B(/(o) + C(oo), 

 where A, B, and C are factors dependent on the distances w hicb 

 separate the atoms; now we have seen that [ft A) and (00) 

 probably have small values, but {ho) is not small, hence th : 

 attraction between molecules of water should be far greater than 

 that between molecules of oxygen, and the boiling-point much 

 higher, a result which is in accord with fact. The boiling-point of 

 water would probably be much higher than it is, were it not tha - 

 the attractions between H and O are exerted over comparatively 

 large distances, owing to the hydrogen of one molecule shielding 

 its companion oxygen from the approach of other hydrogen. In 

 the similarly constituted body, PES, the value of the molecular 

 attraction will be — 



4 A {hh) + 4Ws) 4- C{ss), 

 in which expression A, B, and C may be supposed to have values 

 not differing exce-sively from those which hold good in the case of 

 water (the sulphuretted hydrogen being supposed liquid). The 

 value [ss) is in itself small, and since the force is exerted 

 between two atoms which cannot approach each other very closely, 

 C is :'!-" small. The affinity of hydrogen for sulphur being aKo 

 feeble, the whole value of the molecular attraction is small ; 

 sulphuretted hydrogen should therefore be an extremely volatile 

 body, which is actually the case. 



With hydrochloric, hydrobiomic, and hydr.iklic acids we have 

 for the molecular attraction the several values — 

 A(kh) + 2B(/;<r/) + C{clcl\ 

 A'(h/,) + 2Y,'(hb,) + C'(iror) 

 A"(hh) + 2B"(/</) + C"(ii). 

 As the three bodies are similarly constructed we may assume that 

 A, A', A", &c, do not materially differ. As the third terms of 

 these expressions increase the secmd terms diminish ; we sh mid 

 therefore expect that there might be no greit difference in the 

 vapour-tensions of tbe three substances ; experiment proves tint 



