I have mentioned these recent contributions to science, not only with the object of indicating that our knowledge of crystals is steadily increasing, but also in order to point out that little has yet been done to explain the mysteries of their growth. All that has been effected up to the present is an attempt to explain how they are constructed, not the process by which the construction takes place. It is as though we were to analyse the form and structure of animals and plants and never to watch them as they grow, but only to study them from fossils or from museum specimens. And I believe the reason to be this. All the speculations concerning crystals persist in regarding the particles of which they consist as fixed and immovable: the theories are all statical. And yet we know that the particles of matter, whatever they may be, are really in lively movement. Is it not possible that in order to get a correct understanding of the growth of a crystal we should take account not only of the positions, but of the movements, of its particles? Without some knowledge of these we are not able to approach the problem, or to ascertain how a crystal either of nitrate of soda or of Iceland spar draws the nitrate of soda out of the solution and makes it grow into a solid.
Remember that when we say we know how the particles of a crystal are arranged we really know nothing about their nature, and can only represent them by spheres, or solid figures, or cells, or even points, in order to get a representation of their arrangement. But we might arrange in the same way a number of bodies each of which is whirling in a fixed orbit, like a planet, about the corresponding point, or vibrating about it like the prong of a tuning-fork, or pulsating like a breathing animal; and so far from the arrangement being independent of the movements it may be due to them.
If I may seek another analogy, let me take a group of figure skaters; their centre remains fixed at the orange, maybe, about which their figures are executed, but the group of skaters is at one moment extended when they circle out to their furthest sweep, and at another moment concentrated when they converge to the centre; and this alternating expansion and compression occurs in a regular rhythm.
Imagine a pond covered by a number of such groups of skaters; the manner in which they will fit in, and have to arrange themselves, will depend both upon the dimensions and the rhythm of their curves, and they may even interlace and become part of one great figure system covering the whole pond. May not the growth of a crystal be something of this sort?
All the devices which I have quoted for picturing to ourselves the architecture of a crystal I would regard as merely models representing something that may be really quite different. But I venture to suggest that the time has come when we should make use of moving and not stationary models.
One need not go further than a spinning top for an illustration of stability due to movement, and there is nothing unreasonable in the suggestion that the rigidity of a crystal structure may be due to the motion of its parts.
One curious observation which I have made is suggestive. I have many times noticed that when the appropriate crystal is introduced into a supersaturated solution, which is not strong enough to crystallize spontaneously, it may cause crystals to grow not only in actual contact with itself, but at some little distance in its neighbourhood. If this be so, then the crystallizing force, the power of propagating crystal growth, is not merely a frontier problem, but can be exercised through the liquid to a distance. If I try to picture to myself what is happening, I must again have recourse to analogy; I can only think of the manner in which a string or a tuning-fork is set in vibration and responds to a similar string or tuning-fork which is giving out its note at the other end of the room; and so is it not possible that the movements, whatever they are, vibrations or pulsations or regular oscillations of some sort, which constitute crystalline growth may be communicated through the almost crystallizing liquid, and culminate at some point where they set similar material vibrating, that is to say, crystallizing, in the same way?
I remember, when I first began to be interested in crystals in undergraduate days, reading in Nature an account of two papers which seemed to have a possible bearing on the subject. One is an example of a stationary arrangement of rigid bodies. The other illustrates the principle which I am now suggesting, and is an arrangement of pulsating bodies whose positions are due to their movements. The first was a description of Mayer’s experiment on floating magnets which was, if I remember right, shown here in a Boyle Lecture by Sir Joseph Thomson a few years ago; an experiment in which magnets suspended in water by corks and with their North poles projecting so that they repel one another, are brought together by the attraction of a large magnet held, with its South pole downwards, above the surface of the water; under the joint influence of this attraction and their own mutual repulsions, they group themselves into a number of interesting geometrical figures very suggestive of the geometrical regularity of crystal structure. Indeed, attempts have been made by Lehmann to explain the architecture of a crystal by a grouping of magnetic systems. And the other was the experiment of Bjerknes in 1876 (described in Nature in 1881), in which a number of hollow elastic balls were made to expand and contract by means of air tubes attached to them. These pulsating balls, when placed in water, attracted and repelled one another like magnets, and arranged themselves in a regular manner; thus suggesting that there may be many unexpected ways in which rhythmical motion can exercise an attractive and directive action such as is required to produce a crystal.
Let me illustrate what I mean by another crude analogy. Take a room full of dancers; if they are all dancing to different times and in different ways there will be no order or arrangement in the crowd, and it will remain an incoherent jostling assembly of independent persons. But if there are among them those who are moving to the same step and in the same manner, and if they come together, they can become partners and can continue to dance together; and if the room be filled with such dancers, then the whole assembly can grow together into an orderly movement, and only those whose step does not fit into the dance will be ejected and left out. Or take another example: soldiers marching together have not ceased to be individual men, but when they fell into step they became in addition an organized body with a structure and a coherence that does not belong to a miscellaneous crowd. Even so may the particles of dissolved salt be endowed with a movement which enables them to enter into partnership and cohere, and so to build themselves into the orderly structure which makes up a crystal. And even so do crystals grow out of a mixed solution as a pure and homogeneous substance and reject the other materials which are dissolved in the liquid.
One other suggestion. If the growth of a crystal is really the coming together of vibrating particles which cohere because they are in tune with one another and so enter into a partnership like that of the dancers or the figure skaters, then is it not possible that we may be able to communicate these vibrations to a supersaturated solution, which is so densely crowded that it is ready to crystallize, by some other means than by inoculating it with the appropriate crystal? I think that the time has come when we may be able to get some knowledge of the manner of these movements by experimental methods; perhaps by studying the sort of shock or movement, if there be any such, which starts crystallization in a supersaturated solution; perhaps by finding other substances, whose movements we understand, which are able to start the crystallization when they are introduced into certain solutions.