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ART. XIV. On Saline Crystallization. By ANDREW URE, M. D. Professor of the Glasgow Institution, and Member of the Geological Society.
It is well known that water and solutions of some salts will, under certain circumstances, preserve their fluidity, though cooled down many degrees below their ordinary congealing or crystallizing temperature. The most convenient, and perhaps striking mode of exhibiting this phenomenon, is to fill a phial with a hot saturated solution of sulphate of soda, to cork it well, and set it to cool in a quiet place. When the cork is afterwards withdrawn, the liquid stratum at the surface instantly becomes a solid, with a pearly lustre ; then the parts beneath successively assume the same form, and thus the whole solution in a few seconds changes into a confused crystalline mass. At the instant of solidification the temperature arises from 30 to 40 degrees, affording a fine illustration of the doctrine of latent heat. Should the mere extraction of the cork fail to occasion this curious transition from the liquid to the solid state, it will be infallibly produced by touching the surface of the solution with a crystal of the salt.
This phenomenon has been ascribed by some, to the sudden abstraction of a portion of heat from the liquid, on the admission of the external air; whence the crystallization is determined at the surface, and proceeds gradually downwards; other hypotheses have also been offered; but I am not aware of any experiments having been made, with the view of elucidating the connection of this phenomenon with the atmospheric air and other causes. The following, which were executed about two years ago, may perhaps serve to open some new trains of investigation.
1st. An 8 oz. phial filled with a saturated solution of sulphate of soda in boiling water was cooled down to 38° Fahr., (the temperature of the apartment,) without crystallizing, the mouth of it being well secured with ox-bladder, which the pressure of the atmosphere had deeply hollowed on the contraction of the liquid volume by cold. The phial was placed under the receiver
of an air-pump. When the quicksilver in the two legs of the syphon gauge stood nearly on a level, the bladder became convex, though it had been strained flat across the orifice when the phial was brimful of the boiling hot solution. On piercing the bladder with a sharp pointed rod, which passed air-tight through a collar of leathers in the top plate of the receiver, no change whatever took place; the point was then pushed down into the liquid without any further effect. To try the influence of vibratory agitation of the particles, the handle of the air-pump was briskly moved backwards and forwards; when immediately a portion of the solution was thrown out of the phial with an explosive effort upon the receiver. The liquid instantly shot into a confused crystallization, and at the same time began to boil, from the heat evolved during solidification. The temperature of the saline matter in the phial, when it was withdrawn at the end of a few minutes, was still 74°, though it must have lost much heat from the ebullition in vacuo.
2. The same experiment was tried a second time; and though no effect took place on piercing the bladder, yet on afterwards passing down the steel point into the liquid, crystillization, with the usual pearly appearance, commenced at the surface, and descended to the bottom. I believe that in this case the steel point at its first descent to puncture the bladder had touched the liquid, and thus became tipped with a little saline matter, which drying rapidly in vacuo, served at the second plunge as a nucleus of crystallization.
3d. The bladder was freely punctured in vacuo without effect; the air was slowly admitted into the receiver till it was of the atmospheric density. No change ensued at the end of two hours. The receiver was withdrawn, and the punctured hole enlarged by scissars without any result. But on slightly shaking the phial, the liquid passed speedily into the solid state. 4th. The sulphate was again liquified in the same phial, which was now closed with a perforated cork. Into this was fitted a glass tube inch internal diameter, and 4 inches long. As the tube passed only half way down through the cork, no liquid could
enter it. After standing for fourteen hours exposed to the action of the external air,* through the tube, its liquidity was unimpaired. The glass tube was then withdrawn, and next the cork, without any change ensuing; when finally, on agitation, it solidified
5th. A phial was nearly filled with a similar solution of sulphate, on the surface of which was placed a little olive oil. It cooled without crystallizing. When smartly agitated, it became solid with the usual phenomenon of the crystals shooting from the top downwards. This phial had been first placed on a vibrating glass plate, without effect. This experiment was repeated with a like result, though the phial stood two days.
