CONTRIBUTIONS TO SCIENCE. 567 



mines pressure and viscosity ; and (3) the phenomena 

 depending on the average value of the cubes of the 

 velocities, on which depends the diffusion of energy or the 

 conduction of heat in the gas. In this paper Maxwell 

 showed that if a number of gases be enclosed in any space 

 under the action of any external forces, each gas will arrange 

 itself as if the others were absent, and this result is independ- 

 ent of the law of action of the particles on each other. This 

 is the arrangement which Dalton suggested would probably 

 exist in the atmosphere if the effect of winds could be an- 

 nulled. He also showed that in a vertical column of air in 

 equilibrium under the action of gravity the temperature must 

 be the same throughout. This was contrary to the then cur- 

 rent opinion that the temperature must diminish as we ascend, 

 but Maxwell showed independently that it is a consequence 

 of the principle of the dissipation of energy. Boltzmann was, 

 however, the first to show how, in dealing with a collection of 

 molecules, to take account of external forces acting upon 

 them. In discussing the diffusion of energy Maxwell showed 

 that the thermal conductivity of iron at 25 C, as determined 

 by Principal Forbes, is 3525 times that of air at 16*6 C. 



In the paper " On the Dynamical Evidence of the Mole- 

 cular Constitution of Bodies," published in the Journal of 

 the Chemical Society, June 1875, Maxwell introduces 

 Clausius's conception of the " virial " of a system, a quantity 

 of which great use has been made in the discussion of 

 molecular theories since a name has been given to it by 

 Clausius. This quantity is the sum of the products of the 

 attractions between each pair of molecules into the distance 

 between them. Maxwell commences by explaining Clausius's 

 equation, which states that the energy of a quantity of gas 

 is the sum of two quantities, one of which is the virial, and 

 the other is proportional to the product of the pressure and 

 the volume. Now Joule showed that when a gas expands 

 into a vacuum its temperature remains sensibly unaffected, 

 though no energy is communicated to or taken from it, and 

 since the gas obeys Boyle's law the product of its pressure 

 and volume is constant. Hence the energy being unchanged, 



