Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/706

672 CKYSTALLOGKAPHY cold in the lofty mountains makes the ice so dry that it congeals into crystal (ex illo sicco coagulat glaciem in crystallum). Agricola, three centuries later, knew little more, though affirming that crystallus was not ice but rather succus frigore densatus. Still he indicates some simple forms of crystals, and notes the fissile structure of some stones, as the lapis specularis (probably gypsum or mica), a property which as cleavage soon exercised much influence. Nieolaus Steno, the Dane, born in 1638 at Copenhagen, but for a time resident at Florence, amidst his well-known studies in anatomy, found leisure also to speculate on questions concerning the structure of the earth and the nature of gems and precious stones. As his treatise De solido intra solidum naturaliter contento, published in 1669, anticipated in geology some modern speculations and theories subsequently confirmed, so it also contains the germs of important facts in crystallography. It was still the wondrous rock-crystal with the polished sides of the middle prism and the terminal points of the pyramids, joined by the central axis of the crystal, that formed the starting-point of his speculations, and led him to introduce some new notions and terms into the science. How these crystals originated was doubtful, but they evidently grew, not from within like plants, but from without, by the addition of new layers of minute particles carried to the crystal by a fluid, and laid down specially at the ends, as shown by the fine striae that are never wanting on the middle planes. His rejection of extreme cold as the causa efficiens, for something similar to magnetic power, is again a suggestive idea, and not less his conclusion that crystals therefore were not formed only at the first beginning of things, but continue to grow even at the present day. Still more important as a step in the progress of the science would be his assertion that the number and length of the sides in the plane of the axis may vary widely without change in the angles (in piano axis, lalerum et numerum et longitudinem varie mutari non mutatis angulis), could we regard it as having a wider application than to the case in hand. It was perhaps more a deduction from the mathe matical form of the body than a generalization from observed facts. But some of his other descriptions show great powers of observation, and in his notice of the cleavage of calcspar, and its division into other rhomboidal bodies, we have again a fact that in other hands was to bear important fruits. Erasmus Bartholinus, another Dane (born 1625, died 1698), made known, in his Experimenta Crystalli Islandifi disdiaclastici (1670), another property of the same mineral, very remarkable in itself and its results to science. This was the double refraction of the beautifully transparent variety sent from the Rodefiord in Iceland to Copenhagen. In the same tract, it may be mentioned in passing, occurs the first reference to the blowpipe as a mean of applying heat to minerals. But the optical fact, turning the atten tion of mathematicians to crystals, had more direct influence on our science. The celebrated Huyghens described the same miranda refractio, and pointed out its laws ; hs also measured the angles of the rhomboids with a close approximation to truth, and remarked the occurrence of a less distinct double refraction in quartz in crystallo duplex esset re/radio. He likewise observed the peculiar cleavage of calcspar, which he tried to explain by building up the crystals of spheroids. Leeuwenhoek also, in his Arcana Naturce (1695), mentions cleavage in gypsum and Muscovy glass, and tried to estimate the thickness of the laminae, which Newton in his Optics in 1706 showed could be calculated from his doctrine of the colour of thin plates. In the same work Newton gives an account of the double refraction of Iceland spar and the laws it follows, and, observing the changes to which the rays were subject, asks if these rays of light may not have different sides, with different properties the first anticipation of the polariza tion of light, so important in this science. Returning to Leeuwenhoek, we find him showing salts of various forms, growing up in solutions under his microscopes. About this time too Guglielmini in his treatise on these bodies, De Salibus Dissertatio Ejyistolaris (1707), tried to prove that they could all be divided into molecules of a few regular forms, and affirms as a consequence that the inclination of the planes and angles is always constant. At a somewhat earlier period the celebrated Robert Boyle had published a treatise on precious stones, in which he describes many properties of crystals and their peculiar forms which he compares to those of salts. He also pointed out the crystallization of bismuth from fusion by heat a fact often overlooked by later observers. The attention of men of science was now thoroughly directed to the forms and origin of these bodies, and many curious observations might be collected from the writings of De la Hire, Woodward, Cappeller, Henckel, and others. But we pass on to Linnaeus, whose Systema Naturce formed, Lin in this as in other departments of natural science, the com mencement of a new period in its history. In his first edition in 1736 he gave a classification which, as he says himself, though far from perfect and often blamed, had enabled others mounted on his shoulders to see wider. Some of these successors he enumerates in the twelfth edition of his System (1768) among them Wallerius, Swab, and Cronstedt. He admits in the preface that he had laid aside the study of stones in which he once delighted, and therefore could not boast of his knowledge of lithology. Lithologia mild cristas non eriget, lapides enim quos quondaiie, in deliciis Jiabui, tradita dcmum aliis o 7 is- ciplina, sepos^l^, are his characteristic words Still there is much that was important in his work. Thus he distin guishes figured stones from those that are amorphous, and notes the difference of the tessellata or cubical from the prisma or long columnar and the pyramis or pointed forms. Then he figures rudely, it may be, and describes some forty common forms of crystals, and gives examples of minerals in which they occur. His table of " Affmitates crystallorum" is even more suggestive, and could scarcely fail, if followed out, to lead to further advances. The use he made of these forms as important characters in describing and classifying minerals was well calculated to promote their study. Even the fact that he cut out models in wood of the forms he saw, shows in what a truly practical manner he regarded the subject. His notions regarding the formation of crystals were, however, very imperfect. Salt, he affirms, is the only known natural cause of crystallization, and con sequently the forms of the crystals of other substances were determined by the salts in union with them. This is the more remarkable, as he refers to an anonymous author in his own country, to whom he applies the words of Isaac, vox Swabii, manus Cronstedti, as refuting this theory from the fact that crystals of metals were produced by fusion. The advanced character of these views of Linnaeus appears more strongly when we contrast them with those of his great rival Buffon. According to him crystals are only a result of organization, so that the prisms of rock crystal, the rhombs of calcspar, the cubes of sea salt, tha needles of nitre, and others are produced by the motions of organic molecules, and specially of those derived from the remains of animals and plants found in calcareous masses, and in the layer of vegetable earth covering the surface of the globe. Hence he takes no note of crystallization among the characters of minerals. Very different was the influence of Linnaeus on Rome" Delisle (born 1736, died 1790), Ddi.- whose Essai de Cristallographie appeared first in 1772, and

in an enlarged form in 1783. Working 1 in the spirit of his