134 CARNEGIE INSTITUTION OF WASHINGTON. 



(472) Temperature changes accompanying isentropic, isenergic, and isenkaumic expan- 



sion. Leason H. Adams. J. Wash. Acad. Sci., 12, 407-411. 1922. 



When the pressure on a substance is released, the change in temperature 

 depends on the method by which the pressure is dissipated. In this note the 

 three principal types of expansion are compared, and equations and numerical 

 examples are given. 



(473) The jades of Middle America. Henry S. Washington. Proc. Nat. Acad. Sci., 11, 



319-326. 1922. 



In this paper are described the various materials known as jade, of which 

 are made many ancient objects found in Mexico and Central America. The 

 descriptions are based largely on a considerable number of such objects found 

 at Chichen Itza in Yucatan. It is shown by microscopical and optical exami- 

 nation and by many chemical analyses that these jades form a series of peculiar 

 rocks running from a pure diopside-jadeite to nearly pure albite. A pure 

 diopside-jadeite (called tuxtlite) is best represented by the material of the 

 Tuxtla statuette described in a previous paper ("The jade of the Tuxtla 

 statuette," Proc. U. S. Nat. Mus., vol. 60, art. 14, 1922; see abstract No. 438, 

 Annual Report, 1922). This combination of diopside and jadeite appears 

 to be hitherto unrecognized in mineralogy, and various series of determina- 

 tions of optical characters and densities are given to show its peculiarities. 

 Also, this peculiar albite-tuxtlite series of rocks seems to have been hitherto 

 overlooked. With scarcely an exception, the Middle American jades belong 

 to it, so that it is characteristic of American material. Minerals of this series 

 seldom or never are found among Chinese or other Asiatic jades, so that it 

 seems clear from the present studies that the provenance of the jade of the 

 ancient Middle American objects is either in Mexico or Central America and 

 that it has not been brought from Asia. The archaeological bearings of 

 this are briefly discussed. 



(474) On the crystal structure of ammonium chloride. Ralph W. G. Wyckoff. Am. J. 



Sci., 4, 469^75. 1922. 



It is shown that the Laue photographic data obtained from crystals of the 

 low-temperature form of ammonium chloride are in agreement with the 

 powder data in assigning to it a structure containing one chemical molecule 

 within the unit cube. The consequences introduced by the disagreement 

 between the symmetry of this uniquely determined structure and the syni- 

 metry as obtained by ordinary crystallographic means are discussed. It is 

 pointed out that it is not permissible to accept etch-figure data and face 

 development as definite indications of the symmetry of the arrangement of 

 the atoms within a crystal. 



(476) A theory of the structure and polymorphism of silica. Robert B. Sosman. J. 

 Frankhn Inst., 194, 741-764. 1922. 



There exists a wide variety of experimental data on the forms of silica, and 

 particularly on quartz, which have never been assembled and explained on the 

 basis of a single consistent set of hypotheses as to the ultimate structure of this 

 substance. This paper attempts to provide such a set of hypotheses, based 

 upon the general knowledge already gained concerning the structure of matter 

 in general and silica in particular. 



It is believed that the silica atom-triplet maintains a certain degree of 

 individuality in its amorphous and crystalline states as well as in its com- 

 pounds, and the freedom of its oxygen atoms to change their positions with 

 respect to the silicons is restricted. The triplets are assumed to assemble 

 into chains or threads in the liquid and glassy states, and a thread structure 

 persists in the crystalline states. The three principal crystalHne modifica- 



