claims that the capillary action existing between the liquid and the crystal intervenes, an effect varying with the nature of the faces belonging to the diverse forms and with the nature of the liquid. Basing his belief on Gauss's theory of capillarity, he concludes that such faces develop or require the minimum expenditure of capillary energy. The dominant forms must consequently be conditioned by those faces the constant capillarity of which is the least. The addition of a foreign substance altering the different capillary constants may consequently induce modifications of form. It appears, indeed, that the capillary forces must act, but up to this time there is no fact known which proves that they intervene sufficiently to modify the forms, in spite of the experiments of M. Berent carried out in the laboratory of Sohncke; moreover, I shall describe later an observation showing they are without influence. IV. The crystals of one substance rarely form synchronously with those of another dissolved in the same mother liquid, and it is on this property that chemists base their action when they attempt to purify bodies by repeated crystallizations; but there are exceptions, as in the well-known coloration of hydrated nitrate of strontium by extract of logwood, which was accomplished by Senarmont. Since then M. Lehmann and I have proved a few other cases of coloration of crystals by artificial organic dyes. By making use of the artificial coloration of crystals so as to indicate the presence of foreign matter which has crystallized with the colorless substance I have been enabled to show that the absorption caused modification in form. The absorption of foreign matter by crystals in process of formation is accomplished in two different ways: First, the coloring matter enters into the composition of the crystal, whatever may be its degree of dilution, and is shared between the crystal and the liquid; second, the coloring matter is taken up by the crystal only when the liquid becomes saturated. The two processes may go on simultaneously. The study of certain properties of colored crystals, particularly polychroism, and the law of division, shows that the coloring substance in the first case is found in the crystal in the same state as in the liquid; in the second, on the contrary, the coloring matter is in the crystalline state, and we have to do then with a regular grouping of the crystalline particles of the colorless substance with those of the coloring material added to the mother liquor. Lead nitrate is colored by methylene blue in the second manner; it appears in cubic crystals with the triglyphic striæ of pyrite in stead of in octahedrons. The modifications in the crystals of this salt produced in a mother liquor which holds methylene blue in solution, show that capillarity does not intervene to produce them. Indeed, in a solution depositing lead nitrate and saturated with methylene blue, without, however, giving crystals of this latter substance, the crystals of the nitrate are not at all modified. They are in octahedrons and colorless, but as soon as the coloring matter begins to crystallize synchronously with them the cube faces appear and finally are formed to the exclusion of all others. Nevertheless, the surface tension can not have been changed since the quantity of methylene blue has remained the same in solution. An interesting fact is the inequality of absorption of the foreign matter dissolved in the mother liquor by the different faces of the crystal. Thus, on the octahedral faces of the lead nitrate the methylene blue is not at all deposited, but only on the faces of the cube and the pentagonal dodecahedron. Similar examples can be cited which explain the appearance, frequent in minerals, known as hour-glass structure. In the case of cubic crystals, of all the possible faces it is only those which absorb the foreign matter which will develop. The idea which first comes to mind is that the molecular structure of the crystal plays an important part in this synchronous crystallization. It is not so at all; different foreign substances may be absorbed by different faces, and in such cases the habit of the crystal is dependent on these diverse substances. If one causes the colorless substance to crystallize in a solution containing two colors, each one giving characteristic forms, the crystallizations thus obtained will be the two combined forms, so that one and the same crystal is composed of pyramids or of prisms of different colors. Thus the crystals of urea nitrate colored by methylene blue and picric acid show, if the crystallization has been carefully conducted, yellow triangular prisms corresponding to the faces g' and h', and blue triangular prisms corresponding to the prismatic faces m of the monoclinic system. Not only may foreign crystalline matter be absorbed but also the liquid matter added to the mother liquid, and even the molecules of the latter may also pass regularly into the crystal to modify its form. I have been able to show this fact by crystallizing phthalic acid in a solution containing ethyl alcohol." This explains why a crystal obtained from different solvents may show different faces. Consequently a crystal, very pure in appearance, transparent, and without inclusions, may contain foreign matter, and in the case where a To show this, it is enough to take