Art. LXXXI.–On the Cause of the Movements of Camphor when placed upon the Surface of Water.
[Read before the Wellington Philosophical Society, 31st August, 1878.]
It has long since been known that camphor in small pieces describes rapid and very eccentric movements when placed upon water, the surface of which is free of oily matter. This phenomenon is so singular,* and is, besides, so striking to him who for the first time witnesses it, that such an one can hardly help feeling anxious to become acquainted with its cause,
[Footnote] * I have since discovered that the liquid bi-sulphide of camphor behaves in this respect like camphor. If the water used with it is quite clear from greasy matter, it spreads in various directions by a series of explosive efforts; but if the water contains a minute quantity of grease (as it will do if especial precautions are not taken), the sulphide of camphor, after a little while, rotates slowly round its centre, then rotating progressivel aster, it at last strikes off in a straight line, leaving line, greasy narrow streak behind, which is permanent.
and the more so when, on enquiring into this, he learns that no one has yet published anything in explanation of this phenomenon which is backed either with any kind of authority or, what is more, by a weight of evidence sufficient to recommend it for his unqualified belief.
It was just under these incentives to research that, a little while ago, I commenced investigating this phenomenon, and with the additional one of ultimately finding something in common between it and that of the motion of minute particles in certain liquids–one known by the appellation of “Browinian movements”–the Pedetic movements of Prof. Jevons.
After I had made a great number of experiments with camphor, however, I could find nothing which gave any proof in favour of a common origin for the two phenomena. Neither could I find anything in support of the theory popularly assigned for the explanation of that exhibited by camphor, but instead, evidence of a very decided character, pointing, as I think, very clealy to quite a different origin for it than that which is assigned by the theory in question. This I now ask leave to submit to your inspection, and, if you will allow me, I will do this in the same order, or nearly so, in which it was educed.
First, I will remind you what this theory is, which is thus endorsed by popular opinion. It is based upon the fact that camphor gives off vapour at common temperatures, and it is to the unequal impingement of this vapour upon the water around it that the movements in question are ascribed, the camphor being held, as I suppose, to move in the direction of least evaporation.
Now this does, I allow, appear at first thought quite explanatory to anyone who will observe the effect which camphor vapour, or vapours, generally have upon water, the surface of which is prepared so as to indicate any modification it thereby undergoes. Such a surface, in the case of water, is easily got by dredging a little very finely-powdered resin evenly upon it. *
Camphor suspended close to a surface of this kind produces, as you see, an instantaneous recession of the resinous particles from the point immediately under it; the same effect is also produced by those substances generally which give off vapour in suitable quantity at the temperature at which they are used; for instance, alcohol or ether.
It does, indeed, appear from this, that the popular theory which we are attacking is, after all, correct. One sees the resin quickly dashed away from the camphor, etc., and in consequence conclude that, as action and reaction
[Footnote] * This surface so well indicates (by the displacement of resin) the presence of oily matters that these can readily be detected, as they escape from the finger applied thereto, even immediately after a thorough wash.
are equal, each of the substances whose vapours we are operating with, is at such times subjected on all sides to a force tending to drive it in, that is, towards its centre; a force which, as it cannot be persistently equal around it, will certainly move it from its normal position.
This is, I allow, a conclusion which one is at first inclined to form; but I will now show to you an extension of this experiment yielding results which will, I think, at once prevent anyone previously so inclined from this conclusion.
The camphor is now only one-sixth of an inch above the water, and the diameter of the cleared space below it is about half an inch; I now lower it to within one-fortieth of an inch of the water, but you observe that the area of cleared space is not perceptibly increased, thereby showing that this is very closely upon the maximum of that which can be got by placing the camphor at an infinitestimal distance from the water. I now allow it the slightest contact with the water which I possibly can, and you observe that there is instantly a very large increase of cleared space, whose diameter is certainly not less than four inches, representing, therefore, an area no less than sixty-four times that which we had before.