6th. A corked phial full of the hot solution was tied down to the plate of the air-pump, so that the cork could be drawn in vacuo by a sliding steel rod and hooked extremity. When the cork was pulled, no change ensued; but agitation congealed the
7th. The corked phial was cooled in a horizontal position; on inverting it quickly, the liquid struck against the glass, as in the water hammer. By brisk agitation in the inverted position, congelation began, first below, and ascended to the top of the liquid. This experiment was repeated, with the same result. No particle of air was left in the phial; a cork of the best quality being pressed on the surface of the liquid, and forced in as the liquid contracted its volume, on immersing the phial into a basiu of cool water.
8th. A glass tube twelve inches long, and one inch in diameter, furnished with a brass cap and stop cock at one end, and a tight cork at the other, was filled with the hot solution. When it was cold, each end was opened, and the crystallization began instantly at the two extremities, and proceeded to the middle.
9th. Same tube filled and cooled, with a platina wire passing through the cork. On applying to each end, the opposite
Temperature of the apartment about 40° Fabr.
electric influences of a voltaic battery of 50 pairs of 4-inch plates, the pearly crystallization commenced at the negative end, and proceeded slowly to the positive, at which no symptom of spontaneous congelation could be perceived. The platina wire was positive, and evolved oxygen pretty copiously.
10th. A large egg-shaped vessel, holding about two pints, and terminated at each end with cylindrical apertures of one inch diameter, was filled with the hot saturated solution. Through the cork of each end was passed a platina wire. The vessel, after having been cooled in a bason of cold water to the temperature of 42°, was placed in a horizontal position, and the solution was subjected to the action of a battery of 60 pairs of 4-inch plates. From the oppositely electrified wires, hydrogen and oxygen gases were copiously evolved. The quantity of gas was much more abundant than I ever observed it from pure water with the same voltaic power. Hence, a saline solution affords a better medium for the popular exhibition of this fundamental electro-chemical fact, than water alone. After a few seconds, the pearly lustre appeared at the negative end of the vessel, and the crystallization proceeded slowly and steadily towards the positive end, the plane of demarcation between the congealing and liquid part being perfectly smooth and vertical. No tendency to solidification was observed at the positive end, though gas freely flowed from its platina wire during the whole time that the process of crystallization was advancing from the one extremity to the other. This interval was about 15 seconds.
10th. The above experiment was repeated with a small cylinder with tubular extremities. The negative wire projected internally only to one half the length of the positive wire, in order to see whether it was merely the greater quantity of hydrogen evolved, or some difference in the electrical properties that determined the crystallization at the negative pole. Here again, as before, the pearly appearance commenced at the negative extremity, and proceeded beautifully to the positive.
It was impossible now to doubt, that there existed some relation between negative or resinous electricity, and saline crystallization.
11th. Two glass capsules were then taken. Into each an equal quantity of a tepid solution of pure nitre was put. They were placed along side of each other, and the liquids were connected by a slip of clean filtering paper, moistened with pure water. The power of 60 pairs in moderate action was applied, through the medium of a platina wire dipped into the centre of each solution. In a short time small needles were seen collecting, and attaching themselves around the negative platina wire, which soon increased so as to float through the whole liquid. After a much greater interval a few crystals were perceived forming on the margin of the liquid in the positively electrified capsule, but none near the immersed platina wire. In equal times the quantity of crystals in the negative capsule was quadruple of that in the positive capsule. There was found in the former a very slight excess of alkali, and in the latter of acid, but such as in ordinary circumstances has no influence on the formation of crystals.
12th. A tin flask was filled with the same hot solution, and having its mouth secured with a slip of ox-bladder, it was suffered to cool. It was then placed on the top of a delicate condensing electroscope; and the bladder being pierced with a needle insulated at the end of a glass rod, no divergence of the gold leaves could be observed, even when both the large and small condensing plates had been folded back. I am not certain that in this case the liquid had remained uncrystallized till the instant of piercing the bladder. I shall repeat and vary the experiment, and from the elevation of temperature accompanying the solidification, I shall be able to ascertain whether the experiment has been successfully conducted, and whether any general inference can be drawn from it.
I think it probable from the above detail, that negative electricity may be found a useful agent in promoting the crystallization of saline matter, and may perhaps be employed by Nature in her crystalline formations.