670 SUMMARY OF CURRENT RESEARCHES RELATING TO 



hastens the operation. The edges of the section should be bevelled to 

 avoid tearing the polishing papers and cloths. 



Fine Polishing. — The author insists on the importance of using 

 powders of uniform dimension of grain. The time spent in their pre- 

 paration is fully repaid by the increased rapidity of polishing. For iron 

 and steel three powders are used, sieved emery, levigated emery (finer), 

 and washed alumina. The author's methods for the preparation of these 

 are given in detail. Fine flannel maintained in. a state of tension on 

 glass is used as supporting medium for the polishing powders. Filtered 

 soap solution serves to fix the powder to the cloth. Surfaces thus 

 prepared may be used for polishing dry or damp. To shorten the time 

 occupied in polishing, revolving wooden discs, covered with fine cloth, 

 or felt discs, may be used in the final stage when alumina is employed. 



Methods of Etching. — A 5 p.c. solution of picric acid in alcohol has 

 come into general use. Two reagents recommended by Kourbatoff are : 

 (1) amyl alcohol containing 4 p.c. nitric acid ; (2) 4 p.c. solution of nitric 

 acid in ordinary alcohol 1 part, saturated solution of nitrophenol in 

 ordinary alcohol 3 parts. Cementite is readily coloured, other con- 

 stituents not being affected, by a solution containing 25 p.c. sodium 

 hydrate and 2 p.c. picric acid, at 100° C. 



Microscope. — The author has abandoned the use of the mercury arc 

 lamp, owing to the difficulties of manipulation and the long exposure 

 required, though excellent photographs were obtained by its aid. A 

 Nernst lamp with two thick filaments, so placed that their light is 

 superposed on the illuminator of the Microscope, gives good results ; 

 the source of light is sufficiently broad to eliminate the interference 

 fringes which give trouble when an ordinary Nernst lamp with a thin 

 filament is used. For steel sections exposures of 2-5 minutes are 

 usually sufficient. Several modifications in the Microscope and camera 

 used by the author are described. It is more satisfactory to obtain high 

 magnifications by employing objectives of higher power than by in- 

 creasing the distance between plate and eye-piece. 



Alloys of Copper and Aluminium.* — L. G-uillet confirms the 

 melting point curve (liquidus) of the copper-aluminium alloys obtained 

 by H. le Chatelier, with some slight differences. To determine the 

 curve of the " solidus " he has investigated the cooling curves of different 

 alloys and the micrography of alloys quenched at varying temperatures. 

 The alloys containing 8 p.c. to 14 p.c. aluminium have one and frequently 

 two critical points. The author distinguishes seven constituents in all, 

 three of which are compounds — Al 2 Cu, AlCu, AlCu 3 (?)— the others 

 being solid solutions. Their characteristics and conditions of formation 

 are described in detail. 



Constitution of Iron-Carbon Alloys.f — In an important paper deal- 

 ing with Roozeboom's application of the phase doctrine to the iron- 

 carbon system, E. Heyn points out that the science of metallography 

 has advanced enormously with the development of the theory of solutions 



* Rev. Metallurgie, ii. (1905) pp. 567-88 (4 diagrams, 2S photomicrographs), 

 t Iron aud Steel Mag., ix. (1905) pp. 407-17 and 510-18; x. (19(>5) pp. 42-52 

 (27 figs.). 



