344 



PROFESSOR W. C. ROBERTS-AUSTEN ON MECHANICAL PROPKRTIKS 



cohesion is also thereby altered : the degree of cohesion may thus be investigated 

 either by change of temperature or by mechanical stress, being, as it is, some function 

 of each of these. It might have been well to ascertain the amount of change in the 

 melting-point of gold produced by the presence of different elements in small quantity, 

 but, unfortunately .slight variations in high melting-points are very difficult to determine 

 with any approach to accuracy, and it appeared to be better to ascertain the effect of 

 metallic and other impurity on the cohesion of the gold as indicated by the amount of 

 force externally applied in an ordinary testing-machine, and in that way to ascertain 

 whether the effect of added metals is amenable to any known law. 



In the following experiments only the purest gold I could prepare was employed. 

 It broke with a load of 7 '00 tons per square inch, and elongated 30'8 per cent, (on 

 3 inches) before it fractured, as recorded in the diagrams, Plate 18, Nos. I. and II., 

 subsequently referred to. The effect on the tenacity of gold produced by adding 

 to it about -fa per cent, of various metals and metalloids is shown in the following 

 Table, in which the results are arranged according to the tensile strengths : 



The figures given in the fourth column show that there is some divergence in the 

 amounts of impurity added to the gold. That this is not of much importance in 

 these preliminary experiments may be inferred from the fact, already mentioned (on 

 p. 342), that the -^o^th part of lead appears to produce nearly the same diminution in 

 tenacity as the 3-Qijth part ; a mere trace of certain metals, moreover, will greatly 

 diminish the tenacity of gold. 



[* This is the value given by LOTHAR MEYER. MALLETT'S determination of the density of pure 

 aluminium would give 10'45. 



t These test-pieces drew out after the manner of pitch, that is, as a viscous solid. August 4, 1888.] 