Here, then we have, in one moment, an accession to our knowledge of a kind which teaches us that, whatever the direct radiation of camphor vapour may have to do with the production of the cleared space last got, there is some reaction of camphor with water of quite a different nature which has very much more to do with it; so great, indeed, is the effect produced by the merest contact as compared with that obtained by suspension in the most favourable position, that it really becomes a moot point whether any space at all is cleared by the direct impingement of this vapour on the water surface.
To settle this point by a demonstration I now reproduce certain expeiments of mine:—This small sphere of camphor I suspend over water (prepared as above) within one-sixth of an inch of its surface, and across the cleared space produced, close to the camphor and closer to the water than the camphor is, I place this thin bar; now this clearing should have its shape materially altered, and its area much curtailed if it has been produced by the mere impingement of vapour on water; but you cannot see that any variation occurs whether in shape or area.
Again, I place a small piece of camphor on this prepared surface, and put a wide bar close to one side of it and very near to the water; now, action and reaction being equal there should occur a marked recession of the camphor from the bar if the evaporation theory is correct, for in the direction of this bar is the greatest resistance to the escaping vapour; but you cannot observe that anything of this kind happens.
Evidence of this sort, indeed, is to be had in every exhibition of these movements of camphor, for whenever the camphor gets to the veśssel's side, it remains there motionless, whereas it should (according to the theory we are examining) rebound therefrom, with great force.
But apart from and independent of these results, it is, as I conceive, very questionable whether any vapour emanating from a substance which is of the same temperature as its environment, and in presence of air, has a direct repellent effect thereon. However, this is a question of so general a character that I cannot well extend the scope of this paper so far as to take it into consideration now, but if you will allow me I will trench upon it so much as to inform you of a few very interesting facts which are connected with it, and also in an especial manner with the particular phenomena we are considering.
Experiments.–Two pieces of camphor swung in air in close contiguity, at the ends of very fine and long threads, do not manifest any repellent effect in regard to each other, while, if placed upon water, though at first there is an appearance of a mutually repellent force in action, there is, after a short time has elapsed, an appearance of a mutually attractive force–they actually, as you see, move toward each other and close together. Now, you will allow that these results are not consistent with the theory which credits vapour with direct repulsive effects. The case of turpentine is something similar to the above. It is a substance which gives off vapour more freely than camphor, yet when one sprinkles fine particles of cork upon it and applies its vapour thereto, one cannot observe that the cork is at all affected thereby. But not only this, one can get the very reverse of repulsion during the emission of vapour; thus, upon either turpentine or kerosene a small piece of cork is placed, and a rod moistened with ammonia is then brought to within about half an inch of it, when the cork may be observed to positively rush to the rod and follow it about as obediently as a needle can respond to the movements of a magnet. *
Taken as a whole, the results which I have just described or exhibited to you, do, as I conceive, indisputably show that this recession of particles under the influence of camphor is, appearances notwithstanding, not due to the direct impingement of vapour on them, but rather to some effect following thereupon; and it seems therefore necessarily to follow that the movements of this substance (camphor) on water, are also not the direct result of impingement, but the result of something which follows it.
And now, with the popular theory thus disproved before you, it may occur to some one here, as it did to me, that possibly these movements of
[Footnote] * Volatile acids, also water, applied in this way have the reverse effect, while turpentine is neutral. The rod alone, if warm, has also an apparent repulsive effect on the cork.
camphor are the effect of electrical reactions, but that this is not so appears from the fact that neither of the poles of a six-pair Grove battery, in full action and in good working order, at all effects these movements when applied close to the camphor. The poles were pointed in order to be in the most favourable condition for effect.
The ground being thus cleared, it remains for me to tender for your approval a theory which, in my opinion, explains the phenomena in question. This I will do, and along with it I will describe ordemonstrite, as the case admits, the experimental results upon which, in conjunction with those related above, this theory is based.
You will perhaps remember that in the introductory part of this paper Is tated to you the well-known fact that, for the exhibition of the movements, which I now desire to explain, it is necessary to have the water-surface free from oily matters. Now a knowledge of this is highly suggestive; it is as you will find the key to the question before us. A drop of oil (as you observe) stops in a peremptory manner all camphor movements, and it is now our proper course to enquire how it effects this. It can only, so far as I see, effect this in two ways, either by enfilmning the camphor and so preventing evaporation, or it is not as a suspensive medium favourable to the continuance of such movements; that this last is the case is shown by the following experiment.
