I need not weary this audience with any description of these experiments, which doubtless seem more interesting and important to their author than to anyone else. But I wish to draw your attention to the result that came out of them. I found that it was possible with the same apparatus to measure the refractive power of the liquid in absolute contact with the growing crystal and from this to calculate the exact strength of the solution at that spot. It was thus possible to prove that the solution in contact with the crystal is rather stronger than at a very short distance from it, and to know exactly how much stronger. In other words, while a crystal of, say, alum is growing, the liquid in contact with it contains more particles of alum and less particles of water, or is richer in alum, than the liquid at a short distance from it. I will ask you to bear this in mind in what follows. There is another curious fact connected with this subject. Crystals of nitrate of soda have almost exactly the same shape and almost the same physical properties as crystals of the common mineral calcite, which, in its purest and most perfect form of transparent glassy crystals, is known as Iceland spar. Now, when a perfectly clean crystal of Iceland spar is immersed in a strong solution of nitrate of soda, although it is not dissolved by the liquid itself and therefore cannot crystallize out of it, the Iceland spar actually continues to grow, and becomes enveloped by the nitrate of soda so as to form what is apparently a single crystal. It appears, therefore, highly probable that a crystal of nitrate of soda and a crystal of Iceland spar behave alike in this respect when placed in a strong solution of the nitrate; each draws to itself the liquid nitrate in the solution, then draws it out of the solution in the solid state, and further arranges the particles upon its own surface in a perfectly regular manner, so that the arrangement of the particles in the shell of nitrate is the same as the arrangement of the particles in the spar which it surrounds; just as a bricklayer sets upon the rising wall new bricks arranged in the same way as those which he has already laid. I remember that on LORD KELVIN’S last visit to Oxford, shortly before his death, mindful of his BOYLE Lecture, I showed him, in company with my pupil, Mr. BARKER, this beautiful experiment, with which he was at that time not familiar, and I shall never forget the interest and enthusiasm with which he witnessed the beautifully regular and instantaneous growth of the nitrate crystals. He always was as enthusiastic and inquiring as a boy, and these characteristics were exhibited on that occasion in his old age. It is an experiment which sets one thinking, and I have no doubt that, if LORD KELVIN, even at that advanced age, had set his mind to consider it, he would have been able to deduce far more than it has yet suggested to those who have witnessed it.[3]