November 6, 1919] 



NATURE 



231 



most striking features of the change of outlook 

 since Nature made its first appearance in 1869. 

 Vague ideas of their actual individual mass, size, 

 shape, and constitution have been or are being 

 replaced more and more by exact quantitative 

 knowledge, which invites our literal acceptance 

 and grows in fruitfulness the more implicitly it 

 is used as the basis for further investigations. 



But the latter half of the period under review 

 witnessed an even greater change of outlook. 

 The atom, since the discovery of radio-activity in 

 1896, has ceased to be the smallest coin of the 

 realm of material change. The farthings of i86g 

 have proved to resemble lOOoZ. notes, and the 

 potentialities of the world in consequence have 

 been multiplied a million times. The change of 

 the single atom of matter is well within the range 

 of direct perception by the senses, and, stranger 

 still, the change reveals that, under the image 

 and superscription of the same Caesar, coins of 

 different mass and mintage have been circulating 

 unsuspected in the chemist's currency. 



As regards the physical reality of molecules, 

 by no means the least important factor contribut- 

 ing to the result has been the recognition that, if 

 the molecules were not the smallest parts of 

 matter capable of free independent existence and 

 motion, heat would not be the final permanent 

 form which all kinetic energy liberated in the 

 world assumes. The limit that fixes the physical 

 sub-division of matter limits also the sub-division 

 of motion. Though in the real world of matter 

 in bulk, as contrasted with the ideal fictions of 

 mathematics, friction and imperfect elasticity 

 quickly reduce all moving masses to apparent rest, 

 that " rest " is the perpetual heat motion of the 

 molecules, which, literally and necessarily, must 

 be perfectly frictionless and elastic because they 

 are the smallest particles capable of free inde- 

 pendent motion, and no smaller particles exist 

 among which their motion can be further dis- 

 tributed. 



Moreover, in accordance with the law of equi- 

 partition of energy, all molecules at the same 

 temperature, whatever their mass, become, in 

 consequence of their ceaseless mutual collisions, 

 possessed of the same average amount of kinetic 

 energy, and, therefore, of a velocity of translation 

 inversely proportional to the square-root of their 

 mass. This serves to clarify the conception of 

 the real molecule from misnomers still unthink- 

 ingly retained. 



For example, it is a pure survival of past con- 

 fusion to speak of the molecule of a crystalline 

 solid, if not of any solid, for in such the smallest 

 parts are not free to move, but are anchored in 

 fixed, unchanging positions in the crystal space- 

 lattice, as the resolution of X-rays by the crystal 

 structure has shown. It is, similarly, always a 

 pure misnomer to give the name " molecule " to 

 the least number of atoms which represent the 

 chemical composition and properties of a sub- 

 stance, in the absence of experimental knowledge 

 of the molecular magnitude, and therefore of any 

 knowledge as to whether such a particle really 

 NO. 2610, VOL. 104] 



exists in a form capable of free independent move- 

 ment. 



Cleared of these ambiguities, the conception of 

 the individual molecule has become very real. 

 We have been led by Perrin, and the mathe- 

 matical physicists who paved the way for his 

 experimental work, to recognise the Brownian 

 movement as but one aspect of the perpetual 

 motion of the molecules, which, though invisible 

 to the naked eye, becomes swift and ceaseless for 

 particles even of the scale of minuteness resolved 

 by the microscope, and we can extrapolate with 

 assurance to the minuter world which science had 

 long before visualised by faith. 



Or, again, we may follow Langmuir, with none 

 of the feeling of hesitancy and diffidence that 

 would have held back an earlier generation, into 

 the explanation of catalysis, adsorption, and 

 allied phenomena, as caused by surface layers of 

 molecules "one molecule thick." Nor do we con- 

 sider it fanciful to explain the spreading of animal 

 and vegetable oils upon water and the non-spread- 

 ing of mineral oils, as due to the attempt, in the 

 first case, of the one end, the soluble glycerine 

 ester end, of the rod-like molecule to dissolve in 

 the water, and the refusal of the other end, the 

 insoluble, hydro-carbon, or oily end, to do so. 

 Wherefore the molecules of such oils stand up 

 on end and cover the surface with a one-molecule 

 thick layer of the oily ends of the molecules, 

 whereas the mineral oils, with molecules oily at 

 both end?, do not spread ! Real in one sense as 

 the structural formulae of organic compounds have 

 been for many decades, an earlier generation 

 would scarcely have thought of this. 



The discovery that the X-rays are of a char- 

 acter identical with light, but of wave-length of 

 the order of one ten-thousandth of that of light 

 of the visible spectrum, has made the structure 

 of crystalline solids as open to direct examination 

 as the ten-thousand-fold coarser structure of the 

 Rowland grating, ruled by the dividing engine, 

 is by means of ordinary light. In this way many 

 of the space-lattices hitherto arrived at only by 

 the aid of the second-sight of the mathematical 

 crystallographer have been tested and found 

 real. 



Since the explanation by Le Bel and van't Hoff 

 of optical isomerism as due to structural differ- 

 ences of the arrangement of the atoms in the 

 molecule, of the kind that exist between an asym- 

 metrical object and its mirror-image, and therefore 

 only capable of representation in space of three 

 dimensions, chemists have, not without reproach, 

 used model carbon atoms in building up the struc- 

 ture of organic compounds, and have found them 

 capable of accounting, for example, in cyclic 

 structures, for many of the properties of these 

 coinpounds far removed from the field of optical 

 activity. That the real carbon atom should possess 

 any resemblance to these little wooden balls bear- 

 ing four spokes radiating symmetrically from the 

 centre may have appeared to many too crude a 

 conception for literal belief. Yet when the char- 

 acter of the space-lattice of the diamond crystal 



