348 PROFESSOR W. C. ROBERTS-AUSTEN ON MKCHANICAL PROPKKTIKS 



the point of fracture, and, to some extent, therefore, affected the tenacity. With 

 reference to this point, it is worthy of remark that Professor BAUSOHINGER, of 

 Munich, has demonstrated, by the aid of a remarkable series of test-pieces of iron 

 and steel exhibited at the Nuremberg Exhibition of 1882, that the presence of a 

 minute defect at the point at which fracture ultimately takes place, while not greatly 

 affecting the tensile strength of the test-piece, may nevertheless prevent the metal 

 from contracting to so small an area as would have been the case if the metal had 

 been perfectly sound. The elongation of the test-pieces (given in the third column 

 of the Table, p. 344) and the atomic volume of the added impurity are plotted in 

 diagram No. II., Plate 18, which agrees closely with diagram No. I., representing tensile 

 strength and atomic volume. Cadmium exhibits marked irregularity in both diagrams, 

 but the only striking difference between the two diagrams is presented by tellurium and 

 bismuth, the former of which seems to be more prejudicial to the elongation of gold 

 than to its tenacity. It may be added that some of the metals, zinc and rhodium 

 for instance, although possessing smaller atomic volumes than gold, appear to diminish 

 its elongation while they increase the tenacity of the precious metal. This diminu- 

 tion, though not very marked, causes an irregularity in the portion of the diagram 

 occupied by metals with smaller atomic volumes than gold. 



Mode of Purifying the Gold employed in the Experiments. 



The gold employed in the foregoing experiments was purified by a method which 

 was adopted, after much careful consideration, by the author of this paper in the 

 preparation of the " Trial Plate " of gold which, by the direction of the Lords 

 Commissioners of H.M.'s Treasury, was to supplement the "Standard Trial Plate," the 

 use of which, for verifying the composition of the coinage, has been prescribed by 

 law since the 17th year of King Edward IV. The purity of the gold so prepared 

 has been recognised by M. STAS, and Mr. LOCKYER has also satisfied himself of its 

 high degree of purity by a comparison of photographs of its spectrum. The gold, 

 having been used in the Assay Office of the Mint throughout a long period of years, 

 had already been purified many times, and the only metallic impurities liable to have 

 become associated with it were silver, platinum, and lead ; and those only in very 

 minute quantities. This gold was dissolved in nitre-hydrochloric acid, the excess of 

 acid being driven off by slow evaporation. Platinum and the allied metals were 

 carefully sought for, but were not detected, and the chloride of gold was then 

 dissolved in a large quantity of distilled water, so that each gallon contained about 

 one ounce of metal. This solution was allowed to rest for three weeks, when the 

 finely-divided chloride of silver was separated by careful decantation of the 

 supernatant solution. The last traces of chloride of silver are only thrown out of 

 solution when chloride of gold is rendered very dilute. A warm solution of oxalic 

 acid was then added, to precipitate the gold ; the first and last portions of the gold 



