September 12, 1901] 



NA TURE 



475 



stronger proof of their reality could be desired. Yet there is 

 every reason to believe that something very like this has been 

 accomplished by Mr. C. T. R. Wilson and Prof. J. J. Thomson. 



It is known that it is comparatively difficult to produce a fog 

 in damp air if the mixture consists of air and water-vapour 

 alone. The presence of particles of very line dust facilitates 

 the process. It is evident that the vapour condenses on the 

 dust particles, and that a nucleus of some kind is necessary on 

 which each drop may form. But electrified particles also act 

 as nuclei ; for if a highly charged body from which electricity is 

 escaping be placed near a steam jel, the steam condenses ; and 

 a cloud is also formed in dust-free air more easily than would 

 otherwise be the case if electricity is discharged into it. 



Again, according to accepted theory, when a current of elec- 

 tricity flows through a gas, some of the atoms are divided into 

 parts which carry positive and negative charges as they move in 

 opposite directions, and unless this breaking-up occurs a gas 

 does not conduct electricity. But a gas can be made a conductor 

 merely by allowing the Rcintgen rays or the radiation given off 

 by uranium to fall upon it. A careful study of the facts shows 

 that it is probable that some of the atoms have been broken up 

 by the radiation, and that their oppositely electrified parts are 

 scattered among their unaltered fellows. Such a gas is said to 

 be ionised. 



Thus by these two distinct lines of argument we come to the 

 conclusions : — 1st, that^he presence of electrified particles pro- 

 motes the formation of mist, and 2nd, that in an ionised gas 

 such electrified particles are provided by the breaking-up of 

 atoms. 



The two conclusions will mutually support each other if it 

 can be shown that a mist is easily formed in ionised air. This 

 was tested by Mr. Wilson, who showed that in such air mist is 

 formed as though nuclei were present, and thus in the cloud we 

 have visible evidence of the presence of the divided atoms. If, 

 then, we cannot handle the individual molecules, we have at 

 least some reason to believe that a method is known of seizing 

 individuals, or parts of individuals, which are in a special state, 

 and of wrapping other matter round them till each one is the 

 centre of a discrete particle of a visible fog. 



I have purposely chosen this illustration, because the e.x- 

 planation is based on a theory — that of ionisation — which is at 

 present subjected to hostile criticism. It assumes that an elec- 

 trical current is nothing more than the movement of charges of 

 electricity. But magnets placed near to an electric current 

 tend to set themselves at right angles to its direction ; a fact on 

 which the construction of telegraphic instruments is based. 

 Hence if the theory be true, a similar effect ought to be pro- 

 duced by a moving charge of electricity. This experiment was 

 tried many years ago in the laboratory of Helmholtz by Row- 

 land, who caused a charged disc to spin rapidly near a magnet. 

 The result was in accord with the theory ; the magnet moved 

 as though acted upon by an electric current. Of late, however, 

 M. Cremieu has investigated the matter afresh, and has obtained 

 results which, according to his interpretation, were inconsistent 

 with that of Rowland. 



M. Crcmieu's results are already the subject of controversy,' 

 and are, I believe, likely to be discussed in the Section of 

 Physics. This is not the occasion to enter upon a critical dis- 

 cussion of the question at issue, and I refer to it only to point 

 out that though, if M. Cremieu's result were upheld, our views 

 as to electricity would have to be modified, the foundations of 

 the atomic theory would not be shaken. 



It is, however, from the theory of ions that the most far- 

 reaching speculations of science have recently received unex- 

 pected support. The dream that matter of all kinds will some 

 day be proved to be fundamentally the same has survived many 

 shocks. The opinion is consistent with the great generalisation 

 that the properties of elements are a periodic function of their 

 atomic weights. Sir Norman Lockyer has long been a pro- 

 minent exponent of the view that the spectra of the stars indicate 

 the reduction of our so-called elements to simpler forms, and 

 now Prof. J. J. Thomson believes that we can break off from an 

 atom a part, the mass of w-hich is not more than one-thousandth 

 of the whole, and that these corpuscles, as he has named them, 

 are the carriers of the negative charge in an electric current. If 

 atoms are thus complex, not only is the a priori probability 

 increased that the different structures which we call elements 

 may all be built of similar bricks, but the discover)- by Lenard 



1 See Phil. Mag., July 1901, p. 144 ; and Johns Hopkins University 

 Circulars, XX. No. 152, May-June iqoi, p. 78. 



