144 CARNEGIE INSTITUTION OF WASHINGTON. 



(10) The measurement of the refractive index of a drop of liquid. Fred. Eugene Wright. 



J. Wash. Acad. Sci., 4, 269-279 (1914). 



The measurement of the refractive indices of irregular mineral grains 

 measuring 0.01 mm. in diameter is best accomplished by means of the immer- 

 sion method, by which the refractive indices of the mineral grain are compared 

 with that of the liquid in which it is immersed. The measurement of the 

 refractive index of a single drop of liquid can be accomplished by a number of 

 different methods. The accuracy of the standard methods (with the excep- 

 tion of the Abbe refractometer methods) for this purpose is discussed and 

 certain new modifications are suggested which render possible the application 

 of such methods to a single drop of liquid. The use of a diffusing screen in 

 front of monochromatic light sources is emphasized. A new type of hollow 

 prism is suggested which has proved satisfactory and convenient, and with 

 which accurate measurements (accurate to the fourth decimal place) can be 

 made either by a method of autocollimation or by the minimum deviation 

 method or by Gifford's method. The use of a cover of specially prepared 

 tinfoil on a drop of liquid to be measured on the Abbe-Pulfrich crystal total- 

 refractometer is suggested as a satisfactory method for obtaining the phe- 

 nomena of grazing incidence even on a thin film of liquid. Five new methods 

 are described for measuring the refractive index of a drop of liquid with the 

 petrographic microscope. Of these methods, that which requires simply a 

 plane-parallel glass plate of the high refractive index, with one edge beveled 

 at an angle of 60°, is the simplest and most convenient; with it the refractive 

 index of a drop of liquid can be easily measured to the third decimal place, 



(11) An occurrence of pyroxenite and hornblendite in Bahia, Brazil. Henry S.Washington. 



Am. Jour. Sci. (4), 38, 79-90 (1914). 



The igneous mass occurs near Maracas, in the State of Bahia, intruded into 

 gneisses. The central part is a hornblendite and the outer a pyroxenite, both 

 being exceptionally fresh. Complete analyses of the tAvo are given and their 

 relations to other occurrences are discussed. Both rocks are notable for the 

 large amount of manganese which they contain, which is correlated wdth the 

 abundance of manganese ores in this part of the State of Bahia. There is also 

 a notable amount of copper. 



(12) Hewettite, metahewettite, and pascoite; hydrous calcium vanadates. W. F. Hille- 



brand, H. E. Merwin, and Fred. E. Wright. Proc. Am. Phil. Soc, 53, 31-.54 

 (1914). 



Two apparently different calcium vanadates are described, which resemble 

 each other very closely and have the same composition (CaO. 3V2O5. 9H2O) 

 when holding their maximum water-content at room temperatures. One of 

 them, hewettite, occurs at Minasragra, Peru, and has been noticed on a single 

 specimen from Paradox Valley, Colorado. The other, metahewettite, occurs 

 at numerous localities in western Colorado and eastern Utah. Both minerals 

 are sparingly soluble in water. 



A third calcium vanadate, pascoite (2Ca0.3V205.11?H20), is also described. 

 This occurs with hewettite at Minasragra. It is very soluble in water. 



The first and second minerals are regarded as hydratcd acid hexavanadates 

 (CaH2VGOi7.8H20), the third as a normal hexavanadate (Ca2V60i7.11?H20). 



The reasons for specific separation of hewettite and metahewettite are set 

 forth in detail. The two minerals are so sensitive to changes in atmospheric 

 humidity that their water-content varies within wide limits at different times 

 of the year. The removal of all or nearly all the water does not result in 

 breaking down of the crystal structure, and until this has occurred the water 

 is wholly or in great part taken up again when opportunity is offered. 



