Then he spoke of the sunrise, and the glowing sunset; and of the delicious hours of night; and of tent-life in the Desert; and of wandering Arabs, who revere the grave and silent man; and of the charms of an encampment in some green “oasis;” and then, strongest commendation of all, he said, “I am going back, among the children of the Desert!” CHAPTER VII. CONCERNING CERTAIN NATURAL PHENOMENA.—ORIGIN OF THE DIAMOND.—FORMATION AND COLOURING OF GEMS.—INFILTRATION OF PEBBLES.—CAUSE OF TRANSPARENCY.—INTEGRITY OF THE FOSSIL-NODULES.—THEORY OF THE SHATTERED FLINTS.—CONCLUDING OBSERVATIONS. The charm which wins and rivets our attention in such pursuits as those of Geology or Mineralogy, is not that the phenomena which we meet with are capable of being classified, and of forming a scientific system. All this is, no doubt, one day instructive and interesting; but it was an afterthought, the result of experiment. The first charm lay in that silent mystery which broods over every part of creation, a veil as yet unpenetrated by Science: it lay in our instinctive consciousness that Nature, in what are, perhaps, her simplest movements, still transcends all the master-pieces of Art. Take, for instance, the crystalline gems. These are the most remarkable substances with which we are acquainted; they are also among the most simple. Chemically speaking, the metals and gases may claim to be regarded as simpler bodies; but then it must be remembered that we do not meet with them in nature thus unmixed. Gases vary, both in volume and character, every instant; and ordinary metallic ores are penetrated and disguised by foreign matter; whereas faultless crystals, once perfected, appear to be unalterable. Some of these, as the DIAMOND, would almost seem to be an elementary substance; and yet this can hardly be the actual case. The philosophical account of a crystal, as “some substance, all the particles of which, being free to move, have been operated upon in the way of a chemical, perhaps an electrical change,” certainly makes against it; for, according to this definition, such “substance” is a first desideratum, without which we cannot have the “crystal.” Thus, oxygen and hydrogen, blended together in certain definite proportions, yield the fluid substance, WATER; and the “crystal” of water is ICE. In like manner, common white carbonate of lime, and the fluor spars, whose ingredients we know, are in daily process of formation. The philosophical idea, so accurately expressed in the above definition, is, doubtless, correct, if once we assume a past history for some substance, and take the crystal before us as its result. But, whether all bodies which we now meet with under the form of crystals _have, in point of fact, had such a history_, is quite another question.