PREFACE. xxiii 



between the two above mentioned theories, and the reader is referred to his paper for 

 further information*. Maxwell in the memoir before us has also applied his theory to 

 the passage of light through crystals, and gets rid at once of the wave of normal vibrations 

 which has hitherto proved the stumbling block in other theories of light. 



The electromagnetic Theory of Light has received numerous developments at "the hands 

 of Lord Rayleigh, Mr Glazebrook, Professor J. J. Thomson and others. These volumes 

 also contain various shorter papers on Electrical Science, though perhaps the most complete 

 record of Maxwell's work in this department is to be found in his Treatise on Electricity 

 and Magnetism in which they were afterwards embodied. 



Another series of papers of hardly less importance than those on Electricity are the 

 various memoirs on the Dynamical Theory of Gases. The idea that the properties of 

 matter might be explained by the motions and impacts of their ultimate atoms is as 

 old as the time of the Greeks, and Maxwell has given in his paper on "Atoms" a full 

 sketch of the ancient controversies to which it gave rise. The mathematical difficulties of 

 the speculation however were so great that it made little real progress till it was taken 

 up by Clausius and shortly afterwards by Maxwell. The first paper by Maxwell on the 

 subject is entitled "Illustrations of the Dynamical Theory of Gases" and was published 

 in the Philosophical Magazine for January and July, 1860, having been read at a meeting 

 of the British Association of the previous year. Although the methods developed in this 

 paper were afterwards abandoned for others, the paper itself is most interesting, as it indicates 

 clearly the problems in the theory which Maxwell proposed to himself for solution, and so far 

 contains the germs of much that was treated of in his next memoir. It is also epoch-making, 

 inasmuch as it for the first time enumerates various propositions which are characteristic 

 of Maxwell's work in this subject. It contains the first statement of the distribution of velo- 

 cities according to the law of errors. It also foreshadows the theorem that when two gases 

 are in thermal equilibrium the mean kinetic energy of the molecules of each system is the 

 same; and for the first time the question of the viscosity of gases is treated dynamically. 



In his great memoir " On the Dynamical Theory of Gases " published in the Philo- 

 sophical Transactions of the Royal Society and read before the Society in May, 1866, he 

 returns to this subject and lays down for the first time the general dynamical methods 

 appropriate for its treatment. Though to some extent the same ground is traversed as in 

 his former paper, the methods are widely different. He here abandons his former hypothesis 

 that the molecules are hard elastic spheres, and supposes them to repel each other with 

 forces varying inversely as the fifth power of the distance. His chief reason for assuming 

 this law of action appears to be that it simplifies considerably the calculation of the 

 collisions between the molecules, and it leads to the conclusion that the coefficient of 

 viscosity is directly proportional to the absolute temperature. He himself undertook an 

 experimental enquiry for the purpose of verifying this conclusion, and, in his paper on the 

 Viscosity of Gases, he satisfied himself of its correctness. A re-examination of the numerical 



* British Association Report, 1885. 



