636 SUMMARY OF CURRENT RESEARCHES RELATING TO 



excess of the cobalt solution and adding N/10 silver nitrate, th6 bases 

 of the zeolites are soon replaced by silver (diluted with potassium 

 chromate), whilst the basic cobalt salt becomes black, owing to the 

 formation of Co (0H)3, and this quickly reduces the silver salt, producing 

 a deposit of metallic silver on the calcite. 



Metallography, etc. 



Ancient Peruvian Bronzes.* — A detailed description is given by 

 C. H. Matbewson of the results of a metallographic examination of a 

 number of ancient bronze articles collected at Machu Picchu, in Peru, 

 by the National Geographic Society — Yale University Peruvian Expedi- 

 tion of 1912. In the first part of the paper the author points out the 

 possibilities and limitations of metallographic investigations in regard to 

 the determination of the past history of such specimens. The investi- 

 gation has revealed a considerable amount of knowledge concerning the 

 methods used by the Incas in producing the articles — e.g. conditions of 

 casting, mechanical working, and annealing — but it is not possible by 

 metallographic methods alone-to establish their age. Chemical analyses 

 of the articles studied are given. They were irregular in composition, 

 ranging from 3 to 13 p.c. tin, but they were remarkably pure. Their 

 purity, together with the fact that one article consisted of practically 

 pure tin, leads to the conclusion that the Incas were acquainted with tin 

 in an elementary state, and that the bronzes were made by alloying 

 purified copper and tin and not by smelting the mixed ores. All con- 

 tained a certain amount of sulphur, some much less than others, recog- 

 nizable under the microscope as cuprous sulphide. There is evidence 

 that the Incas had attained considerable skill in the art of casting, but 

 they were not acquainted with the use of addition agents for deoxidizing 

 purposes, as the castings are frequently unsound and consequently lacking 

 in hardness and strength. Perforations were obtained by casting rather 

 than by mechanical means, thus avoiding the difficulty of obtaining tools 

 to pierce such a tough material as bronze. No evidence was obtained 

 that the articles had undergone any special heat treatment. The struc- 

 tures were generally non-uniform and of small grain size, such as is 

 characteristic of bronze worked at low temperatures. The author dis- 

 cusses in general the relation between the rate of grain-growth in bronze 

 and temperature of annealing, time of annealing, and extent of cold- 

 work previously received, in connexion with its bearing upon the 

 interpretation of the structures of the ancient bronzes. Numerous 

 photomicrographs of etched sections taken from the articles investigated 

 are given, together with diagrams illustrating the articles, and the posi- 

 tions to which the etched sections correspond. 



Methods and Results of Etching.f — A general account of the 

 methods of metallographic etching and etching phenomena is given by 



* Amer. Journ. Sci., xl. (1915) pp. 525-98 (46 figs.). 



t Stahl und Eisen, xxxv. (1916) -pp. 1073-8, 1129-35 (21 figs). 



