Art. LXIII.—On the Mode in which Oil acts as a Nucleus in Super-saturated Saline Solutions; with Notes on the Mode of Action of Solid Nuclei.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 12, 1879, Page 407

Art. LXIII.—On the Mode in which Oil acts as a Nucleus in Super-saturated Saline Solutions; with Notes on the Mode of Action of Solid Nuclei. By William Skey, Analyst to the Geological Survey Department. [Read before the Wellington Philosophical Society, 21st February, 1880.] In a paper read before the Royal Society of England “On a relation between the Surface Tension of Liquids and the Super-saturation of Saline Solutions,” by Charles Tomlinson and G. Van der Mensbrugghe,* See also London Chemical News, Vol. 25, p. 281. it is stated that certain oils, both volatile and fixed, when applied to any of these solutions, “act as powerful nuclei;” that is, they cause the solidification of these solutions, by determining a crystallization of a portion of the salt thereof, which, when started even on the most minute scale, progresses sometimes slowly, sometimes at a speed giving an appearance of instantaneous effect, until the whole of the solution has solidified; a phenomenon which, I may remark, is of a somewhat striking character when witnessed and considered upon. This phenomenon is held by these investigators to be explained upon the theory, which they propound, “that whatever tends greatly to lower the surface-tension of a super-saturated saline solution, causes a separation of salt, and at once puts an end to the condition of super-saturation.” The theory is backed by formulæ, by frequent determination of tensions, and by experimental results of a very solid and varied character. But there are other results of theirs which do not appear in accordance with it, and the endeavours made to show them to be otherwise have the effect only of rendering this apparent unconformity the more conspicuous to me, and, besides that, of creating towards the theory itself a feeling unfavourable for its reception. It was under such an impression as to the insufficiency of the tension theory to account for all the facts given in their paper that, for a better understanding of the matter, I referred back to an investigation of mine, communicated to you last year; and by this I feel strongly persuaded that a factor in the problem—why oils sometimes act as nuclei?—has been left out of it—and a factor of such high value, as to reduce the part which tension plays therein (if, indeed, played at all) to one of a very subordinate character. This factor which I would bring to your notice is that of chemical affinity,—the affinity of one or more constituents of the oil used as the nucleus for the water of whatever solution is operated with. In the communication referred to “On the Nature and Cause of Tomlinson's Cohesion Figures,”† Trans. N.Z. Inst., Vol. XI., p. 490. you may remember I stated the constitution of these figures to be fundamentally different from that which is assigned

them,—e.g., their annular markings being hills of oil, and the ground in which they are set a resin; and that, further, I stated these figures to be caused by the resin dissolved in these oils leaving them for the water, the surface of which they monopolize to the comparative exclusion of the oil, owing to their affinities for it being greater than those of oil for the same liquid. Now, granted all this, and I have not yet heard of anyone disputing it, the explanation of the problem set us—the nuclear action of oils in super-saturated solutions—becomes an easy matter. Such solutions, as they are most likely to be made, are super-saturated to the highest degree possible at a given temperature, in short are saturated super-saturated solutions; and so, for the initiation of the change in view, require divesting of but the minutest portion of water, and this will hardly fail to be immediately effected by the application thereto of any old vegetable or animal oil, as such contain resinous matter much of which is so far metamorphosed as to be of a decidedly acidic nature. The affinities for water of these resinoid matters may be feeble, but they certainly have to contend with affinities also feeble; nor, it should be remarked, is the resulting product entirely or even notably soluble in water, or such resin might behave only as salts, which when added thereto are ineffective as nuclei; but water is taken away from the solution to be incorporated with such matters in a solid form, and so a portion of salt is liberated to form the nucleus for a general solidification. But it should follow as a consequence that, if resin is the cause of nuclear action, a freshly made oil may not have nuclear action of the kind at all; and this is exactly what we are told by these investigators, they find that while “turpentine old but bright and clear acted powerfully as a nucleus” it afforded when distilled a liquid which was not then active. Another example they give is that of an old oil of bitter almonds which they experimented with. This oil though strongly nuclear gave a distillate which was non-nuclear; and again we learn from them that the oils of commerce as we have them, even those of the same kind and presumably of the same quality apparently, any way having surface tension very similar in degree, vary in the same way that the oils named above do. Now all the cases occurring in which oils are non-nuclear are explained I believe by my theory upon either the very probable supposition or the fact that they do not contain resinous matter; but these cases are certainly not explained by Professor Tomlinson and his associate. They show that in the case of active and inactive oil of turpentine their tensions were to each other as near as 2·2 to 2·4, a difference admittedly insufficient to account for their mutual diversity in relation to the saline solution upon this tension theory. With all deference therefore to the opinion of these eminent

