On the transmission colours of sulphur suspensions

In the ‘Proceedings of the Royal Society,’ Keen and Porter described some interesting optical effects exhibited by suspensions of finely-divided sulphur, obtained by adding dilute sulphuric acid to a weak solution of sodium thiosulphate. As is well known, the solution (which is at first perfectly transparent) becomes turbid when the particles form in it, and the transmission of light by the suspension gradually diminishes in intensity. The colour of the transmitted light, which is at first white, also changes, becoming yellow, orange, red, and then deep crimson red. Finally, the solution (if in a sufficiently thick layer) becomes almost completely opaque. This had been previously supposed to terminate the sequence of phenomena. Keen and Porter observed, however, that after further lapse of time, light begins again to be transmitted by the suspension, the colour of the light which passes through being at first indigo, then blue, blue-green, greenish-yellow, and finally again white. This remarkable reappearance of the transmitted light was quantitatively studied by them, measurements being made of the intensity of the transmitted light in the various stages of the experiment, red and blue glass plates being used to approximately monochromatise the light of the source. Keen and Porter published curves showing the fraction of the incident light which is transmitted as a function of the time, and found that the shape of the curve is different for different parts of the spectrum, which is of course to be expected, in view of the colours exhibited by the solution when the incident light is white. The diameter of the sulphur particles was also found to increase with time. No attempt was, however, made to explain the observed phenomena on theoretical principles. The problem was then taken up by the late Lord Rayleigh, who attempted to investigate the effects on the basis of the mathematical theory of the scattering of light by small transparent spheres. The explanation of the phenomena observed in the earlier stages of the experiment presented no difficulty. As the particles grow in size, the suspension refuses to transmit, first the shorter waves, and then, finally, the whole visible spectrum. This is precisely what is to be expected, in view of the fact that the scattering power of the particles grows rapidly as their size in relation to their wave-length increases and the transmitting power of the suspension decreasespari passu with the increase in the proportion of the energy scattered. Lord Rayleigh did not, however, find it possible to explain the reappearance of the transmitted light in the later stages studied by Keen and Porter, and he went so far as to suggest that there might be some doubt whether the effect was really due to transmitted light in the technical sense of the term.