192 Mr. Charles Tomlinson on [Mar. 14, 



woolly-looking crystals after a slight rubbing, and although the wire 

 was rapidly withdrawn as soon as the crystals began to appear, it left 

 behind it a curious kind of trail, consisting of an axis of the form of 

 the wire, repeating its bends and irregularities, while small acicular 

 crystals started out at right angles to this line, and thickly studded 

 it soon after the whole of the solution became solid. Zincic acetate 

 (1 to 1) did not display crystals as soon as the side was rubbed, but 

 slowly, in the course of half-an-hour or so, the part rubbed was oc- 

 cupied by a dense crop of beautiful crystals, the points of which 

 passed beyond the axis of the tube. 



Sodic acetate solution does not readily yield to the effort of rubbing, 

 except in certain states of the weather when the crystallizing force is 

 strong ; but a case occurred to me some years ago which exactly suits 

 my present purpose. Several flasks containing a highly supersaturated 

 solution of sodic acetate, to which oil had been added without any 

 nuclear effect, were emptied into a small, stout cylindrical glass, where 

 it remained during some weeks without change, the layer of oil on its 

 surface preventing it from absorbing moisture. It was several times 

 stirred with the finger without effect, for this, on being introduced, 

 became coated with oil and so prevented contact. The finger was 

 then pressed against the strong side of the vessel with a considerable 

 amount of force and drawn slowly from the bottom upwards. The 

 solution immediately became solid. I attributed this effect at the 

 time to nuclear action, but I now regard it as an excellent illustration 

 of the point which I am seeking to establish. 



A supersaturated solution of sodic carbonate is very sensitive to any 

 interference with its adhesion. A speck of carbon or of ferrous oxide, 

 coming to the surface within the tube, is sufficient, on cooling down 

 the solution, to cause the salt to start from the speck in crystalline 

 lines in all directions, producing immediate solidification.* 



In addition to amber and resin some tubes were coated with other 

 substances, such as sulphur, shellac, stearine, and paraffin; but as 

 some of these, in addition to the objection of being opaque, melted 

 when the boiling solution came into contact with them, I need 

 not refer to them any further, except perhaps to paraffin, with 

 which an interesting result was obtained. A large tube was lined 

 with paraffin and passed, telescope fashion, over a smaller tube, con- 

 taining a nearly boiling solution of sodic sulphate. When cold, the 

 tubes were slowly reversed, so as to pour the solution into the coated 

 tube ; but it solidified as soon as it came into contact with the paraffin. 

 If, however, it were poured in while still warm, the solution accommo- 



* In the experiments which I had the honour of exhibiting to the Society after 

 the reading of my first paper (28th May, 1868), a case of this kind occurred and was 

 pointed out by my late lamented friend, William Allen Miller, as " a very prettj 

 effect." 



