342 PROFESSOR W. C. ROBERTS-AUSTEN ON MECHANICAL PROPERTIES 



Percentage of lead. 



its tenacity more than -y</b~oth part. Professor KENNEDY has kindly made further 

 experiments on pure gold to which yg-oth part of lead had been added ; and his results 

 substantially confirm those just referred to, for he found that the breaking load of 

 the contaminated gold was 1'84 ton per square inch, and the elongation unmeasurable. 



In the experiments which follow, pure gold was in all cases melted under a pure 

 form of charcoal ; a certain amount of the metallic impurity to be added was carefully 

 weighed, on a delicate assay-balance, and tightly wrapped in pure gold foil. This 

 little packet was held in a charred splinter of wood, and rapidly submerged in the 

 molten gold. It was found better not to stir the gold, but to thoroughly mix the 

 contents of the crucible by giving it a swinging motion. The gold was then poured 

 through an atmosphere of coal gas into an iron mould lined with lamp-black, and the 

 resulting bars had the following dimensions : 3'5 inches length, - 295 inch breadth, 

 0'205 inch thickness, the sectional area of the bars being therefore 0'0605 square 

 inch. The length varied in some cases. The results of analyses of different parts of 

 the bars showed that the mixing had rarely been defective, but there was evidence 

 in some cases that liquation had disturbed the homogeneity of the metal, though not 

 to any serious extent. 



The test-pieces were made in the Assay Department of the Royal Mint, by 

 Mr. GROVES, whose careful manipulation and great experience in melting the precious 

 metals were of much service to me. 



In the case of very volatile metals, such as potassium and cadmium, a rich alloy 

 with gold was first prepared by fusion in an atmosphere of pure hydrogen. The alloy 

 so formed was analysed, and the necessary amount of it added to pure gold in the 

 manner already described so as to give approximately y - per cent, of foreign metal 

 in the solidified mass. It may here be pointed out that the purity of the gold 



