ARTICLE II.

Observations on Crystallization. By John Redman Coxe, M. D.
Professor of Chemistry, Philadelphia.

THE efficacy of temperature in augmenting the solvent power of liquids is laid down by most chemical writers. This is more especially the case with the class of salts; to which, however, some exceptions occur, as in muriate of soda, which is nearly equally soluble in boiling water, and in water at the common atmospheric temperature. There is, nevertheless, something as yet not well understood, that appears to me operative in such cases, independent entirely of temperature, even in the instances of our most soluble salts, as Glauber's, or the sulphate of soda and some others.

It is almost universally asserted by authors on the subject, that atmospheric pressure is essential to the crystallization of salts; and the proof advanced is, that if a phial, nearly filled with a boiling saturated solution of Glauber's salt, be closely corked whilst filled with vapour, so as to exclude the atmospheric pressure; this solution will remain, even when cold, perfectly fluid, and may be shaken without becoming solid: but if the cork be withdrawn, the sudden impulse, from the air rushing into the phial, immediately induces the crystallization of the mass, with a sensible evolution of heat.

Now this beautiful and interesting experiment, which is usually shown in every course of chemical lectures, certainly at first sight appears to prove the position advanced. There are, however, numerous objections to its truth; yet so numerous are the anomalies that present themselves in experimenting upon this subject, that I am unable to form any theory or speculation on their causes.

1. If the above position were true, then certainly, by a parity of reasoning, we should expect every other saline solution, in which a boiling heat is employed to promote its fullest state of saturation, to be affected in a similar way; but this is not the case as far as I have tried it. Nitrate of potash and muriate of ammonia, both nearly as soluble as Glauber's salt, when secured from atmospheric pressure, by corking the phial, or tying a bladder over the mouth, precipitate in regular crystals as the solution cools. This fact alone is sufficient to overturn the theory advanced to explain the case stated of the Glauber's salt;-but,

2. A perfectly saturated solution of Glauber's salt, thus carefully corked at a boiling heat, has repeatedly crystallized throughout, without any exposure to the atmospheric pressure; whilst a solution of equal strength, and prepared and secured in every respect as the former, has, whilst standing beside it, remained perfectly fluid.

3. Saturated solutions of salts as above, uncorked, evince the

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same results. I have kept some vessels thus exposed to the full atmospheric pressure for three days, without any consolidation; and others, during all the intermediate periods, with similar results. Sometimes one or more will crystallize, whilst others continue fluid. I have made these experiments in phials holding from two drachms to 16 ounces; in receivers of a globular and oval shape, from half a pint to half a gallon; some with short, and others with long necks; and in open glass jars of one to two inches diameter, and eight or nine long; so that the form of the vessel in no way appears to influence the result, Nor has the quantity of solution in the vessel any influence, since it is the same when filled to the top, of when only filled to one-fourth or one-fifth part. The result was the same when I employed the common Glauber's salts of the shops, the native, or the artificial, thade by the direct combination of the constituents. In one experiment made with the artificial sulphate I filled three equal phials, two were closely corked, the third remained open, and all were placed beside each other to cool. In four hours one of the corked solutions was regularly crystallized in solid transparent crystals, one-fifth only of the mass being in a liquid state, which did not consolidate by shaking, or by withdrawing the cork. The contents of the other corked, and of the uncorked phial, both continued fluid; and both became solid by shaking, without withdrawing the cork of the closed one.

4. Solutions as above, after remaining exposed, have even not crystallized when briskly shaken, and some time afterwards without any apparent cause, have assumed the solid form.

5. Solutions as above, and closely secured, have failed to become solid, when the cork has been drawn, or the bladder punctured, for some moments, and even minutes; and in a few cases when even agitation was employed in addition: and these, in like manner, when least expected, have suddenly crystallized.

6. Solutions as above, both corked and uncorked, have gradually deposited regular transparent firm crystals,* in some instances two inches in length; in others, in irregular masses, at the bottom of the vessel-the fluid above, in these cases, continuing clear and saturated; and when shaken, sometimes consolidating in the usual way.

