172 



NATURE 



\yune 27, 1872 



Young, Fresnel, FoucauU, and Fizeau, are shown to be unten- 

 able. A very able American metaphysician, in meeting an 

 objection brought by Huxley agiinst the views of Comte, has 

 strongly expressed his unqualified dissent ;* nevertheless, the 

 hypothesis that light, heat, and acinism are propagated by the 

 undulations of a subtle all-pervading fluid, is the only one which 

 sitisfactorily accounts for a certain class of phenomena, and it is 

 accepted by all the prominent experimental physicists of tlic 

 present day. 



The vast difference in density indicated cannot be apprehended, 

 because numerical eomparisons utterly fail to raise in the mind 

 any clear conception regarding a fluid so attenuated ; yet it 

 naturally suggests the idea that there must be many intervening 

 conditions of matter in which it exists in successive degrees of 

 increasing density, and that these conditions form the connecting 

 links, so to speak, between its apparently imponderable and 

 iis ponderable states. Something like this opinion seems to 

 have been maintaine<l in a curious work published in En-^land 

 many yeai-s ago. f Tlie reverend author, vie^'ing the universe 

 as a systematic manifestation of the Divine Wdl, a-sumes that 

 the medium of light is the mother element from which by pro- 

 gressive steps the chemical elements have been evolved. Pro- 

 ceeding from the first lines of morphology he arrives at the pri- 

 mitive form which cannot be isolated ; then by an exceedingly 

 ingenious synthetic process he represents by diagrams his ideal 

 structure of different kinds of atoms, all of which are duplici- 

 tions of the tetrahcd'"on. Thus he claims to reveal the unit, by 

 multiples of which the atomic weight of all chemical elements 

 may be expressed, and so arrives at a result which will be recog- 

 nised as simoly a modi^cition of the so-callei law of ProuC. 

 Although this, and oth-r remarkable surmises by Macv'car are, 

 for reasons which need not here be adduced, quite untenable, lie 

 seems to have led the way to an assumption which has recently 

 met with some favour, namely, that the chemical atom, althougli 

 indivisible, is a collection of smiller particles. However, in 

 following this author towards the infinitesimxl, we only realise 

 more fully the truth that above and below the narrow zone of the 

 visible are objects too far off and too fine for human scrutiny. 

 Although the seeming all K'. rounded by intimation of other and 

 brighter regions, Science can never compass tliem by any exten- 

 sion of her domain ! In those unsounded depths which form the 

 boundary and background of the known, thought grown di/zy 

 finds no support ; and even the positivist turns back bewildered 

 when mensuration fails and computations end in surds ! 



On examining the numerous works on chemistry published 

 within the last twenty years, one cannot fail to notice a gradual 

 change in the expressions employed in describing reactions. The 

 word " equivalent " seems to have lost the meaning originally 

 assigned to it by WoUaston, and the terms "comljining weight" 

 and "combining proportion " are now used less frequently than 

 "atomic weight" and "atom." This abandonment of old 

 forms of expression doubtless indicates a gradual change of 

 opinion among leading chemists, a change which may be ascribed 

 )i\rtly to an accumulation of facts tending to confirm the atomic 

 theory, and, partly, to the promptings of that mysterious intuition 

 which, overleaping the limits of logic, often arrives at correct 

 conclusions even before their truth has been demonstrated. 



During all the discussions on " atomicity " hardly a doubt has 

 been raised as to the actual existence of the atom. It was not, 

 therefore, surprising that the chemical world received a sensible 

 shock at the stand made by IJrodie in 18(58. J However, a carc- 

 f j1 examination of his paper is likely to lead to the conclusion 

 that the objections to the atomic theoi-y therein enumerated are 

 not more formidable than those which can be urged against his 

 own ingenious, but complicated niethod of chemical operations. 

 Precision in signs and definitions leads to exact results in the 

 abstract, nevertheless a mathematical formula often requires 

 modification to meet the varying conditions found in actual 

 practice, and even then it only gives a near approximation to the 

 truth. 



Renewed attention to this subject was doubtless the means of 

 drawing from the then President of the London Chemical Society 

 a paper " on the Atomic Theory," which is generally regarded 

 as the best exposition and defence of the doctrine yet made, and 

 which may be consulted with profit by those desiring to obtain a 



* Eleventh Harvard lecture, by Prof. John Fiske. Cambridge, Mass., 

 1SC9. 



t "Elements of the Economy of Nature." By J. G. Macvicar, D.D. 

 (London : Chapman and Hall.) 1856. 



t "The Cilculn; nf Cliemicnl Operations" I!y Prof B. C. Brodic. 