a colored liquid, but with the exception of bromine there is no liquid which has a proper color at the ordinary temperature. it is the mother liquid which is absorbed its purification is impossible. The solvent must be changed. When the crystals of a determined substance obtained from two different solutions do not present the same forms, it is incontrovertible that in one of the cases, perhaps in both cases, since we do not always know the form of the pure crystal, there has been absorption of the molecules of the mother liquor. Sometimes it is the water which is absorbed, and this water has been regarded as water of crystallization or as water of constitution, according to the temperature at which it is driven off. When purification is attempted by recrystallization, if the foreign substance which passes into the crystal is present in small quantities in the mother liquid, the first or the last crystals formed, according to the mode of synchronous crystallization, will be the purest. In case there is a division of the foreign matter between the crystal and the liquid, if the coefficient of its solubility in the crystal and the liquid are known, the number of crystallizations demanded for the purification of the crystals may be calculated under proper conditions. V. The natural crystals appear in such varied habits that before Romé de l'Isle no one could see the constancy of forms, and the genius of Hauy was necessary to establish their derivation. It is known that ordinarily the crystals of the same deposit and of the same generation are identical, and that those of successive deposits or generations may have different dominant forms. All these differences may be explained by the rapidity of crystallization, but especially by the constant presence of foreign substances. Unfortunately it is difficult to determine the nature of the latter, since the results of analyses made up to the present time have little value in solving this problem. Indeed, a very small quantity of matter is required to modify the forms of a crystal; sometimes an amount even less than one-one thousandth of the weight of the latter is sufficient. In every case, whether we have to do with natural or artificial crystals, we need to determine their form in the pure state, a form which is constant and which I have called fundamental. It may be distinct from the primitive form chosen by crystallographers. In closing, I shall observe that the substances prepared in laboratories seem rarely to show the numerous modifications of form, so frequent in the natural crystals. This is due to the fact that the artificial crystals are prepared almost always in the same manner, with the same reagents and consequently with the same foreign substances in the mother liquor. In nature, on the contrary, as the analyses of mineral waters show, the composition of the solutions which deposit the crystals of a given substance varies from one region to another as much in the quality as in the quantity of the different elements dissolved. But all crystals do not lend themselves with the same facility to these modifications of the faces; and just as there exist in nature bodies like calcite which possess the most varied habits, there exist also artificial compounds, the crystals of which may appear in a great number of forms depending on the condition of crystallization, as, for example, phthalic acid, meconic acid, nitrate and oxalate of urea. THE DISTRIBUTION OF THE ELEMENTS IN IGNEOUS ROCKS.@ By HENRY S. WASHINGTON, New York, N. Y. (Chattanooga meeting, October, 1908.) I. INTRODUCTION. During the last twenty years or so the chemical investigation of rocks has made great advances, and it is now generally recognized that a knowledge of the chemical composition is as essential as that of the texture or mineral composition, if not more so, for the proper classification of rocks and study of their origin and relationships. Rock analyses have vastly increased in numbers and, what is of greater importance, in quality. New and improved methods permit of greater accuracy than was possible in the early days, and the list of chemical constituents frequently determined has risen from the seven or eight of the greater part of the nineteenth century to twenty or more. Indeed, rock analyses with determinations of so many constitutents are now commonly made by the chemists of the United States and Australia, while in Germany, Great Britain, France, and Italy the rarer constituents are determined more frequently than formerly. As a consequence of this modern, accurate work, it has been discovered that some elements which were formerly supposed to be rare are of widespread occurrence and are often present in considerable amount. The fact is further being developed that the elements tend to show certain relations of occurrence or abundance in connection with each other. This is a fact which is applicable to the rarer elements, and which also finds a broad geological and petrological expression in the recognition of petrographic provinces. We are beginning to obtain some definite, though as yet rudimentary, knowledge of the distribution of the elements among igneous rocks. a Reprinted by permission from Bi-Monthly Bulletin of the American Institute of Mining Engineers, New York, No. 23, September, 1908, pp. 809-838; also in Transactions, pp. 735-764. 45745°- 279 |