I float a small piece of cork upon turpentine, on this cork I place camphor, and you observe that we get no camphoric movement. * † Clearly then it appears that whether we are to get the phenomena or not depends upon the nature of the surface of the liquid which we use for flotation, and this independent of any effect it may have upon the solid camphor in preventing its evaporation. The knowledge of the fact that oils generally are, in this particular respect, antagonistic to camphormovements I sought to amplify, and in this I have succeeded. Thus I find that besides them alcohol, ether, bi-sulphide of carbon and ammonia, applied even as vapours, and only in minute quantities, arrest camphoric movements very quickly. I further find that such movements are very much less rapid and prolonged if the water used is charged heavily with either acids or salts.†
Being thus acquainted with the fact that so many and such diverse substances as those just above cited, render water unfavourable for camphoric
[Footnote] * This fact, by the way, is corroborative of the correctness of the opinion I have ventured above, viz., that camphor movements are not caused directly by the impingement of its vapour upon the liquid which floats it.
[Footnote] † I have since found that, in singular opposition to these vapours, etc., gasoline not only refuses to stop or retard camphoric movements, but even starts them in case of camphor rendered stationary in this way.
movement, it occurred to me to try camphor itself. I placed a large piece of it upon water in a vessel closed from the air, and found, after the lapse of about four hours, that it was stationary, and that fresh pieces put therein would not move. I then took out the camphor and exposed the water freely to the air for a short time, when I found that camphor would then describe very lively movements upon it. The same kind of effects are not produced if the camphor is kept wholly immersed in the water, not even if the time of contact is prolonged to a week. I further found that a watersurface, which has been rendered unfavourable for these movements by camphor, is also in a very unfavourable condition for the spread of oils thereon, so much so that some of them, when so placed, that is, of course, when used in small quantity, keep nearly to the drop form.
The very pertinent facts are, then, now distinctly shown, that many vapours are obstructive to camphoric movements when condensed within or upon the upper stratum of any water on which camphor is, moving, and that among these substances is camphor itself. Now we have already seen that all the substances just named above (and this includes camphor) are capable under certain circumstances of giving very decided and rapid movements to solid particles when they occupy a water-surface; and we have also seen that these vapours do not effect this directly by what I may here designate vapour force, but rather by surface modification. The conclusion, therefore, which one is led to by a consideration of all this is, that the movements of camphor in question are the effect of surface modification and modification which it produces itself.
The analogy is, I hold, complete between resin upon water moving away from that which has been modified by vapour, and camphor upon water moving away from the water it has modified, except that we have one substance less in the latter than in the former case–thus, camphor placed upon water gives up a part of its substance to the upper stratum thereof; this part represents the vapours, whether of alcohol, ether, or camphor, which are resident in the clearing they have made amongst the particles of resin with which the water has been laden; the remaining part of the camphor represents the resin which has been removed to make this clearing.
Such is the theory which I submit to you as explanatory of the phenomena we have been observing. I think you will readily allow that it is at all events the most probable of any which you have now had for this purpose, and now it only remains for me to state to you what I conceive to be–
The precise nature of the surface modification which I have credited camphor with producing.
The manner in which a surface so modified forces camphor to move.
First then in regard to the nature of this modification. Camphor, I should remind you, has properties affecting this question as follows:—It is soluble in water, and so without doubt combines with; it is not decomposed by water, even in conjunction with air that is at common tempertures, and it is a resin slowly volatile at such temperatures.
