GEOPHYSICAL LABORATORY. 137 



is more than half fused at 1150°. Yet the inclusions as actually found show- 

 no indication of fusion or flow. The most probable explanation of this 

 discrepancy is that the fusion temperature of the diabase has been lowered 

 at the time of intrusion, by water or other volatile components which are no 

 longer present in the rock. 



(17) An electrical goniometer fiirnace for the measurement of crystal angles and of 



refractive indices at high temperatures. Fred. Eugene Wright. J. Wash. 

 Acad. Sci., 3, 396-401. 1913. 



In this furnace the effort has been made to produce an instrument of fair 

 precision which may serve not only for the measurement of the interfacial 

 angles of a crystal, but also for the direct measurement of its refractive 

 indices at any temperature up to 1225° C, at which temperature the light 

 of the furnace becomes relatively intense and the measurements are corre- 

 spondingly less satisfactory. 



The furnace fits as an attachment on the Goldschmidt two-circled gonio- 

 meter. It consists of two flat disks of alundum (7 cm. in diameter and 5 mm. 

 thick), on one side of which a spiral of 1.75 mm. pitch is stamped; into 

 its grooves platinum wire 0.4 mm. thick is wound and then covered with 

 a thin layer of alundum cement and baked at 1200° to 1300° C. These 

 disks are backed by magnesia powder and mounted in hollow cylindrical 

 water-jackets. Their edges are shielded from direct contact with the water- 

 jackets by asbestos-wool packing. Since alundum is a good conductor of 

 heat the alundum segments tend effectively to render the heat distribution 

 uniform at the center of the furnace, while the outside asbestos rings are 

 poor conductors and tend to confine the heat properly. Temperatures are 

 read by means of the thermo-element in contact with the crystal, either on 

 a millivoltmeter of the Siemens and Halske type or on one of the poten- 

 tiometer-galvanometer systems of this laboratory. 



After the proper electric and water-jacket connections have been made, 

 the furnace can be carried to any temperature up to 1225° and the crystal 

 angles or the refractive indices of a prism measured by the ordinary room- 

 temperature methods. As the crystal is inclosed in the furnace a dark- 

 room is not necessary for the measurements. 



It may be of interest to note that recent preliminary measurements in the 

 goniometer furnace on a cleavage rhomb of calcite indicate that the cleavage 

 angle of calcite at 600° C. is 75° 52', while at room-temperature (30°) it is 

 74° 55', a change in the cleavage angle of nearly 1 degree during a tempera- 

 ture rise of about 600°. At 700° the calcite crystal faces lose their luster and 

 become white (formation of CaO) and are valueless for goniometric work. 

 The above change in cleavage angle indicates an average increase of 1 minute 

 in angle for every 10° temperature rise. It would seem, therefore, that the 

 practice of expressing crystal angles to seconds of arc without giving the 

 temperature at the time of measurement can serve little purpose and is in 

 fact illusory as regards the actual accuracy implied. 



An extended series of measurements of the change of the optical proper- 

 ties and crystallographic angles of the rock-making minerals with tempera- 

 ture rise has been commenced at the Geophysical Laboratory. 



(18) The volcanic cycles in Sardinia. H. S. Washington. Advance publications, 



XII Congres G^ologique International, Toronto, 1913. 



The volcanoes of northwestern Sardinia were studied in the autumn of 

 1905 as part of a research under Grant No. 95. They belong to three 



