III. Fresh-water Ice. Flame 6 inches from nearest Thermometer.

IV. Fresh-water Ice. Flame 6 inches from nearest Thermometer. Temperature of outer air 34°. January 11, 1876.

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V. Fresh-water Ice. Flame 6 inches from nearest Thermometer.

VII. Sea-water Ice. Flame 8 inches from nearest Thermometer. Temperature of outer air 55°. February 29, 1876.

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VIII. Fresh-water Ice. Flame 6 inches from nearest Thermometer. Temperature of outer air 37°. January 3, 1876.

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III. "On the Function of the Sides of the Vessel in maintaining the State of Supersaturation." By CHARLES TOMLINSON, F.R.S. Received February 21, 1878.

Before any consistent theory can be framed of all the phenomena of supersaturated saline solutions, it is necessary to determine whether the sides of the vessel bear any, and what part, in maintaining the state of supersaturation.

It is remarkable that among the multitude of memoirs and papers that have been published on the subject of supersaturation generally, and of special phenomena in particular, my reading should not have made me acquainted with any special experimental researches conducted with the view of determining the point in question.

In 1819, Gay-Lussac* seeing how easily a supersaturated solution of sodic carbonate can be made to solidify, by merely shaking the vessel, expressed his opinion that the state of supersaturation depends, not on a chemical, but a purely mechanical force. "We cannot," he says, "fix the point at which supersaturation ends, since it is entirely accidental in each experiment, depending, as it does, on the nature of the vessel, its polish, its conducting power, and the agitation of the air."

In 1851 Lamyt arrived at the conclusion that the form, thickness, and nature of the vessel, and the quantity of solution contained therein, have no influence on the state of supersaturation.

* Ann. de Ch. et Phy., 2e Serie, xi, 303.

+ Comptes Rendus.

Löwel, whose researches extend over about ten years, namely, from 1848 to 1857, makes frequent reference, in the course of his six memoirs, to the sides of the vessel. He is of opinion that the asperities of the sides exert no action in determining crystallisation; that nothing certain can be affirmed as to the size of the tubes in producing such an effect; that it is not a mechanical action, as Gay-Lussac supposed; that heat deprives the sides of their active, or as he calls it, dynamic power; that even in an open tube the sides are not in a passive state; that nothing is positively decided as to the action of the sides. He also refers to that mysterious unknown force which holds the crystals in an abnormal state; he also speaks of the inner surface of the flasks recovering that particular property of determining crystallisation which heat had deprived it of, the cause of which is unknown; and in his last memoir he supposes that the sides determine the formation of the normal salt by an action of contact.

I have already expressed my opinion† that had Löwel worked with chemically clean flasks and tubes, he would not have encountered so many contradictory results, or have expressed so many uncertain and conflicting opinions as those above quoted.

In 1866 Jeannel threw out an opinion that the state of supersaturation is maintained in closed vessels by reason of the attraction of the sides and the saturation of the interior air.

I had long ago formed an opinion that the adhesion of the solution to the walls of the vessel is an important function in maintaining the state of supersaturation; and it seemed probable if this were so, that any force that could effectually detach the solution from a small portion of the side, below the surface, would cause the whole system to break down.

The most obvious mode of experiment seemed to consist in rubbing the interior surface with a clean wire. About five years ago I obtained a number of results in this way, but did not think them sufficiently important or trustworthy for publication. One source of fallacy soon became obvious. On moving the wire up and down in a nearly vertical direction, a film of the solution is being constantly dragged above the surface, which film, in consequence of rapid evaporation, disengages a molecule of the salt, and this acts as a nucleus to the whole of the solution. Attempts were made to prevent such an effect by tying a piece of linen dipped in hot water loosely round the neck of the tube, and passing the wire through the cloth into the tube, but the result was not satisfactory; for unless the crystals could be seen actually to start from the place rubbed and to spread from this point alone through the solution, no fair conclusion could be formed as to the effect of the rubbing. Friction with platinum, brass and steel wires seemed to * Ann. de Ch. et de Phy.

+ Proc. Roy. Soc., xvi, 408.

Comptes Rendus, 2nd January.

have little or no effect on solutions of sodic sulphate and sodic acetate. The most satisfactory results were obtained with magnesic sulphate; but with this and other solutions the results varied with the state of the weather as the crystallising force varied, and also with the nature of the rubber, whether a glass rod or the wires just named.

At length it occurred to me to line the tubes with some substance which is not wetted by the solution or imperfectly so. Accordingly, two large tubes (5 oz. and 3 oz.) were made chemically clean, and coated with a solution of amber in chloroform. They were three parts filled with a solution of sodic sulphate (2 to 1), covered over, and left until the next day. A stout platinum wire was taken out of hot water and introduced into each tube, when moderately hard friction was made in a vertical line about one-sixth or one-eighth of an inch in length. A crystalline brush immediately diverged from the part rubbed, and spread all through the solution, the mass of which was sufficient for the eye to note the details, and to convey the assurance that crystallisation set in from the part rubbed and from that only.

The advantage of using amber is that the coating, being transparent, allows the progress of the experiment to be watched. The same advantage may, however, be obtained at less cost and trouble, by means of resin, a small quantity of which, in powder, together with a few drops of spirits of wine, heated in the tube over a spirit lamp, enables the operator easily to spread a coat evenly upon the inner surface by moving the tube about while it is still warm.

A number of one-ounce tubes, coated in this way, were filled with a stronger solution of sodic sulphate (3 to 1) than that before employed. The result was satisfactory: the solutions often became solid in cooling, and always when the inner surface was rubbed with the platinum wire.

The experiment was also made comparative by placing an uncoated by the side of a coated tube, and so leaving them to cool covered with glass capsules, or small beakers. The solution in the coated tube often solidified in cooling, and always remained liquid in the uncoated tube,

The results as obtained with supersaturated solutions of other salts also showed how greatly they depend for their stability on adhesion. The system always broke down when a portion of the solution was detached from the side, by the rubbing action of the wire. The solution of each salt, however, presented those characteristic features of its own, which form one of the principal charms in the study of natural objects. In the ammonia alum solution (1 to 1) the octohedral crystals, under the action of rubbing, seemed to bound forth towards the axis of the tube. In the magnesic sulphate solution (3 to 1) the disengaged crystals were too minute to appear, except as a chalky-white line following the motion of the wire. Zincic sulphate (3 to 1) formed small