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86 for these researches, was able to direct the manufacture of chemical and optical glasses in the United States in a most successful manner, and performed very valuable services to American industry.

Theory of Ternary Mixtures. The theory of ternary mixtures has been very fully worked out mathematically by Willard Gibbs, Backhuis Roozeboom, Schreinemakers, Smits, Kuenen, Tammann and others, and may be said to be well understood in its main ap- plications. Experimental investigation has also made great progress. It is a laborious matter, requiring great skill and very elaborate appliances. The diagram we have given showing the tertiary system CaO, AU Oj, Si O 2, required over 7,000 experiments, in some of which the heating had to be continued for many hours and even for days; and the thermal results had usually to be checked by microscopic examination of the product. Other systems equally important have received very careful study. These are not so simple as the instance quoted, because mineral transformations, either during or after crystallization of the melts, often give rise to many complications. Thus, for example, CaSiOj crystallizes as pseudo-wollastonite at high temperatures, which may change to wollastonite at low tem- peratures. MgSiO 3 has four forms kupferite, magnesia amphibole, enstatite and clinoenstatite. Where both these substances separate from the melt, none of the above-named minerals appears, but a diopside clinoenstatite solid solution or isomorphous mixture, which belongs to the common group of minerals known as the pyroxenes.

These facts are of the greatest interest to both mineralogists and petrologists. They have introduced many new minerals (artificial) to our knowledge, and taught us their relationships at atmospheric pressures in certain dry meTts. They have also enabled us to under- stand many of the peculiarities of the minerals that occur in rocks, and the method of their origin.

As another instance we may quote the relations of fprsterite (Mg 2 SiO 4 ) and rustalite (MgSiOj). Forsterite may crystallize from melts of suitable composition and after a time it may become unstable in the residual magma; then it is dissolved up or "cor- roded " and clinoenstatite replaces it. Again, if an enstatite (MgSiOj) mixture be fused and allowed to cool, forsterite begins to separate out ; it will tend to be reabsorbed and converted at a lower temperature to clinoenstatite, but if this be prevented the mixture solidifies as forsterite, clinoenstatite and quartz. To the petrologist this is of great interest, because forsterite is one of the constituents of olivine, a very common mineral of the basic igneous rocks. Now olivine generally occurs along with some member of the pyroxene group, and the olivine crystals show rounded outlines, which have been taken to indicate corrosion or resorption by the magma after crystallization with concurrent formation of pyroxene. In some rocks there is clear evidence that olivine crystallized early and sub- sequently was entirely dissolved. In other rocks olivine is found enveloped in clusters of enstatite crystals which have evidently formed at its expense; and it is a very frequent characteristic of the olivines of gabbro and norite that they are surrounded by " reac- tion rims " or " corrosion borders " consisting of enstatite, tremolite and other magnesian silicates that have a higher percentage of silica than olivine itself. The meaning of these phenomena was fairly well apprehended by petrologists, and now their conclusions have been confirmed by experimental evidence. Another feature of some interest is that, in the ternary system under consideration, it has been shown that forsterite may crystallize early, then dis- appear by corrosion, and a second generation of forsterite may sub- sequently crystallize. Whether this is a common phenomenon is not yet known, and its exact relation to the frequent appearance of minerals in two generations in rocks remains to be demonstrated.

The number of ternary systems that have as yet been fully in- vestigated is small, though the results are of the highest importance. Those which contain the alkalis potash and soda present certain special difficulties, such as the volatility of part of the mixtures and the difficulty of crystallizing some of the minerals. Rapid progress is being made and it is unlikely that experimental difficulties will retard the advance of knowledge. A special case, of the greatest possible interest to the petrologist, arises where one or more of the com- ponents are volatile. Natural magmas are probably always richly charged with gases. The theory of such systems has been explored by Schreinemakers, Smits and others. Experiments with mixtures enclosed in steel bombs which can withstand great pressures at high temperatures have been very successful in certain cases, but as yet only the borders of this field have been explored. A good deal of work has been done on the dissociation pressure of sulphides of the metals, a subject of great importance as regards the paragenesis of natural sulphides and the conditions under which mineral veins have been formed. The effect of steam in the formation of silicates is under investigation and during the next few years will probably be the subject of much research.

Quaternary Mixtures. We have seen that systems of three com- ponents are much more complicated than systems of two com- ponents; and the addition of a fourth component greatly increases the difficulties. The theory of such systems is not as yet fully ex- plored; in fact, it is quite probable that many of the problems will not be solvable by mathematics. No four-component system has yet been completely studied, though parts of such systems have re- ceived investigation. As a diagram a solid figure bounded by three equilateral triangles may be used, and projections similar to those

employed to represent the faces of crystals have been tried. Systems more complicated than the quaternary systems are at present be- yond mathematical and experimental investigation.

