The Action of Very Minute Particles on Light

Abstract

THE action upon transmitted light of very minute particles suspended in a transparent medium is very well known, thanks to the investigations of Brücke, Tyndall, and others, up to a certain point. That is to say, that white light, passing through varying depths of a medium with such particles more or less thickly interspersed, is known to emerge coloured yellow, orange, or red, according to the extent of the action in question. Wishing to illustrate this phenomenon experimentally, I employed a very dilute solution of sodium thiosulphate (hyposulphite), which was acidified with hydrochloric or sulphuric acid, and then allowed to stand, observing from time to time the appearances when examined by transmitted light. The solution mentioned is admirably adapted for the purpose, inasmuch as the precipitation of the sulphur proceeds gradually; and, according to the greater or less dilution at starting, the completion of the reaction can be spread over a long period of time, in some of my experiments occupying more than forty-eight hours. For a while no turbidity whatever is visible; then a faint opalescence makes its appearance, and these exceedingly minute particles grow gradually in size, remaining, however, quite uniformly suspended for a considerable period, until a dimension is reached which causes them to settle out of the liquid. In this way I observed with unfailing regularity, and in unvarying order, though with various degrees of rapidity, an extension of the series of colours, which, so far as I am aware, had not previously been noticed, or at any rate published. From orange, the tint passed successively through rose red, purplish rose, to a full purple; then by insensible gradations to a fine violet, blue, green, greenish yellow, neutral tint, &c.