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ATOM. 



different combining weights, and probably different chemical properties of other 

 k Aa no chemist has yet obtained specimens of hydrogen differing in this 

 way faxn other |HHff' M "-, we conclude that all the molecules of hydrogen 

 are of wnsibly the same mass, and not merely that their mean mass is a statis- 

 tical constant of great stability. 



But as yet we have not considered the phenomena which enable us to 

 form an estimate of the actual mass and dimensions of a molecule. It is to 

 Clau*iu* that we owe the first definite conception of the free path of a molecule 

 and of the mean distance travelled by a molecule between successive encounters. 

 He shewed that the number of encounters of a molecule in a given time is 

 proportional to the velocity, to the number of molecules in unit of volume, 

 and to the square of the distance between the centres of two molecules when 

 they act on one another so as to have an encounter. From this it appears 

 that if we call this distance of the centres the diameter of a molecule, and 

 the volume of a sphere having this diameter the volume of a molecule, and 

 the sum of the volumes of all the molecules the molecular volume of the gas, 

 then the diameter of a molecule is a certain multiple of the quantity obtained 

 by diminishing the free path in the ratio of the molecular volume of the gas 

 t-i the whole volume of the gas. The numerical value of this multiple differs 

 slightly, according to the hypothesis we assume about the law of distribution 

 of velocities. It also depends on the definition of an encounter. When the 

 molecules are regarded as elastic spheres we know what is meant by an 

 encounter, but if they act on each other at a distance by attractive or 

 repulsive forces of finite magnitude, the distance of their centres varies during 

 an encounter, and is not a definite quantity. Nevertheless, the above state- 

 ment of Clausius enables us, if we know the length of the mean path and 

 the molecular volume of gas, to form a tolerably near estimate of the diameter 

 of the sphere of the intense action of a molecule, and thence of the number 

 f molecules in unit of volume and the actual mass of each molecule. To 

 complete the investigation we have, therefore, to determine the mean path and 

 the molecular volume. The first numerical estimate of the mean path of a 

 gaseous molecule was made by the present writer from data derived from the 

 internal friction of air. There are three phenomena which depend on the 

 length of the free path of the molecules of a gas. It is evident that the 

 greater the free path the more rapidly will the molecules travel from one 

 part of the medium to another, because their direction will not be so often 