Now camphor, as we have seen, modifies a very much larger extent of surface when placed in contact with water than when suspended over it. We may, therefore, be certain that the modification in question is not occasioned simply by a deposit of condensed camphor vapour upon the water surface, but rather by some combination of it with water. This combination, however, is not that which is obtained by saturating water with comphor in the ordinary way (a compound containing one part of camphor to one thousand parts of water), as is shown by the fact I have already stated to you, that a saturated solution of camphor in water allows camphor to describe movements upon its surface; this compound, therefore, is one new to us, it can only exist as a thin surface film, and is therefore doubtless one far richer in camphor than that which we already know of, a compound indeed so highly camphoretted that it, in all probability, partakes in an eminent degree of the characters of an oil. * However, it is impossible for me as yet to obtain direct evidence as to the nature of a film so exceedingly thin as this. We must therefore for the present rest content with the indirect evidence which we now have.
Granted, then, that the modification effected upon a surface of water by camphor is owing to a combination of the two for the production of a highly camphoretted oil, I have only now to show the precise manner in which this compound induces camphor to move.
For this purpose I will refer you to the effect which a drop of oil has when placed upon water laden with an indicatory substance, such as clay in fine particles; the oil spreads quickly and regularly around, and in the act urges the clay to the side of the containing vessel, where it becomes stationary. Now the clay and the oil here are, I hold, fairly representative of the camphor and its oily compound; there is this difference, however, in the movement of the oils, and it is an important one, it is a difference upon which all camphor movement depends,–the spread of oil in the case of camphor is not even around it, as is that of the other oil; were it so there would be no such phenomenon as that we are investigating. Regularity of spread is in this case impossible, because the production is irregular, owing to the highly crystalline nature of camphor, and its great fragility, whereby
[Footnote] * I have lately observed that water thus modified, when compared side by side with pure water, exhibits a higher lustre than the pure water does, a fact greatly in favour of this view.
one cannot obtain a homogeneous sphere of it for use, or if one could it would at once lose its character and shape. The oil then being of necessity produced irregularly around the camphor spreads unequally from it, and in the act urges it in a direction which is away from that side on which there is the greatest output of oil; thus the camphor breaks through the oily film at its weakest part, and sets itself upon the edge thereof, which position it retains so long as there is motion produced. The movements of camphor are, in short, the joint results of the outward spread of oil along the water surface–the inertia or adhesiveness of this oil as regards what surface it thus occupies–and, lastly, the antipathy, as it were, which exists between the two, the camphor and its oil, whereby they refuse to associate.
But the questions will now without doubt present themselves to you–Why should the oil spread so determinately over the water and retain the position thus gained so obstinately? and why should it appear to repel camphor? Unto such questions I might with propriety reply, that to entertain them here is not incumbent upon me, as I have now completed the task I set myself in this paper, by showing that camphor moves, as we have seen, upon water, for the same reason that solid particles occupying a water-surface move when oil is added. To treat such questions is really to take up another subject, and one which includes within its scope the behaviour of oils generally with water when in presence of it–a subject, moreover, which I had reserved for a further communication to you; but rather than leave the matter in hand in a state which may to some appear unfinished, I will trench upon these subjects so far as to make a few general observations thereupon in elucidation of these questions. It is, however, proper that I should premise these observations with a short statement of the prevailing opinion as to the reason of the extensive spread of even minute quantities of oil upon water under favourable circumstances, and their refusal to mix under other circumstances.
According to these opinions, and these are both the popular and scientific ones, the spread of oil upon water is simply the result of gravitation in conflict with the cohesiveness of the oil, and the apparent antipathy which they manifest towards each other, is the result of an exertion of a repulsive property innate in one or the other, or in both. Thus it seems to me that the possibility of chemical reactions being concerned in each of these operations has not been contemplated, and so, as I am persuaded, an important factor in both these problems has been left out of consideration. In opposition, then, to such opinions, I will maintain here that both the spread of oil upon water in thin films, and the apparent repulsion which may be seen to occur between the two, are brought about mainly through the satisfaction of chemical affinity.
I will only now take to task, in a direct way, the idea that oil and water mutually repel each other.
If any one will put a slender stick of some solid fat into water and then remove it so that the lowermost point thereof is about one-twelfth of an inch above the water-line, he will upon due examination find that a portion of water adheres to the fat and joins it to the remaining water. Some fats may be lifted out of water to a much greater distance than this before connection therewith is broken. Now, you will perceive that these effects are quite incompatible with this idea of repulsion; could fats repel water, they would rather depress water when applied thereto than raise it.
