Septembeb 3, 1909] 



SCIENCE 



295 



and to determine the magnitude of the 

 electric charge carried by each. 



A few examples will now be given which 

 illustrate the older methods of estimating 

 the mass and dimensions of molecules. As 

 soon as the idea of the discrete structure of 

 matter had taken firm hold, it was natural 

 that attempts should be made to estimate 

 the degree of coarse-grainedness of matter, 

 and to form an idea of the dimension of 

 molecules, assuming that they have exten- 

 sion in space. Lord Rayleigh has drawn 

 attention to the fact that the earliest esti- 

 mate of this kind was made by Thomas 

 Young in 1805, from considerations of the 

 theory of capillarity. Space does not allow 

 me to consider the great variety of methods 

 that have later been employed to form an 

 idea of the thickness of a film of matter in 

 which a molecular structure is discernible. 

 This phase of the subject was always a 

 favorite one with Lord Kelvin, who devel- 

 oped a number of important methods of 

 estimating the probable dimensions of 

 molecular structure. 



The development of the kinetic theory 

 of gases on a mathematical basis at once 

 suggested methods of estimating the num- 

 ber of molecules in a cubic centimeter of 

 any gas at normal pressure and tempera- 

 ture. This number, which will throughout 

 be denoted by the symbol N, is a funda- 

 mental constant of gases ; for, according to 

 the hypothesis of Avogadro, and also on 

 the kinetic theory, all gases at normal pres- 

 sure and temperature have an identical 

 number of molecules in unit volume. 

 Knowing the value of N, approximate esti- 

 mates can be made of the diameter of the 

 molecule ; but in our ignorance of the con- 

 stitution of the molecule, the meaning of 

 the term diameter is somewhat indefinite. 

 It is usually considered to refer to the di- 

 ameter of the sphere of action of the forces 

 surrounding the molecule. This diameter 

 is not necessarily the same for the mole- 



cules of all gases, so that it is preferable to 

 consider the magnitude of the fundamental 

 constant N. The earliest estimates based 

 on the kinetic theory were made by Lo- 

 schmidt, Johnstone Stoney and Maxwell. 

 Prom the data then at his disposal, the 

 latter found N to be 1.9 X 10". Meyer, 

 in his "Kinetic Theory of Gases," dis- 

 cusses the various methods of estimating 

 the dimensions of molecules on the theory, 

 and concludes that the most probable esti- 

 mate of N is 6.1 X 10". Estimates of N 

 based on the kinetic theory are only ap- 

 proximate, and in many cases serve merely 

 to fix an inferior or superior limit to the 

 number of the molecules. Such estimates 

 are, however, of considerable interest and 

 historical importance, since for a long time 

 they served as the most reliable methods of 

 forming an idea of molecular magnitudes. 

 A very interesting and impressive method 

 of determining the value of N was given by 

 Lord Rayleigh in 1899 as a deduction from 

 his theory of the blue color in the cloudless 

 sky. This theory supposes that the mole- 

 cules of the air scatter the waves of light 

 incident upon them. This scattering for 

 particles, small compared with the wave- 

 length of light, is proportional to the fourth 

 power of the wave-length, so that the pro- 

 portion of scattered to incident light is 

 much greater for the violet than for the 

 red end of the spectrum, and consequently 

 the sky which is viewed by the scattered 

 light is of a deep blue color. This scatter- 

 ing of the light in passing through the at- 

 mosphere causes alterations of brightness 

 of stars when viewed at different altitudes, 

 and determinations of this loss of bright- 

 ness have been made experimentally. 

 Knowing this value, the number N of mole- 

 cules in unit volume can be deduced by aid 

 of the theory. From the data thus avail- 

 able, Lord Rayleigh concluded that the 

 value of N was not less than 7 X 10". 

 Lord Kelvin in 1902 recalculated the value 



