papers the ideas here originated received further development. Meanwhile other phenomena were interesting him. He had already (1855) written on the theory of colours in relation to colour-blindness, and in a paper on ‘Experiments on Colour as Perceived by the Eye’ (Phil. Trans. Roy. Soc. Edin. vol. xxi. pt. ii.), he had investigated the effects of combinations of various colours by means of the rapid rotation of discs coloured differently in different parts. Maxwell's colour-top is now well known. The main results of his work on colour are summed up in his paper ‘On the Theory of Compound Colours,’ read before the Royal Society 22 March 1860 (Phil. Trans. 1860). His instrument, the colour-box, by which he investigated the effect of mixing in given proportions light taken from different parts of the spectrum, is first described, and then it is shown that any given colour sensation may be produced by combinations in due proportion of rays taken from three parts of the spectrum, and also that if we select three definite rays as standards, all other colours may be produced by proper combinations of these. In the most general case it may be that, to produce a given colour, we should have to subtract a certain amount of the third colour C, from the two other colours A and B, taken arbitrarily. This would mean that the effect of mixing the given colours, and a proper amount of C, just matches the mixture of A and B, but it is further shown that there are three primary colours by arithmetical addition of which, in proper proportions, any other colour may be produced. Probably these three different elements of colours correspond to three different sensations in the eye, and a body appears to us of a definite colour because it excites these sensations each in its proper proportion. The experiments tended to confirm the conclusion that colour-blindness is due to the absence of one of the three primary sensations. For this work Maxwell was awarded the Rumford medal of the Royal Society 30 Nov. 1860.
Meanwhile Maxwell had been engaged on his essay ‘On the Stability of Motion of Saturn's Rings,’ which gained the Adams prize in 1857. Laplace had shown that the ring could not be solid, for if so it would be unstable, the slightest displacement of its centre from the centre of the planet would originate a motion, which would ultimately destroy the whole.
Maxwell considered the effect of loading the ring at one or more points, and showed that if the load were great enough we could account for the motion on known laws, but if this were so, the load must be so great, that it would be visible as a satellite, and this is not the case. There then remained the assumption that the ring is fluid, or else consists of a large number of very small separate solid particles. Either of these hypotheses was proved to give a possible form of motion, and the latter in all probability is the nature of the ring.
It may have been the discrete particles of Saturn's rings that led Maxwell to study the kinetic theory of gases. According to this theory, the pressure which a gas exerts is due to the impact of its molecules on the walls of the enclosing vessel; the temperature depends on the average energy of the motion. This had been clearly pointed out by Herapath in 1847, and in 1848 Joule, assuming that all the molecules of the gas possessed the same velocity of agitation, determined the relation between the velocity and the pressure, and calculated the former for hydrogen and other gases at a definite pressure and temperature. Clausius in 1857 and 1859 extended the work, making the same hypothesis as to the velocity of the individual molecules, and introduced the idea of the mean free path.
Maxwell's first papers on the subject appeared in the ‘Philosophical Magazine’ (January and July 1860). He pointed out that the velocities of the different molecules, even if equal to start with, would become unequal immediately in consequence of the collisions. He therefore devised the statistical method of treating the problem. On this method the whole number of molecules are divided into a series of groups, the velocities of all the molecules constituting a group, being the same within narrow limits, and the average velocity of each group is considered. He also found the law connecting this average velocity with the number of molecules in the group, and showed that when a state of permanence, that is of uniform temperature, has been reached, in the case either of a single gas or of a mixture, the average energy of agitation is the same throughout. From these considerations and on the supposition that the mean energy of agitation measures the temperature, the laws of Gay Lussac and Charles are deduced. The theory of diffusion had been given by Herapath, Maxwell extended it, and by applying similar reasoning to the diffusion of the momentum and the diffusion of the energy, explained the phenomena of viscosity and of conduction of heat respectively. The law of Dulong and Petit connecting the specific heat and the molecular weight was shown to follow, but difficulties of a serious nature were met with when the theory was applied to deduce the elation between the specific heats of a gas at constant pressure and volume respectively.
