GEOPHYSICAL LABORATORY. 153 



(3) The -prism method for the observation of interference figures. — A small 

 doubly reflecting prism of special shape is introduced in a sliding carriage 

 directly beneath the eyepiece and serves in the examination of interference 

 figures by the Lasaulx method. Its use obviates the necessity of removing 

 the eyepiece each time an interference figure is observed. Its small size, 

 moreover, functions as a diaphragm and enables the observer to examine the 

 interference figure from a single small mineral grain. 



(4) A device for use in the accurate measurement of extinction angles. — An 

 extension-arm mth a needle at its outer end is attached to the microscope- 

 stage. Settings on maximum darkness of a given mineral plate between 

 crossed nicols are recorded by pressing down the needle-point into a piece of 

 coordinate paper. The settings are made rapidly and are free from the 

 personal element which is introduced when angular readings are taken. 

 After a certain number of settings have been made with clockwise and counter- 

 clockwise rotation of the stage the eye can estimate the average center of a 

 series of points with sufficient accuracy for practical purposes and the angular 

 reading of this center may then be taken. 



(22) The petrographic microscope in aualvsis. Fred. E. Wright. J. Am. Chem. Soc, 38, 



1647-1658 (1916). 



In this paper the function of the petrographic microscope as applied to 

 certain classes of problems of a chemical nature is discussed in a general way 

 and its usefulness in such work is emphasized. Attention is directed to the 

 difference betw^een the ordinary microscope, which is only a magnifier, and the 

 petrographic microscope, which, in addition, serves for the determination of the 

 optical properties of minute crystal grains and plates measuring at least 0.01 

 mm. in diameter. The several optical properties thus used in diagnostic work 

 are described briefly and the mode of their determination by means of the 

 petrographic microscope is indicated. 



(23) Some reactions involved in secondarj' copper-sulphide enrichment. E. G. Zies, E. T. 



Allen (Chemical Study), and H. E. Merwin (Microscopic Study). Econ. Geol., 

 11,407-503 (1916). 



(1) The reactions of a number of natural sulphides Vv'ith copper-sulphate 

 solutions have been quantitatively investigated. Attention has been confined 

 to the following : chalcocite (CU2S) , covellite (CuS) , bornite (Cu5FeS4) , chalco- 

 pyrite (CuFeS2), pyrrhotite (FeS(S)2), pyrite (FeS2), sphalerite (ZnS), and 

 galena (PbS). In all cases a copper-enrichment product is formed, either a 

 sulphide which varies with the conditions, or as a special case, metallic copper 

 and cuprite. In all cases the sulphate of the metal contained in the original 

 sulphide is also formed, and usually sulphuric acid as well. This acid is 

 derived from the oxidation of the sulphur in the sulphide by cupric sulphate. 

 In these reactions cupric sulphate plays the role of an oxidizing agent, not only 

 at elevated temperatures, but at lower temperatures as well. 



(2) The sulphide-enrichment products are crystalline and all adhere 

 firmly to the altered sulphide as in nature. When cupric sulphate is the 

 enriching agent, pijrite alters to covellite and chalcocite. It has been shown 

 that the alteration to chalcocite is represented by the following equation: 



5FeS2+14CuS04+12H20 = 7Cu2S+5FeS04+12H2S04 



and that in the alteration to covellite the following equation in all probability 

 represents the reaction 



4FeS24-7CuS04+4H20 = 7CuS-|-4FeS04+4H2S04 



The evidence is good that this reaction is involved when pyrite alters to 

 chalcocite. Pyrrhotite alters to chalcopyrite and very probably to bornite. 