A cohesiveness so strong as this can, as I believe, only be explained * by assuming that a chemical combination has taken place between the fat and the water, a combination not of masses but of surfaces, because of the insoluble nature of the product in relation to both the substances furnishing it.
What is true here of fat will undoubtedly be true of the oil it furnishes.
Allowing, then, that oil and water have affinities for each other, these will certainly come into play to a very great extent as regards the oil when a very thin film of it has contact with water, such as obtains when a drop of oil is suffered to extend itself upon water unchecked, and the fact that oils thin out in this way, and so rapidly as they do, upon water, I would attribute in largest measure to successful exertion on their part to satisfy this affinity. Corroborative of the truth of this opinion is the fact which I have ascertained that oils spread far more rapidly and extensively upon water than upon mercury, a substance which as far as we know has not any affinity for them; and in further corroboration of this, oil, as we have already seen, does not spread at all when applied in small quantity to water which is covered with hydrated camphor; still, each of these surfaces–the metallic and the camphorotted–may appear to us as smooth as that of the purest water.
And now applying the knowledge of these results and the deductions they seem to allow us to make to the elucidation of the questions which I have proposed on your behalf, I would maintain that this camphor oil, though in part composed of water, has still an unsatisfied affinity for water by which it is urged to extend itself around in search of it; it occupies firmly the surface it has thus overrun by reason of its internal cohesiveness, its inertia, and its affinity for water.
It forces the parent piece of camphor into movement, because being saturated with camphor there is no unsatisfied affinity existing between
[Footnote] * That this effect is not produced by atmospheric pressure is certain from the fact that the whole of the portion wetted may be open to the air.
them, so each of them–that is, the oil and camphor–can only respond to their affinities for water; to the water therefore they both keep, for its possession they fight. It is a running fight, in which the oil having a motion of its own communicates a part of this to the camphor, and so appears as the pursuer.
Summarizing, now, all that we have here arrived at, by way of emphasizing the points I consider as fundamental to the theory I have proposed, you perceive that I have maintained upon evidence, much of which has been experimentally demonstrated, that the movements described by camphor occupying a surface of pure water, are neither due to the direct impingement of vapour upon such surface, nor yet to any electrical effect, but to the production of a compound of camphor with water, which, being of an oily nature, spreads upon the surface of the water, and, in the act, forces the camphor to describe the movements in question, precisely in the same manner that oils generally, in their flow along the same kind of surface, can urge away from them any solid particles resident thereon. In the actual process, directly that the camphor touches the water there is a considerable but unequal output of this oil therefrom, and as this does not dissolve in the water, nor volatilize, at nearly the speed at which it is formed, it spreads principally upon the water, and retaining for the time possession of all it thus overruns, it urges and keeps the solid camphor away.
As I have already stated, were the production of this compound equal around the camphor, we should not see the camphor move as we have done; but this is in the nature of things impossible, as there is certain to be more of it made at the instant of immersion upon one side of the camphor than upon any other of equal extent, and so an initial movement and direction is given to the camphor. When this direction changes, as it frequently does, the output of oil has become greater upon a different side, or the camphor, in its course, has got into contact with its trail; when all movement ceases, the whole surface of the water, or at least that within a considerable distance of the camphor, has got enfilmed with the oily compound, and the camphor thus becomes oil-bound.
As to the reason why the oil of camphor when in motion, or, indeed, any oil in motion, should thus urge camphor about, I have attempted to show that this is due to the fact that they have no affinity for each other, but that each has an affinity for water, and so they have both a tendency to keepin contact with it–in fact, they are adhesive in relation to the water, but neutral in regard to each other, hence the appearance of direct repulsive effect–a kind of effect, which by-the-way, I believe to be always due to secondary action.
Now, if the conclusions stated above are in the main correct, any substance floating upon a clear surface of water and discharging oil thereon, should describe motion, and motion of the same character as that which you have seen camphor make. Well, this is, I find, precisely what does happen when the experiment is performed.
