878 TRANSACTIONS OF SECTION A 
required to form a known volume of gas has been measured. May we not 
conclude that the gas is discrete in structure, and that this number repre 
sents the actual number of atoms in the gas? 
We have seen that under special conditions it is possible to detect easily 
by an electrical method the emission of a single a particle—i.e., of a single 
charged atom of matter. This has been rendered possible by the great 
velocity and energy of the expelled a particle, which confers on it the power 
of dissociating or ionising the gas through which it passes. It is obviously 
only possible to detect the presence of a single atom of matter when it is 
endowed with some special property or properties which distinguishes it from 
the molecules of the gas with which it is surrounded. There is a very 
important and striking method, for example, of visibly differentiating 
between the ordinary molecules of a gas and the ions produced in the gas 
by various agencies. C.T.R. Wilson showed in 1897 that under certain 
conditions each charged ion became a centre of condensation of water 
vapour, so that the presence of each ion was rendered visible to the eye. 
Sir Joseph Thomson, H. A. Wilson, and others have employed this method 
to count the number of ions present 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 extension in space. Lord Rayleigh has drawn attention to the 
fact that the earliest estimate 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 favourite one with Lord Kelvin, who developed 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 number of molecules in a cubic 
centimetre of any gas at normal pressure and temperature. This number, 
which will throughout be denoted by the symbol N, is a fundamental con- 
stant of gases; for, according to the hypothesis of Avogadro, and also on 
the kinetic theory, all gases at normal pressure and temperature have an 
identical number of molecules in unit volume. Knowing the value of N, 
approximate estimates can be made of the diameter of the molecule; but 
in our ignorance of the constitution of the molecule, the meaning of the 
term diameter is somewhat indefinite. It is usually considered to refer 
to the diameter of the sphere of action of the forces surrounding the 
molecule. This diameter is not necessarily the same for the molecules 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 Loschmidt, Johnstone Stoney, and Maxwell. From the data then at 
his disposal, the latter found N to be 1°9 X 10". Meyer, in his ‘ Kinetic 
Theory of Gases,’ discusses the various methods of estimating the dimen- 
sions of molecules on the theory, and concludes that the most probable 
estimate of N is 61 x 10’. Estimates of N based on the kinetic theory 
are only approximate, and in many cases serve merely to fix an inferior 
or superior limit to the number of the molecules. Such estimates are, how- 
ever, 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 
