The Boltzmann-Maxwell law applies only to cases in which the temperature is uniform throughout. In a paper published in the Philosophical Transactions for 1879, on “Stresses in Rarefied Gases Arising from Inequalities of Temperature,” Maxwell deals, among other matters, with the theory of the radiometer. He shows that the observed motions will not take place unless gas, in contact with a solid, can slide along the surface of the solid with a finite velocity between places where the temperature is different; and in an appendix he proves that, on certain assumptions regarding the nature of the contact of the solid and the gas, there will be, even when the pressure is constant, a flow of gas along the surface from the colder to the hotter parts. Among his less important papers bearing on molecular theory must be mentioned a lecture on “Molecules” to the British Association at its Bradford meeting; “Scientific Papers of Clerk Maxwell,” vol. ii., p. 361; and another on “The Molecular Constitution of Bodies,” Scientific Papers, vol. ii., p. 418. In this latter, and also in a review in _Nature_ of Van der Waals’ book on “The Continuity of the Gaseous and Liquid States,”[56] he explains and discusses Clausius’ virial equation, by means of which the variations of the permanent gases from Boyle’s law are explained. The lecture gives a clear account, in Maxwell’s own inimitable style, of the advances made in the kinetic theory up to the date at which it was delivered, and puts clearly the difficulties it has to meet. Maxwell thought that those arising from the known values of the ratio of the specific heats were the most serious. In the articles, “Atomic Constitution of Bodies” and “Diffusion,” in the ninth edition of the _Encyclopædia Britannica_, we have Maxwell’s later views on the fundamental assumptions of the molecular theory. The text-book on “Heat” contains some further developments of the theory. In particular he shows how the conclusions of the second law of thermo-dynamics are connected with the fact that the coarseness of our faculties will not allow us to grapple with individual molecules. The work described in the foregoing chapters would have been sufficient to secure to Maxwell a distinguished place among those who have advanced our knowledge; it remains still to describe his greatest work, his theory of Electricity and Magnetism. CHAPTER IX. SCIENTIFIC WORK.--ELECTRICAL THEORIES. Clerk Maxwell’s first electrical paper--that on Faraday’s “Lines of Force”--was read to the Cambridge Philosophical Society on December 10th, 1855, and Part II. on February 11th, 1856. The author was then a Bachelor of Arts, only twenty-three years in age, and of less than one year’s standing from the time of taking his degree. The opening words of the paper are as follows (Scientific Papers, vol. i., p. 155):--