I rub this small piece of cork with a very little turpentine oil; it has now absorbed this oil, and appears but little different from another piece of cork I have here, which has not been oiled. I place both upon water, and now you can see that while the clean piece of cork remains stationary thereon, the oiled piece moves about in a very vigorous and eccentrie manner, imitating the motion of camphor so closely that, except for the difference which exists between the colour of the two (the camphor and the cork), one would take them as they move for the same thing.
Like effects follow when any non-volatile oil is used in place of turpentine. These experiments are surely crucial tests as regards the correctness or otherwise of the theory I have proposed to you.
I will only add to this by stating that, should further research prove that the several deductions I have here made to you are correct, a decisive blow has been given to the popular theory that vapours are directly repellent, as they are emitted in presence of air, from substances at the same temperature and pressure as that of their surroundings; * and also, as I believe, to the theory that a repulsive property is innate in our oils, and that it is, it were, developed into action whenever water is placed nearly in contact with them. I say nearly in contact, for the contact which ensues on collision of bodies may also cause repulsion, but not repulsion in the sense I use it here, viz., that in which it is used and to which it is limited by physicists when they deal with this particular matter.
In this connection I cannot refrain from stating to you my belief that repulsion and its correlative attraction, whether in reference to electricity, magnetism, or the movements of masses in relation to each other, are not direct results indicating the action of two properties, but are secondary ones brought about by and indicating modification of matter.
I have lately been successful in getting results which, in the first place, demonstrate that the surface of water is chemically affected by camphor, and, in the second place, I got results which completely explodes the current idea that vapours in their emission drive solids away from them, as per se.
I. Water which has had camphor moving over its surface a long time, refuses to gyrate camphor; its surface acquires a somewhat resinous lustre, being, in fact, more refractive of light than
[Footnote] * Vapours, of course, are always in a repellent condition as they rush into a vacuum.
before, as I think (this is, however, for measurement), and such surface will not allow kerosene or turpentine to flow over it; i.e., a drop of either put on keeps in nearly the drop form; in pure water they would flow over instantaneously. I have got the surface to refuse olive oil even, a very searching oil though a slow mover. This shows that such surface is a combination of camphor with water; and I am now enabled to fill up a break you would observe in the continuity of my paper. I now show how camphor forms some compound with water, which will not allow oil to flow over it. This compound is not camphor, because camphor is soluble in oil. I think it is probably the hydrate. If it is the hydrate, the surface should, by continued exposure, again rotate camphor, or allow oil to flow over it; and I find that two hours' exposure of a non-rotating surface does restore to it the property of rotation.
II. That vapour does not necessarily repel, seems shown by the fact that cork, on turpentine, or kerosene, is attracted by ammonia vapour. Now, ammonia is a substance which, being rapidly evolved–far more so than camphor–should repel cork on turpentine strongly. (Camphor, I may say, and ether, and alcohol, do repel cork on this liquid).
Again: Turpentine vapour does not repel cork on turpentine, but it repels cork on water; and water offers more resistance to the movements of substances swimming on it than to substances swimming on turpentine.
Clearly the idea that vapour in transit (molecular movement) can move masses of matter visibly to us, does not receive any support from such results as these.
I am led to hold by researches here, and by thought on the matter, that the generally received idea that vapour exercises a direct repellent force, is entirely erroneous; it appears to do so, but it in reality forms a compound with the liquid it appears to repel. A compound being lighter, or having greater diffusive power than the normal liquid, rolls like a wave away from the supply, and carries any dust mechanically with it, giving the apparent repulsive effect observed.
Repulsion by direct force of vapour is, I hold, an optical illusion, and one which physicists should long since have exploded by force of pure mathematical formulæ, and not left it as a job for chemists.
I shall in a future paper take the larger subject, that no kind of matter can be visibly repelled while intercepting vapour in transit so long as a common temperature is observed and the pressure is not notably irregular.
Lastly: I get a decided attraction (apparently) of camphor for camphor when this substance occupies the surface of water upon which it has rotated for a time sufficient to give a surface about half charged with the camphor compound,