ZOOLOGY AND BOTANY MICROSCOPY, ETC. 83 



Metallography, etc. 



Metallography of German Silver. *— F. C. Thompson has studied 

 the microstructure of commercial specimens of German silver, consisting 

 of the a solid solution of the copper-zinc-nickel system. In one series 

 of experiments four melts of identical composition were made. To one 

 was added ■ 25 p.c. manganese, to another • 5 p.c. aluminium ; while 

 no addition was made to the remaining two. The alloys after casting 

 were rolled into sheets and annealed. The mean area of the crystals of 

 the two alloys which had not been deoxidized was 0*3 sq. cm., while the 

 mean crystal-area of the alloys deoxidized respectively with manganese 

 and with aluminium was ■ 0005 sq. cm. The manganese had passed 

 almost wholly into the slag, hardly a trace remaining in the alloy. A 

 specimen of high nickel content, after annealing at an excessively high 

 temperature, was found to have a very definite " casting " structure. 

 It was then annealed at 750° C. for four hours. The structure after 

 this treatment was normal, and all traces of the dendritic markings had 

 disappeared. The specimen was next reheated to about 1000° C. for 

 one hour : this caused the reappearance of the casting pattern. Th 

 author believes that the casting pattern, reproduced by overheating 

 previously annealed alloys,, is a remnant of the structure of the original 

 ingot, but so faintly preserved that in ordinary circumstances it is not 

 seen. Its reappearance may be due to incipient volatilization of zinc 

 occurring at high temperatures. An overheated specimen showed 

 almost complete absence of twin crystals. A series of seven alloys 

 ranging in nickel content from 7 to 22 p.c. were submitted to ordinary 

 commercial treatment. The size of the crystals diminished notably 

 with increasing nickel content, the mean crystal-area being 0*015 sq. 

 cm. in the alloy containing 12 p.c. nickel, and 0"0005 sq. cm. in the 

 alloy containing 22 p.c. nickel. The sections were etched with 5 p.c. 

 ferric chloride solution. 



Artificial Twin-crystals in Tin.f — The crystals of a block of tin 

 affected by stresses may contain twinned lamellae ; these become ap- 

 parent in an artificially-polished surface etched with hydrochloric acid. 

 P. Gaubert has studied the formation of the twin-crystals in the following 

 manner. A few grammes of tin were melted between clean plane glass 

 surfaces. By pressing the upper glass plate, the layer of tin could be 

 brought to the desired thickness. On solidification a plate of tin was 

 obtained with plane surfaces having a perfect polish. By controlling 

 the rate of cooling during solidification, the crystals could be obtained 



* Journ. Cheiu. Soc. cv. (1914) pp. 2342-9 (7 figs.). 

 t Oomptes Rendus, clix. (1914) pp. 680-2. 



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