Its form is the expression of its permanent composition, and so far as we know has always been the same. Indeed, until it has been converted into its constituent elements and destroyed, it is in a sense not only unchanging, but imperishable. For, take a crystal and break or dissolve it away until only the tiniest fragment remains; that fragment (e. g. sugar), though it may be only an invisible speck, will, if immersed in the appropriate solution, continue to grow again and will once more assume the form of a perfect crystal. Neither does it make any difference if the crystal fragment has been kept for years or even centuries; it will, when supplied with nourishment from the appropriate solution, heal itself and continue to grow as though the process had never been suspended. In a sense it is immortal, for, if not destroyed, it never loses the mysterious power of growth, and is therefore more imperishable than any seed or germ of life.

The main conclusion of my lecture was that, having been led by analogy to compare the growing crystal with the growing plant, one finds the growth and life of the crystal to be totally unlike the growth and life of any organism. Its life is only an unchanging persistence without the display of any struggle for existence, any movement, any adaptation, or response to environment; its growth is only the addition of new material on the surface of the old, without assimilation, so that the crystal remains uniform and increases in size; there is no distinction of any one part from any other; the crystal grows, therefore, without the development of any organs.

One appears to be left with the conclusion that the crystal should be regarded rather as a type of death than of immortality.

Schopenhauer expressed this idea when he said: ‘The crystal has only one manifestation of life, crystallization, which afterwards has its fully adequate and exhaustive expression in the rigid form—the corpse of that momentary life.’

This, however, is not the conclusion which I would have you draw from the facts; and I shall ask you on the present occasion to accompany me in pursuing a little further the inquiry into what we may call the vitality of a crystal and the manner in which it is displayed; although this vitality is not to be confounded in any way with that of a living plant or creature.

It is, I hope, not unfitting that the subject of crystal growth should be dealt with in a Boyle Lecture, for Robert Boyle was himself one of the first to treat it in a scientific spirit. He was the exact contemporary of Nicolas Steno, the famous Danish physician who laid the foundation of modern crystallography; and Boyle, in his treatise on the origin and virtues of gems, was the first to express the conviction which was almost simultaneously expressed by Steno—that gems must have solidified from the liquid state. One of his reasons for thinking so was their crystalline form, which resembles that of salts which crystallize from solutions. Let me quote his exact words: ‘The origin assigned to gems may be countenanced by the external figuration of divers of them. For we plainly see that the corpuscles of nitre, alum, vitriol, and even common salt, being suffered to coagulate in the liquors they swam in before, will convene into crystals of curious and determinate shapes.’ And then he points out that when a salt such as nitre crystallizes in a vessel it is only where it is free to grow in the liquid, and away from contact with the sides of the vessel, that it can acquire this shape. These crystals (he says), ‘having a fluid ambient to shoot in, will have those parts of their bodies that are contiguous to the liquor curiously formed into such prismatical shapes as are proper to nitre.’

When Boyle wrote these words he had not seen the work of Steno, which had just been published.

I may, therefore, claim that he was the first person to deal in a scientific spirit with the subject which I have chosen for this lecture, the ‘Growth of a Crystal.’

Three years before I came back to Oxford—on May 16th, 1893, the subject of crystal structure had been treated in the second Boyle Lecture in a most original and masterly manner by no less a person than Lord Kelvin. I well remember that Lecture, at which I had the good fortune to be present; and to one who was already interested in crystals it was a wonderfully illuminating and inspiring address.

On that occasion my venerable predecessor and teacher, Professor Story Maskelyne, conducted Lord Kelvin to the lecture table.^[1]

At that time, however, there was no laboratory in Oxford for mineralogical and crystallographical research; the Professor only had a lecture-room, he did not reside in Oxford, and his scientific work was necessarily carried on elsewhere. To-day I am more fortunate in being able to draw upon the resources of the well-equipped laboratory of my successor, and former pupil, Professor Bowman, and upon his still more valuable personal assistance and that of Mr. Barker; so that, though I only have to deal with ideas that are simple compared with those which issued with fiery vigour from the fertile brain of the Boyle Lecturer of 1893, I have a better opportunity of showing to an Oxford audience to-day the actual things of which I am speaking, and may help to make my meaning clear to those who cannot know much from personal experience concerning the growth of crystals.^[2] When I delivered my own inaugural lecture I had no means of making visible to an audience the astonishing features of crystal growth. Beautiful effects may easily be witnessed by anyone with a few drops of common solution and a magnifying glass, yet I believe that they are witnessed by comparatively few persons.

I have alluded to laws of crystalline structure, to which Lord Kelvin had directed attention: these are the laws of geometrical arrangement which prove crystals to be constructed in an entirely different manner from the living plants which they may so closely resemble; these laws, however, were not established by observations or experiments upon the growth of crystals. They were the result of a century of patient measurements of the external shape of countless crystals; more than a century of accurate determinations of what happens when heat and light are transmitted through them and of numberless other experiments made by physicists;