CONTRIBUTIONS TO SCIENCE. 561 



of the molecular theory of gases, as well as for its establish- 

 ment on a sound dynamical basis, mainly to the researches 

 of three men Professor E. Clausius, Dr. Ludwig Boltz- 

 mann of Vienna, and James Clerk Maxwell. Maxwell's 

 principal contributions to the literature of this subject are 



papers on "Illustrations of the Dynamical Theory of 

 Gases," presented to the British Association at the meeting 

 in Aberdeen in 1 8 5 9 ; " On the Viscosity or Internal Friction 

 of Air and other Gases," which constituted the Bakerian 

 lecture read before the Eoyal Society on February 8th, 1866, 

 and published in the Phil. Trans, for that year ; " On the 

 Dynamical Theory of Gases," a paper read before the Eoyal 

 Society on May 31st, 1866, and also published in the Phil. 

 Trans. ; the lecture " On Molecules " delivered before the 

 British Association at Bradford in 1873, and which has 

 been referred to in Part I. ; a paper " On the Dynamical 

 Evidence of the Molecular Constitution of Bodies " which 

 was read before the Chemical Society in the Spring of 1875, 

 and published in the June number of its Journal ; and the 

 article " ATOM," in the ninth edition of the Encyclopaedia 

 Britannica. 



According to the molecular theory all bodies are made 

 up of molecules which are more or less free to move relatively 

 to one another. In solids, each molecule can only move 

 through a small distance from its normal position, so that 

 all its movements are essentially of the nature of vibrations. 

 In liquids the molecules are free to move through any 

 distances within the substance of the liquid, but their motion 

 is constantly impeded by neighbouring molecules, from whose 

 interference they are never altogether free, each molecule in 

 its movements resembling an individual endeavouring to 

 work his way through a dense crowd. In a gas each molecule 

 is perfectly free except on tlie occasions, comparatively rare, 

 unless the gas be very dense when it comes into collision 

 with other molecules of gas, or of some other body in contact 

 with the gas. According to this theory, a gas consists of 

 molecules moving in all directions in straight lines, except 

 when they strike one another or some foreign body, when 



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