' Constitution of Rocks. In rocks the least number of components that can be regarded as essential is seven (SiO 2, Al 2 Os, Fe 2 O 3 , CaO, MgO, K 2 O and Na 2 O). In rock analyses as a rule from 12 to 20 components are ascertained, but most of these are in small amount and may be regarded as unessential. To the seven main coinponents we must add the volcanic gases (such as H 2 O, F, Cl, HC1, .SO 2, B 2 O 3 ), most of which appear only in very small quantity in the crystallized rock, but are believed to exert a powerful influence in determining the crystalline phases that appear. High pressures must also be employed, and in some cases it is certain that the pres- sures were by no means constant. For these and other reasons it is clear that the genesis of such a rock as granite or basalt presents problems of the highest order of difficulty.

In the preliminary stages it may be possible to simplify the prob- lem by considering rock magmas, not as a mixture of the seven oxides above named, but as mixtures of minerals. If three minerals be taken a ternary system is the result. For example, a granite might be regarded as consisting of orthoclase, quartz and albite; a nepheline syenite as consisting of nepheline, albite and aegirine; a basalt as composed of olivine, diopside, anorthite. Such systems, of course, are a mere approximation and it is already known that it may not be safe to treat them as simply ternary. In the system diopside-forsterite-silica, for example, it is proved that under certain conditions spinel crystallizes, and the reactions can only be regarded as those of a four-component mixture. Another difficulty that is bound to prove important is the occurrence of isomorphous minerals. In petrology this is practically universal. Except quartz, all the common rock-forming minerals are members of complicated iso- morphous groups (felspars, nepheline, sodalite, micas, pyroxenes, amphiboles, olivines) ; and even in simple ternary systems, if two of the three minerals are isomorphous, there may be, strictly speaking, no eutectic mixture as the last stage of crystallization. It is not too much to say that the crystallization of rocks is very largely a problem of the formation of isomorphous crystals, and it is unsafe to apply to rocks the results derived from mixtures of minerals that do not show similar relations.

BIBLIOGRAPHY. For the recent advances in physico-chemical and experimental petrology no English textbook is available. Marker's Natural History of Igneous Rocks (1909) gives a sketch of the subject from the standpoint of its time, but is now out of date. A more adequate work is Boeke's Physikalische-Chemische Petrographie ('9'5)i which gives full references and is the best general account. Vogt's Silikat Schmeldosungen (1904) is interesting but very specula- tive. His Sulphid Silikatschmelzuneen (in course of publication) deals with some special aspects of the problem. The fundamental works on the theory of equilibrium are Roozeboom's Helerogene Gleichgewichte and its continuation (3rd vol.) by Schreinemakers. See also Bancroft, The Phase Rule, and Findlay, The Phase Rule; Tammann, Krystalliziren und Schmelzen. For English and American students the best authorities are the papers published by the workers of the Geophysical Institution in Washington. See also Niggli, Lehrbuch der Mineralogie (1920), Die Leichtfluchtigen Bestandtheile im Magma (1920); Daly, Igneous Rocks and their Origin (1914);

Iddings, Igneous Rocks (1913); Holmes, Nomenclature of Petrol- ogy (1920) ; Stark, "Petrographische Provinzen" (in Fortschritte der Mineralogie, 1914). (J. S. F.)

PFLANZER-BALTIN, KARL, FREIHERR VON (1855- ), Austro-Hungarian general, was born at Pecs in Hungary in 1855. He served in the cavalry and on the general staff, but in 1914 found himself, on account of precarious health, unattached, and it was only in the autumn of that year, when Rumania appeared to be turning against the Central Powers, that he was charged with the defence of Transylvania. But when the Russians at this period crossed the Carpathians, and there was immediate danger of their eruption into the plains of Hungary, Pflanzer-Baltin, with a division improvised by his brilliant talent for organization out of next to nothing, threw himself on this enemy, and conducted the defensive in the form of a series of daring offensive movements. After fighting with varying success in the southern part of Eastern Galicia and in the Bukovina the VII. Army under his command was driven back by Brussilov's offen- sive in June 1916, whereupon he was relieved of his command. In the summer of 1918 the Austro-Hungarian front in ; Albania yielded before the attack of the Entente army. Pflanzer-Baltin, entrusted with the command in this theatre of operations, won back, after a brief and powerful attack, the old positions south- wards of Fjeri and Berat the last considerable success which fell to the Austro-Hungarian army in the field. (A.-K.) PHARMACOLOGY (see 21.347). The science of experimental pharmacology is a product chiefly of the last half-century, and