NO. 1663, VOL. 64] 



that the ease with which the corpuscles penetrate different bodies 

 depends only on the density of the obstacles, and not on their 

 chemical constitution, is held by Prof. Thomson to be " a strong 

 confirmation of the view that the atoms of the elementary sub- 

 stances are made up of simpler parts, all of which arealike."' On 

 the present occasion, however, we are occupied rather with the 

 foundations than with these ultimate ramifications of the atomic 

 theory ; and having shown how wide its range is, I must, to a 

 certain extent, retrace my steps and return to the main line of 

 my argument. 



T/id Properties of Atoms and Molecules. 



For if it be granted that the evidence that matter is coarse- 

 grained and is formed of separate atoms and molecules is too 

 strong to be resisted, it may still be contended that we can 

 know little or nothing of the sizes and properties of the mole- 

 cules. 



It must be admitted that, though the fundamental postulates 

 are always the same, different aspects of the theory, which have 

 not in all cases been successfully combined, have to be developed 

 when it is applied to different problems ; but in spite of thi 

 there is little doubt that we have some fairly accurate knowledge 

 of molecular motions and magnitudes. 



If a liquid is stretched into a very thin film, such as a soap- 

 bubble, we should expect indications of a change in its proper- 

 ties when the thickness of the film is not a very large multiple 

 of the average distance between two neighbouring molecules. 

 In 1890 Sohncke {Wied. Ann., 1890, xl. pp. 345-355) detected 

 evidence of such a change in films of the average thickness of 

 106 millionths of a millimetre (mm), and quite recently Rudolph 

 Weber found it in an oil-film when the thickness was 115 mm 

 {Annalen der Pkysik, 1901, iv. pp. 706-721). 



Taking the mean of these numbers and combining the results 

 of ditferent variants of the theory, we may conclude that a film 

 should become unstable and tend to rupture spontaneously some- 

 where between the thicknesses of 1 10 and 55 mMi ar"l Prof. Reinold 

 and I found by experiment that this instability is actually ex- 

 hibited between the thicknesses of 96 and 45 fiix (Phil. Trans.., 

 1893, 184, pp. 505-529). There can therefore be little doubt 

 that the first approach to molecular magnitudes is signalled when 

 the thickness of a film is somewhat less than 100 ixji, or 4 

 millionths of an inch. 



Thirteen years ago I had the honour of laying before the 

 Chemical Society a rjsume of what was then known on these 

 subjects [Cheiit. Soc. Trans., liii., March 1 888, pp. 222-262), 

 and I must refer to that lecture or to the most recent edition of 

 O. E. Meyer's work on the kinetic theory of gases ("Kinetic 

 Theory of Gases," O. E. Meyer, 1899 ; translated by R. E. 

 Baynes) for the evidence that various independent lines of argu- 

 ment enable us to estimate quantities very much less than 4 

 millionths of an inch, which is perhaps from 500 to 1000 times 

 greater than the magnitude which, in the present state of our 

 knowledge, we can best describe as the diameter of a molecule. 



Confining our attention, however, to the larger quantities, I 

 will give one example to show how strong is the cumulative 

 force of the evidence as to our knowledge of the magnitudes of 

 molecular quantities. 



We have every reason to believe that though the molecules in 

 a gas frequently collide with each other, yet in the case of the 

 more perfect gases the time occupied in collisions is small com- 

 pared with that in which each molecule travels undisturbed by 

 its fellows. The average distance travelled between two suc- 

 cessive encounters is called the mean free path, and, for the 

 reason just given, the question of the magnitude of this distance 

 can be attacked without any precise knowledge of what a mole- 

 cule is, or of what happens during an encounter. 



Thus the mean free path can be determined, by the aid of the 

 theory, either from the viscosity of the gas or from the thermal 

 conductivity. Using figures given in the latest work on the 

 subject (Meyer's " Kinetic Theory of Gases" ; see above), and 

 dealing with one gas only, as a fair sample of the rest, the 

 lengths of the mean free path of hydrogen as determined by 

 these two independent methods differ only by about 3 per cent. 

 Further, the mean of the values which I gave in the lecture 

 already referred to differed only by about 6 per cent, from the 

 best modern result, so that no great change has been introduced 

 during the last thirteen years. 



' For the most recent account of this subject see an article on '* Bodies 

 Smaller than Atoms." by Prof. J. J. Thomson in the Fopnlar Science 

 Monthly i^\\^ Science Press), .\ugust 1901. 



