426 Prof. W. C. Roberts-Austen. Certain Mechanical [Mar. 15. 



were submitted to the Society. The author states that in his own 

 experiments he has employed gold prepared by himself with great 

 care, the purity of which has been recognised by M. Stas. A 

 portion of this gold was recently used by Professor Thorpe in a 

 determination of the atomic weight of gold. Gold was selected 

 for the experiments for the following reasons : — It can be prepared 

 of a very high degree of purity ; it possesses considerable tenacity 

 and ductility ; the accuracy of the results of the experiments is 

 not likely to be disturbed by the oxidation of the metal or by the 

 presence of occluded gases ; and the amount of impurity added to the 

 gold can be determined with rigorous accuracy. The influence of 

 small quantities of metallic impurity in rendering gold brittle has 

 long been known, and is frequently referred to by the older metallur- 

 gists, especially by Greber, Biringuccio, and Gellert, and by Robert 

 Boyle. The first systematic experiments on the subject were made 

 by Hatchett at the request of the Privy Council, and were com- 

 municated to the Royal Society in 1803. Hatchett concluded that 

 certain metals, even when present in so small an amount as the - x 9 X 

 part of the mass, will render gold brittle, and he stated that " The 

 different metallic substances which have been employed in these 

 experiments appear to effect gold in the following decreasing order : — 

 1. Bismuth; 2. Lead; 3. Antimony; 4. Arsenic; 5. Zinc; 6. Cobalt; 

 7. Manganese; 8. Nickel; 9. Tin; 10. Iron; 11. Platinum; 12. Cop- 

 per; 13. Silver." Mr. Hatchett did not, however, employ pure gold, 

 and in his time the importance of submitting metals to mechanical 

 tests was not understood. 



The author then proceeds to describe the results of his own experi- 

 ments, and he states that in selecting tenacity as the test to which 

 the metal should be submitted with a view to ascertain the effect of 

 the added matter, the following considerations presented themselves. 

 W. Spring has built up alloys by compressing the powders of the 

 constituent metals, and by pointing to the evidence of molecular 

 mobility in solid alloys he has done much to show the close connexion 

 which exists between cohesion and chemical affinity. Raoul Pictet 

 considers that there is intimate relation between the melting points of 

 metals and the lengths of their molecular oscillations, the length of 

 the oscillation diminishing as the melting point rises; and, as Car- 

 nelley has pointed out, " We should expect that those metals which 

 have the highest melting points would also be the most tenacious." 

 It is known that the melting points of metals are altered by the 

 presence of small quantities of foreign matter, and their cohesion is 

 also thereby altered. The degree of cohesion may thus be investi- 

 gated either by the aid of heat or by mechanical stress. It might 

 be well to ascertain the amount of change in the melting point of 

 gold produced by the presence of the different elements in small 



