The Growth of a Crystal / Being the eighteenth Robert Boyle lecture

by Henry Alex Miers

Some two years or so before the time of which I am speaking, Mr. Barker had at my suggestion made an exhaustive study of a great number of different substances in order to ascertain which of them behave towards one another like nitrate of soda and calcite, and why they do so, and he has really discovered the secret. When two substances like nitrate of soda and calcite have nearly the same shape and resemble one another closely in their physical properties, the geometrical laws of crystalline structure of which I have already spoken make it certain that they consist of particles arranged in the same way. We do not know what these particles are or what is their shape or size, but we may be sure that they are arranged in the same way. I am perhaps using the word particles in a loose sense, for they might be hollow cells or they might be the space occupied by moving particles, or anything else, but whatever they may be it is pretty certain that they are arranged in the same way.

Well, Mr. Barker has proved that, if two crystals grow together like nitrate of soda and calcite, their particles are not only arranged in the same way, but they must be of the same size, or at any rate occupy the same space. In more scientific language, the two crystals not only have the same molecular structure, but the same molecular volume. It is, therefore, mainly a question of fitting together, and, if the two structures do not fit, they cannot grow together, like nitrate of soda and Iceland spar, as a continuous crystal.

Let me illustrate by a suggestive comparison. The bee’s cell is one of the most remarkable and symmetrical structures in nature. Its regularity is probably due to the fact that bees of the same size are, in making it, so closely crowded together; there is one bee’s head in each cell, and therefore you may say that the arrangement of the bees is the same as that of the cells. For example, if you place in each cell a ball which exactly fits it and then take away the cells, you have an arrangement of balls which is the same as that of the cells, each being in contact with six neighbours. It may be called a hexagonal arrangement. You have only to push together a number of balls on a table, and they will fall into this arrangement. It was a contemporary and associate of Boyle, and an Oxford man, Dr. Robert Hooke, who pointed out that with balls piled together in this way you can build up the shapes of crystals, and that, for example, a pyramid of cannon-balls stacked together in the manner that I have just shown, has the shape and angles of an alum crystal. Now, if two such arrangements are to fit together, they must be of the same size, whether they consist of bees, or cells, or balls, or molecules. In the same way Mr. Barker has proved that Iceland spar and nitrate of soda must consist of materials which are not only arranged in the same way, but are of the same size, although we do not know in the least what that material is, nor what its actual size may be.

But this, of course, is not the end of the whole mystery: for not only do two such structures fit together, as might be expected, but each has the remarkable property of making the other crystallize and grow, and that means, as I have just explained, that it draws the other out of the solution where it is liquid into the crystal where it becomes part of a solid structure and lays it down in the exact position in which it fits, just as one bee’s cell is added to another in the growth of the comb. We have advanced a step, but only one step, towards the better understanding of the mystery, and I would beg you to note how we are continually led on by analogies which may be quite false, but which are at any rate fruitful.

Now let me pass to another series of researches which were conducted by Miss Isaac and myself for a few years before I left Oxford and have since been carried on by her with conspicuous success. Still experimenting with the same apparatus, and endeavouring to trace how a solution changes in strength while it is crystallizing, we came across some curious and unexpected results. It is, of course, well known that if a crystal, say of alum, is placed in a weak solution of alum it is dissolved, and only has the effect of making the solution stronger, but that at last a stage is reached at which the solution becomes ‘saturated’ and can dissolve no more, just as a stage is reached at which a soaking sponge will hold no more water. At this point a crystal, if put into it, remains unchanged. But it is quite easy to make the solution still stronger, not by adding alum to it, but by taking water away from it by evaporation: it then becomes oversaturated or ‘supersaturated’, and now a crystal of alum dipped into the liquid will at once begin to grow and to make the solution weaker. In fact, this is an unfailing test by which we can tell whether a solution is saturated or supersaturated; and, more than this, until a bit of solid crystal gets into it the liquid does not crystallize. Keep it in a closed vessel so that no speck of alum can fall into it, and it will remain liquid for weeks or years or as long as you please. But let the smallest possible grain of an alum crystal fall into it, and crystallization will be started; inoculate it with an invisible germ of alum dust, such as must be flying about in the air of any room where dry alum is or has been kept, and you will see the life and growth of a crystal begin when that germ is introduced. This is the most extraordinarily sensitive test, and one that can easily be applied.

In many of our experiments we found that during the first day when we were working with some new substance it would not crystallize from an exposed solution; but on the second day, when the air of the laboratory had become impregnated with crystal germs, an exposed solution would begin to crystallize at once.

Dr. Tutton, one of the most accomplished investigators of crystals, whose refined and beautiful researches were for many years carried on in Oxford, has fully described these effects in his two books on crystals just published, and confirms them from his own experience.

Now, Miss Isaac and I have found in the course of our researches that, as the solution becomes stronger and stronger—say, by the evaporation of some of its water, it continues to be in the ordinary state of supersaturation in which it does not crystallize save by inoculation with a germ of solid alum crystal, but at last it suddenly reaches a condition in which it can crystallize spontaneously, and at this moment it is enough to stir or shake the solution, and you will at once witness the birth of thousands of tiny crystals which appear as a cloud in the liquid and begin to grow rapidly.

A very easy way in which to make this experiment is to dip a clean needle into a drop of evaporating solution on a glass plate and to scratch the glass on which the drop lies: for a time nothing happens, and then suddenly, as the liquid passes into the new condition, a chain of tiny crystals appears along the line of scratch.

That this suspended crystallization has a fixed limit was suspected, and had been predicted by the German chemist Ostwald, but could never be proved until we made our experiments, and it was Mr. Hartley who first helped us to interpret our results. He has subsequently, with his pupils, made a number of investigations on the same subject.