August 26, 1909J 



NATURE 



261 



by the hydrogen atom set free by the electrolysis of water 

 is taken as the fundamental unit of quantity of electricity. 

 On this view, which is supported by strong evidence, the 

 charge carried by the hydrogen atom is the smallest unit 

 of electricity that can be obtained, and every quantity 

 of electricity consists of an integral multiple of this unit. 

 The experitiients of Townsend have shown that the charge 

 carried by a gaseous ion is, in the majority of cases, the 

 same and equal in magnitude to the charge carried by 

 a hydrogen atom in the electrolysis of water. From 

 measurement of the quantity of electricity required to set 

 free one gram of hydrogen in electrolysis, it can be 

 deduced that Nc= 1-29x10'" electrostatic units, where N, 

 as before, is the number of molecules of hydrogen in one 

 cubic centimetre of gas, and f the charge carried by each 

 ion. If e be determined experimentally, the value of N 

 can at once be deduced from this relation. 



The first direct measurement of the charge carried by 

 the ion was made by Townsend in 1897. When a solu- 

 tion of sulphuric acid is electrolysed, the liberated oxygen 

 is found in a moist atmosphere to give rise to a dense 

 cloud composed of minute globules of water. Each of 

 these minute drops carries a negative chprge of electricity. 

 The size of the globules, and consequently the weight, 

 was deduced with the aid of Stokes's formula by observing 

 the rate of fall of the cloud under gravity. The weight 

 of the cloud was measured, and, knowing the weight of 

 each globule, the total number of drops present was deter- 

 mined. Since the total charge carried by the cloud was 

 measured, the charge c carried by each drop was deduced. 

 The value of e, the charge carried by each drop, was 

 found by this method to be about 30x10-" electrostatic 

 units. The corresponding value of N is about 4-3x10". 



We have already referred to the method discovered by 

 C. T. R. Wilson of rendering each ion visible by the 

 condensation of water upon it by a sudden expansion of 

 the gas. The property was utilised by Sir Joseph 

 Thomson to measure the charge e carried by each ion. 

 When the expansion of the gas exceeds a certain value, 

 the water condenses on both the negative and positive 

 ions, and a dense cloud of small water-drops is seen. 

 J. J. Thomson found (" = 3-4x10-'°, H. A. Wilson 

 e = 3-ixio-'°, and Millikan and Begeman 4-06x10-'°. 

 The corresponding values of N are 3-8, 4-2, and 3-2x10'° 

 respectively. This method is of great interest and import- 

 ance, as it provides a method of directly counting the 

 number of ions produced in the gas. .'\n exact determina- 

 tion of e by this method is. however, unfortunately beset 

 with great experimental difficulties. 



Moreau has recently measured the charge carried by the 

 negative ions produced in flames. The values deduced for 

 e and N were respectively 4-3x10-'° and 3-0x10'°. 



We have referred earlier in the paper to the work of 

 Ehrenhaft on the Brownian movement in air shown by 

 ultra-microscopic dust of silver. In a recent paper (i()oq) 

 he has shown that each of these particles carries a positive 

 or negative charge. The size of each particle was 

 measured by the ultra-microscope, and also by the rate of 

 fall under gravity. The charge carried bv each particle 

 was deduced from the measured mass of the particle, and 

 its rate of movement in an electric field. The mean value 

 of e was found to be 4-6x10-'°, and thus N becomes 

 2-74x10". 



A third important method of determination of N from 

 radio-active data was given by Rutherford and Geiger in 

 1908. The charge carried by each o particle expelled from 

 radium was measured by directly determining the total 

 charge carried by a counted number of a particles. The 

 value of the charge on each a particle was found to be 

 9-3x10-'°. From consideration of the general evidence, it 

 was concluded that each a particle carries two unit posi- 

 tive charges, so that the value of e becomes 4-65x10-'°, 

 and of N 2-77x10". This method is deserving of con- 

 siderable confidence, as the measurements involved are 

 direct and capable of accuracy. 



The methods of determination- of e, so far explained, 

 have depended on direct experiment. This discussion would 

 not be complete without a reference to an important deter- 

 mination of e from theoretical considerations by Planck. 

