Xll 



may slide over the surface with a finite velocity, and that this velocity 

 and the corresponding tangential stress are affected by inequalities of 

 temperature at the surface of the solid, tending to make the gas slide 

 from colder to hotter places." The latter result, as a deduction 

 from the Kinetic Theory, had been previously obtained in another 

 way by Professor 0. Reynolds. The stress which is the result of 

 this theory would help to account for the radiometer motion. 

 In this memoir are interpolated various notes dated May, 1879, in 

 which the application of Spherical Harmonics is briefly indicated as a 

 means of simplifying the mathematical parts of the "work. The failure 

 of the author's health prevented him from afterwards following out 

 this interesting extension of the subject. 



Another important memoir was presented to the Cambridge Philo- 

 sophical Society in May, 1878, " On Boltzmann's Theorem on the 

 Average Distribution of Energy in a System of Material Points." 

 This paper is remarkable alike for the comprehensive character of the 

 assumptions as regards the forces acting between the points, the close- 

 ness and condensation of the reasoning, and the generality of the 

 theorems proved. It is impossible, however, to describe them with 

 sufficient brevity in a short notice. Passing to the particular problem 

 of the equilibrium of temperatures of two gases, we find the author 

 once more examines Gay-Lussac's law, and points out that the 

 theorem that the average kinetic energy of a single molecule is the 

 same for molecules of different gases, is not sufficient for the 

 equilibrium of a mixture of gases, because we have no means of 

 finding out the separate temperatures of the two ; the case would 

 be different if the gases were separated by a diaphragm. Another 

 interesting application of the general results is made to the theory 

 of Loschmidt's experiments on the diffusion of gases. These ex- 

 periments were made with a tube into which was put a gas or a 

 mixture of gases, and the tube was then whirled about one end ; and 

 the ultimate distributions of the gases, with the times of attaining to 

 them, formed the subject of investigation. 



From this review of Maxwell's chief contributions to the Kinetic 

 Theory of Grases, imperfect though it be, it will be seen that, if he 

 was not the discoverer of this new region of inquiry* he yet explored 

 it more thoroughly than any of his predecessors ; he bore a leading 

 part in evolving the general laws to which it must be subject, 

 reducing it to order, and pointing out in what directions it would be 

 most fruitful. 



Maxwell took up the theory of Colour Perception at the point where 

 Young had left it. According to Young's theory, there are three 

 distinct varieties of nerves, sensitive to red, green, and violet light 

 respectively, and by the superposition of these sensations when 

 aroused in the sensorium the colours of external objects are 



