288 Sub- Lieut. A. L. Williams on the Electrical 



were at room temperatures and when their temperature was 

 gradually raised by means of an electric furnace. The object 

 in view was to see whether the fused mixtures possessed any 

 crystalline structure, ;md if they did whether the increased 

 conductivity observed with them on raising their tempera- 

 ture could be connected in any way with observable modifi- 

 cations in their crystal structure. 



I. Preparation of Specimens. 



In preparing these specimens they were first of all filed 

 oft' to an approximately flat surface. The surfaces were then 

 ground on a carborundum wheel, and after that on several 

 successive grades of aloxite of increasing fineness. The 

 grades used were those commercially known as Nos. 90, 150, 

 220, and 3F respectively. The polishing was then started 

 with optical alundum and finished with jewellers' rouge. 

 The two coarsest grades of aloxite were used on a flat metal 

 plate, while the finer grades and the optical alundum were 

 used on fine even linen fabric stretched over a smooth glass 

 plate. The rouge was used on a piece of soft, smooth broad- 

 cloth stretched over a glass plate. The plates used were 

 fastened on a horizontal revolving table rotated by a small 

 electric motor. In some cases the surfaces were etched with 

 nitric acid of various concentrations ranging from strengths 

 of 10 per cent, to 25 per cent, and even to 50 per cent. 

 Better results, however, were obtained by the use of ammonia 

 in solution, with a specific gravity of about 0*93. With this 

 solution the specimens were found to be uniformly etched 

 by an attack of about one hour. 



II. Optical Equipment. 



The microscope used was one of the instruments especially 

 designed by Bausch and Lomb for micrographic work. For 

 normal illumination the type of illuminator used was the 

 usual reflecting disk of thin cover glass. In this method 

 the light was projected at right angles to the optical axis of 

 the microscope, reflected from the cover glass along the 

 optical axis of the system to the specimen, and then back 

 through the microscope. For visual examination the source 

 of light was a frosted electric light bulb, while for the photo- 

 graphic work a small carbon arc was used. The photographic 

 plates used were rapid panchromatic, and the shorter wave- 

 lengths in the illuminating beam were cut out with a Wratten 

 and Wainwright filter. Oblique, in place of normal illumina- 

 tion, was used in some cases. 