7. Solutions as above, both corked and uncorked, after thus depo siting these regular crystals at the bottom, have, without an appa rent cause, become consolidated above them, whilst remaining untouched.

8. Solutions as above (especially in a mattrass with a neck nearly two feet long), have, after considerable exposure and frequent agitation, refused to crystallize, even although continued at intervals for

The crystals which form suddenly in these solutions are always of a soft, spongy, silky, striated, appearance; and do not exhibit the firm, transparent, glassy, appearance of the common crystals of Glauber's salt.

more than an hour; yet by then turning the vessel, so as to pour out a little from the neck, the crystallization has immediately oecurred.

9. The same solution in the mattrass above mentioned has frequently become completely crystallized when left uncorked; at other times a large mass, equal to half the volume of the solution, has crystallized regularly, in hard transparent crystals, the remainder of the solution continuing fluid.

10. Saturated mixed solutions of nitre and Glauber's salt, corked closely, have allowed the nitre to crystallize regularly at the bottom; whilst the Glauber's salt remained fluid, and on drawing the cork became solid in the usual way.

11. Solutions, by no means saturated, evince similar results with the above fully saturated ones, although not in so strongly marked a

manner.

12. One of the most singular and interesting facts connected with these experiments is, that in those cases in which (either in the corked or uncorked solutions) regular, firm, transparent crystals form, so soon as the residuary saturated solution above them solidifies, either spontaneously, or by shaking, drawing the cork, &c. an immediate (or nearly so) opalescence, or loss of transparency, ensues in those first formed crystals, which gradually increases to a beautiful porcelainous whiteness. This I have almost invariably noticed under the above circumstances: I believe it arises from the gradual abstraction of the water of crystallization of the first formed regular crystals, by the mass of secondary crystals; for in one experiment made, I found the porcelainous mass, when dissolved in water, and regularly recrystallized, afforded a quantity of transparent crystals, superior in weight to those I employed, which could only arise from their re-obtaining their thus lost water of crystallization. How the secondary crystals operate in withdrawing this water from the first, I cannot form the most distant idea.

13. In those solutions in which spontaneous crystals have formed, in the course of a few days, if the secondary crystallization does not take place, a complete truncation of the summits of the crystals occurs, gradually forming a level of the whole, as in common cases; yet in several instances the solution above was sufficiently saturated to consolidate when shaken.

14. In one experiment two equal sized phials were filled to the top with saturated solutions; one was corked, the other was left open in two hours the uncorked one had consolidated; the other was observed to have contracted above one-fourth of an inch, and continued fluid; it crystallized, however, as usual, when briskly shaken, without withdrawing the cork.

It should perhaps be mentioned, that this sudden crystallization always commences at the surface.

I have put the solutions, both corked and uncorked, into cold water, as soon as made, in order to expedite their cooling, and have found the same results generally as when suffered to cool gra»

dually. The solution in open phials has sometimes cooled down to the temperature of the cold water (about 40°), and has then remained fluid in it for two or three hours; it has then sometimes crystallized in the soft spongy mass; at others in firm, well formed regular crystals.

15. Four or five phials have burst in which spontaneous regular crystals had formed, and over which subsequently a sudden consolidation of the residuary solution had taken place, after the change of colour was effected in the first crystals (as mentioned in No. 12), but whether from an expansion in the first or second crystals, I know not, as I was never present when this occurred. * I have never seen this fracture of the phial when only the regular crystals had formed, nor when only the spontaneous solidification took place. It is probably, therefore, somehow connected with the abstraction of the water of crystallization from the regular by the spontaneous spongy mass. In the above instances the crystals which had formed regularly were perfectly white, and were readily separated from the superior spongy ones by a little water gently poured over them, leaving them of the most perfect regularity, and forming a beautiful white crystalline preparation easily preserved, and not efflorescent, as in common cases.

In all the cases thus enumerated, such are the anomalies presented as to prevent my drawing one conclusion from them which could give me any insight into the causes that produce them. In some cases atmospheric pressure seems to operate, in others not; agitation sometimes, but not invariably. The whole series of experiments is so interesting, I trust this account may lead to further investigation, which may finally afford an explanation, and possibly lead to new views on the subject of crystallization generally. I can only add, that I never could promise myself, à priori, that any one case should certainly turn out as I expected; it appeared a matter of chance in a great degree, whether this or the other result should ensue. †

*I apprehend it must occur during the abstraction of the water of crystallization from the primary by the secondary crystals, which must be accompanied by a correspondent expansion.