 From the theory of the distribution of energy in the spec- 

 trum of a hot body, Planck found that e = 4-69X io-'°, and 



NO. 2078, VOL. 81] 



N=2-8oxio". For reasons that we cannot enter into-, 

 here, this theoretical deduction must be given great weight. 



When we consider the great diversity of the theones- 

 and methods which have been utilised to determine the- 

 values of the atomic constants e and N, and the probable- 

 experimental errors, the agreement among the numbers i& 

 remarkably close. This is especially the case in consider- 

 ing the more recent measurements by very different 

 methods, which are far more trustworthy than the older 

 estimates. It is difficult to fix on one determination as- 

 more deserving of confidence than another ; but I may be- 

 pardoned if 1 place some reliance on the radio-active 

 method previously discussed, which depends on the charge- 

 carried by the a 'particle. The value obtained in this way 

 is not only in close agreement with the theoretical estimate 

 of Planck, but is in fair agreement with the recent deter- 

 minations bv several other distinct methods. We may 

 consequently' conclude that the number of molecules in a 

 cubic centimetre of any gas at standard pressure and' 

 temperature is about 2-77x10'°, and that the value of 

 the fundamental unit of quantity of electricity is about 

 4-65x10-'° electrostatic units. From these data it is a 

 siinple matter to deduce the mass of any atom the atomic 

 weight of which is known, and to determine the values 

 of a number of related atomic and molecular magnitudes. 



There is now no reason to view the values of these 

 fundamental constants with scepticism, but they may be- 

 employed with confidence in calculations to advance stilf 

 further our knowledge of the constitution of atoms and", 

 molecules. There will no doubt be a great number of 

 investigations in the future to fix the values of these- 

 important constants with the greatest possible precision ; 

 but there is every reason to believe that the values are- 

 already known with reasonable certainty, and with a 

 degree of accuracy far greater than it was possible to 

 att^ain a few year's ago. The remarkable agreement in 

 the values of e and N, based on so many different theories, 

 of itself affords exceedingly strong evidence of the correct- 

 ness of the atomic theory of matter and of electricity, for- 

 it is difficult to believe t'hat such concordance would show 

 itself if the atoms and their charges had no real existence. 



There has been a tendency in some quarters to sup- 

 pose that the development of physics in recent years has 

 cast doubt on the validity of the atomic theory of matter. 

 This view is quite erroneous, for it will be clear from- 

 the evidence already discussed that the recent discoveries- 

 have not only greatly strengthened the evidence in support 

 of the theory, but have given an almost direct and con- 

 vincing proof of its correctness. The chemical atom as 

 a definite unit in the subdivision of matter is now fixed 

 in an impregnable position in science. Lea-i'ing out of 

 account considerations of etymology, the atom in chemistry 

 has long been considered to refer only to the smallest unit 

 of m.atter that enters into ordinary chemical combination. 

 There is no assumption made that the atom itself is in- 

 destructible and eternal, or that methods may not 

 ultimately be found for its subdivision into still more 

 elementary units. The advent of the electron has shown 

 that the "atom is not the unit of smallest mass of which- 

 we have cognisance, while the study of radio-active bodies 

 has shown that the atoms of a few elements of high- 

 atomic weight are not permanently stable, but break up- 

 spontaneously with the appearance of new types of matter. 

 These advances in knowledge do not in any way invalidate- 

 the position of the chemical atom, but rather indicate its 

 great importance as a subdivision of matter the properties 

 of which should be exhaustively studied. 



The proof of the existence of corpuscles or electrons 

 with an apparent mass very small compared with that of 

 the hydrogen atom marks an important ^ stage in the 

 extension of our ideas of atomic constitution. This dis- 

 covery, which has exercised a profound influence on the 

 development of modern physics, we owe mainly to the 

 genius of the President of this Association. The exist- 

 ence of the electron as a distinct entity is established by 

 similar methods and with almost the same certainty as- 

 the existence of individual a particles. While it has not 

 yet been found possible to detect a single electron by itS' 

 electrical or optical effect, and thus to count the number 

 directly as in the case of the o particles, there seems to 

 be no ' reason - why this should not be accomplished by the- 