 Journal 0/ the CliciuUal So.lcty, Lo.idon, vol. xxi. p. 367. 



clear statement of the principal results of chemical research 

 adduced for its confirmation.' 



A vigorous attack on the atomic theory has since been made 

 by Mills, the real tendency of which is to raise drabts concerning 

 the existence of matter itself.t He quotes with evident satisfac- 

 tion from a work by Digby "on the na'ure of bodies" printed 

 in 1645, wherein ijiiantily is defined " as but one whole tliat may 

 indeed be cut into so many several parts ; but those ]iarts are 

 really not there till by division they are parcelled out ; and then 

 tlie whole (out of which they are made) ceaseth to be any longer, 

 and the parts succeed in lieu of it, and are every one of them a 

 new whole." From this statement proceeds a train of geometrical 

 reasoning concerning extension and division which leads to the 

 old dilemma regarding finite and infinite indivisibles. 



Fortunately a new science, unknown to Digby, has 

 demonstrated that matter has other than mere physi- 

 cal properties which are so clear and well defined as 

 to enable its votaries to determine the ultimate com- 

 position of all bodies. The chemist affirms that, however in- 

 clined we may be to regard a body as a whole, it is in f.ict com - 

 posed of minute parts which may be separated, anrl that in the 

 gr'=at majority of bodies, which are compounds. Nature has her- 

 self made divisions by incorporating unlike parts which may be 

 replaced by other unlike parts. On questions relating to the 

 actual size of the=e parts, their form, their structure, &~., he 

 makes no issue ; he simjily asserts that all these ultimate parts 

 are permanent, and that those composed of the same kind of 

 matter are i 'entical in size and structure. The lioiits proposed 

 for this paper will permit elucidation of this point alone. 



The cleai-es* conception of molecules and atoms will be arrived 

 at by examining the principal phenomena attending the mechani- 

 cal mixture and final chemical union of the lightest and the 

 heaviest of the simple gases. The electro-positive element, 

 hydrogen, is a permanently elastic gas, having a relative dens'ty 

 expressed by r. Its properties are in marked contrast with tho'e 

 of chhirine, a yellowish green gas, which maybe condensed into 

 a liquid, by a pressure of about four atmospheres. Tlie density 

 of this strong electro-negative element is 35 "J. If two vessels of 

 equal capacity, filled with these gases respectively, be placed in 

 the dark, one over the other, and a communication be opened 

 between them, a mutual diffusion of the gases will commence, 

 the relative velocity being inversely as the square root of their 

 densities. The action continues untraversed by the force of 

 giavitation until minute portions of hydrogen and chlorine are 

 equally diffused throughout both receptac'es. This phenomenon 

 cannot be accounted for, excepting on the supposition that minute 

 parts of each gas have undergone complete isolation. If ditTu- 

 sion were effected only through a single stratum or extremely thin 

 layer, it would be possible for two gaseous elements to retain 

 their continuitv by passing each other in intertwining streams, 

 thus forming like threads, a warp and woof; but when diffusion 

 is in every direction it is obvious that these elements must poM- 

 tively separate each other, and thus be divided into extremely 

 diminutive bodies each of the same dimensions. Let / represent 

 the lighter gas, </ the denser, and e the dimensions or size of each 

 isolated portion, then el and ed will denote the dissimilar parts of 

 which the whole gaseous matter is composed. As the phe- 

 nomenon of diffusion occurs under the conditions mentioned, 

 whatever m.ay be the quantity of gases employed, it follows that 

 i'/and I-./ are individual volumes or molecules, invariably of tlie 

 same dimensions. This diffusion of gases may therefore be 

 defined as the uniform intermingling of dissimilar molecules. 



If the molecules el and ed thus commingled while in the dark 

 be exposed to direct sunlight, an instantaneous and complete 

 chemical combination occurs with explosive violence but without 

 condensation ; or if exposed to diffused daylight, the union of 

 elements will be gradual and without explosion ; the resulting 

 compound in each case being hydrochloric acid gas. 



The affinity or force of chemism is generated by the action of 

 light on the coloured gas chlorine, which, by absorbing all the 

 rays and transmitting only the yellowish green, acquires a power 

 which seems to be expended by the union of that element with 

 hydrogen. Early in the present century M. Benard announced 

 that the new properties acquired by chlorine on exposure to 

 light were derived from the violet ray. In 1843 Draper proved 

 by experiment the relative power of each ray in producing this 



* "On the Atomic Theory." By Prof. A. W. Williamson. Jour. C/wiK. 

 Soc. London, vol. x.xii. p. 328 



t "On the Atomic Theory." By Edmund J, Mills, D. Sc rhUoioMikal 

 M.isaziuc, vol. xlii No. 278, p. 112. 