scientists, I will now affirm that such facts are irreconcilable with this theory of theirs. The question I have started as to whether or no surface tension has an effect, even of a mere subsidiary character, in producing these nuclear effects described, I leave for solution to those physicists who for so many years have been working at the mechanical properties of liquid surfaces. But I must remark, ere I leave this part of my subject, in relation to the effect or non-effect of any specific oil in producing crystallization, that in regard to this, not only does the more or less resinous condition of an oil affect it, but also the more or less proneness it has to become resinoid upon exposure to air and water; and this brings us face to face with another fact elicited by these investigators. It is, that in dull, damp, and cloudy weather an oil may not produce nuclear effect; whereas on a fine bright day it may be “particularly active.” This they explain by supposing the surface tension of the solution to be lower upon cloudy than upon bright days. But in connection with my theory (the chemical one) I would point to the well-known fact, that light in conjunction with air favours the change of oil generally to resinous matter, and hence an acquirement of nuclear effect on bright days. Another and the last remark I have to make here is, that in all those experiments with oil where a nuclear effect is produced by these scientists, a flashing or spreading out of such oil from the lens-shape it may first assume, is the constant concomitant or antecedent. Now, this uniformity of action is misleading, for it impresses one with the idea that any oil which spreads over the surface is one which will have, when fairly tried, a nuclear action; and so it has the effect of prepossessing one unduly in favour of the tension theory. As a matter of fact, however, I find that gasoline, an oil of very feeble tension, too, spreads rapidly over such solution without producing any solidifying action, so it is not nuclear, thus breaking the uniformity hitherto observed. On the other hand, however, we may take it for granted that in every case where a nuclear action is excrcised by an oil, there is more or less of a surface spread of the oil accompanying such action. And now in pursuance of the next part of my paper: it is relative to the mode of action of solid nuclei in super-saturated solutions, and is merely a description of certain results I have obtained thereon, while the subject of liquid nuclei was under consideration. Two of these results are, I believe, novel to scientists (Nos. 4 and 5). The others are got by repetitions of experiments by investigations which have often led to results diverging more or less from one another. They are recorded, not under the idea of settling a question about which there is great debate, but only as a small and possibly

a useful contribution to those who have taken this question as an especial subject for research. These results are:— 1st. That crystals of the common hydrated sulphate of copper, epsom salts, nitrate of baryta, borax, and glauber salts, after being sluiced with water, and while still wet all over, do not act as nuclei in a super-saturated solution of sodic acetate when applied thereto. 2nd. That on the other hand a crystal of sodic acetate similarly treated (as above) does act as a nucleus with such solutions. 3rd. That chloride of sodium does not when sluiced and mixed while wet with a super-saturated solution of alum, act as a nucleus thereto, but that a crystal of alum similarly treated does. 4th. That the formation of solid sulphate of baryta formed in any of the super-saturated solutions of a sulphate (by adding baric-chloride thereto) is not attended or followed upon by the crystallization of such solution. 5th. That the electro-deposition of copper in a super-saturated solution of either sodic acetate or alum (charged feebly with cupric sulphate) is also unattended by crystallization in either case. These results show that only those salts* Only those salts are indicated here which have feeble affinities for water; it is always understood, I believe, that those having strong affinities for it are left out of the question. which are of the same kind as that of the super-saturated solution, are nuclear thereto, and they also tend to show that solids having no affinity for water have not any nuclear effect in such solutions, not even when applied thereto as freshly prepared, perhaps I may correctly say in a nascent state. It will be seen, therefore, that so far as these results are indicative of anything at all, their general character is such as to sustain De Gernez† Watts' Dictionary of Chemistry, Vol. V., p. 350. in his assumption that “sudden crystallization of a super-saturated solution is in all cases induced only by contact with a crystal of the same salt” (allowing, of course, that he excludes from his cases those of the oils and alcohols, also salts, having notable affinities for water); and so far they are antagonistic to the results of Jeannel, as stated in the same work, viz., that any solid substance applied to such a solution of sodic-acetate causes crystallization, and so tends to upset the theory by which he seeks to explain the phenomena in question. A due consideration of all which has been now advanced on this subject will, I think, have the effect of inclining you strongly to the following general deductions which I draw therefrom:— 1. That the only substance which acts directly upon a super-saturated solution as a nucleus, is a salt of the same kind as that of such solution,

hydrated to an equal extent with the salt which is separated thereby. It is just probable, too, that such a salt must be in a crystallized form. 2. That in all those cases where substances other than the salt of which such solution is made are nuclear thereto, they act for this in an indirect manner, the first step being a removal of water from such solution, and this causes a portion of salt to separate—to form in its turn a nucleus capable of acting directly on the solution. Nuclear action is therefore, in these cases, always a result of secondary action.