GEOPHYSICAL LABORATORY. 127 



THE TEMPERATURE FACTOR IN THE OPTICAL STUDY OF CRYSTALS. 



Two new pieces of accessory apparatus for use with standard in- 

 struments of petrographic research deserve more than passing notice. 

 The first is a small furnace arranged to be mounted directly on the 

 table of the petrographic microscope. It is completely inclosed by 

 running water, so that no heat escapes to disturb the optical adjust- 

 ments of the instrument. It differs from apparatus hitherto con- 

 structed for the optical study of crystals with changing temperature 

 in that it provides for turning the crystal about an axis perpendicular 

 to the line of sight during the observations. A graduated circle 

 measures the amount of the rotation. The apparatus permits the 

 accurate determination of three optical constants, the birefringence, 

 extinction angle, and optic axial angle, of properly oriented crystal 

 plates up to temperatures as high as 1200°. (Review No. 11 seq.) 



The second apparatus has a similar purpose, namely, to equip 

 the Goldschmidt two-circle goniometer with a similar water-inclosed 

 furnace, through the use of which the interfacial angles of crystals, 

 as well as their refractive indices (minimum deviation method), can 

 be conveniently and accurately measured within the same tempera- 

 ture range. (Reviews Nos. 17 and 44 seq.) 



All optical determinations through which crystalline minerals are 

 classified and their properties defined are subject to variations of 

 unknown magnitude with change of temperature. It will, therefore, 

 be one of the important problems to be actively prosecuted during 

 the coming winter to establish the magnitude of the variation with 

 temperature of a number of these optical constants. These two 

 furnaces together provide for the complete control of this variable 

 and hitherto neglected factor in the optical study of all crystals. 

 The temperature at which crystallographic and optical properties of 

 minerals have a measured value is of the same importance and has the 

 same significance as a knowledge of the temperature corresponding 

 to a given value of the specific gravity or the specific heat. It is not 

 proper to speak of these as characteristic constants except for a 

 definite temperature. 



VOLCANO STUDIES. 



The further prosecution of the field studies at the Kilauea volcano, 

 of which some account VN^as given in Year Book No. 11, page 99, has 

 been interrupted for the moment because of the cessation of the 

 volcanic activity there. It is planned to resume the investigation 

 as soon as a favorable opportunity offers. Meanwhile the laboratory 

 study of the material, both lava and gases, gathered in 1912, is 

 proceeding rapidly. The analysis of the gases shows them to be 

 made up primarily of free sulphur, free hydrogen, CO2, SO2, CO, 

 H2O, and N2, the proportions varying with each bubble which bursts 

 through the liquid lava. 



