4, Moreton Square, W.C.

6 July 1861.

My Dear Sir,

I am much gratified by your attention to the condition of the colloidal solutions described in my paper, and shall take an early opportunity to forward a few specimens, as you desire, for your more particular study.

After reflecting upon your remarks I am inclined to think that you will obtain some light from the observation of silicic acid, one of these solutions which has the advantage of being colourless. The liquid condition of all such bodies, it is to be remembered, is not permanent. Now in silicic acid the transition to the gelatinous (solid) form is visibly preceded by a faint opalescence of the liquid. This gradually increases during a few hours or even days, and is sometimes very beautiful. But it is sure to end, sooner or later, in the somewhat sudden solidification of the mass. The previous opalescence may very well be due to suspended solid matter, like Faraday's highly divided gold, as your theory supposes. It is however the effect of an incalculably minute amount of suspended matter, and does not touch the great mass of colloidal matter present. In short the opalescence may be due to suspended matter, although the colloid is truly liquid. Indeed one or two per cent, of such substances generally produce a firm jelly, on passing from the liquid condition, while the solutions operated upon were perfectly limpid with from 3 to 10 per cent, of substance in solution.

That colloids are really in solution appears also to follow from the fact that they are diffusive. They possess the property, and of several the rate has been accurately observed.