+ In speaking of the effect of atmospheric pressure on saturated solutions of salts, Dr. Higgins details an experiment which he made in a narrow-necked glass mattrass of three gallons dimensions. It was fixed in a vessel filled with a saturated solution of sea salt: a solution of 144 oz. of Glauber's salts in 96 oz. of water, in a separate vessel, was filtered into the mattrass, which was filled twothirds by it, and the whole was made to boil so as to exclude the air by the vapour formed. A strip of wet bladder secured the mouth of the mattrass, and sustained the atmospheric pressure.

Two mattrasses were thus prepared: they stood three days at a temperature between 400 and 50°, and were often shaken without crystallizing; as soon as the bladder was cut a few small concentric spicular crystals formed, and shot rapidly through the liquor till it was almost solid; the caloric evolved, raised the temperature from 60° to 90°, and in one experiment from 40° to 90°.

From this experiment connected with those above detailed, as also from many well-known facts, I am impelled to deny the perfection of Dr. Black's celebrated theory of latent heat. It will be observed that boiling saturated solutions of

..I have tried similar experiments with other salts, of which I shall barely state the ontlines.

1. Sulphate of Magnesia.-Boiling saturated solutions of this salt, corked and uncorked, like the before-mentioned ones, sometimes crystallize, and sometimes continue fluid, I have never observed the beautiful satin-like crystallization perceptible in the sulphate of soda; but the crystals fall down in minute grains, like sand, diffused through the solution, gradually sinking to the bottom.

2. Alum, as above.-Crystals formed at the bottom; the remainder continued fluid, even when shaken; when the cork was withdrawn, shaking produced no effect for nearly a minute, when the same sand-like precipitation ensued, commencing from the top, When this ceased, it appeared nearly solid; but by standing for 24 hours, more than one-half was fluid.

3. Sulphate of Iron exhibited an appearance nearly similar to that of alum.

4. Sulphate of Copper.-The same, with some occasional variation, even in the same solution.

5. Sulphate of Zinc remained fluid for 24 hours, although a boiling saturated solution was employed, and frequent agitation.

6. Subcarbonate of Soda (sal soda) boiling and saturated. In one case (corked) it became nearly solid when cold, from the spontaneous crystallization. The same solution subsequently deposited, whilst corked, a smaller quantity of spontaneous crystals; and after drawing the cork and shaking, small granular crystals speedily clouded the solution. The same resulted in uncorked solutions.

7. Muriate of Lime, saturated and boiling, crystallized, when corked, completely throughout: subsequently, dissolved by heat again, and corked, it remained fluid, until shaken without uncorking, when a crystallization as beautiful, and nearly resembling that of sulphate of soda, took place, with an extrication of more caloric than in any of the preceding cases.

8. Muriate of Ammonia, corked and uncorked; boiling saturated solutions became solid as they cooled, with a firm crystallization.

9. Nitre deposits regular crystals at the bottom, both in corked and uncorked phials; but I never perceived any further result, except by the slow evaporation of the fluid.

I have tried a number of other salts, but the results are not worth

Glauber's salts have repeatedly refused to crystallize, even when exposed to the full pressure of the air, and that for days, Now it is to be remembered that such solutions had cooled from at least 212° to near the freezing point, and yet were enabled to hold that portion of salt in solution, which our theories presume to depend on the additional temperature. What was it that thus enabled the water to maintain its fluidity and transparency, although charged with such a quantity of solid matter, in opposition to atmospheric pressure and a diminished temperature of at least 150°? Can it possibly have depended on a quantity of latent heat only equal in the above experiment of Dr. Higgins to 50°? And is not the fact that water itself has been cooled down to 20° or 25° below the freezing point without congealing, evidence that something more than a certain quantum of latent heat is essential to the fluidity of water, &c. Other objections to this theory present themselves, but this is not the place for considering them.