On the Structure of Metals, its Origin and Changes. 149 

 the product of the elasticity E, when multiplied by a fractional 

 power of the atomic volume , is a constant for all metals, 



1) 



The divergences shown by ssveral metals from this mean value 

 arise from the fact that the presence of small amounts of impurity 

 makes a great difference in their elasticity. 



Sutherland* finds a close relation between the atomic volume and 

 the rigidity of metals, and considers that this rigidity is " in its 

 essence a kinetic phenomenon, almost as simple in character as the 

 elasticity of perfect gases." 



Professor Fessenden,f moreover, has urged that the cohesion of 

 metals is proportional to some power of the atomic volume, and he 

 considers that the rigidity varies as the fifth power of the distance 

 of the centre of the atoms, or as (atomic volume)^. It will be 

 evident, therefore, that the atomic volume of a metal is very impor- 

 tant. 



One of the authors purified gold with great care, and alloyed 

 seventeen separate portions of it with foreign elements in quantities 

 which were in each case close to 0*2 per cent., and from each sample 

 of this alloyed gold, bars were cast, 88 mm. long by 7'5 mm. wide by 

 5'2 mm. thick. The tensile strength, elongation, and reduction of 

 sectional area (striction) were determined, and the results were 

 published in the ' Phil. Traits.' in 1888. These results indicated 

 in a general way, that the tenacity and ductility of gold is increased 

 by the presence of 0*2 per cent, of an added element of smaller 

 atomic volume than that of gold itself, while, on the other hand, 

 these properties are diminished when the atomic volume of the added 

 element is greater than that of gold. 



There are, as might be expected, exceptions and irregularities, but 

 it is strange that they are not more numerous and more marked. 

 Even the purest metals are not, from a mechanical point of view, 

 homogeneous. Under the influence of internal forces which tend to 

 make them crystalline, and of external stresses which are set up by 

 contraction during cooling, the invisible molecules become arranged 

 in visible and more or less highly organised groups. These groups 

 are separated from each other either by planes of cleavage or by joints 

 which are often surfaces of least cohesion, and, therefore, of weakness. 

 This is especially the case when these joints have been accentuated by 

 the evolution of dissolved gas at the moment of the solidification of 

 the metal. In alloys, chemical homogeneity may, in turn, disappear, 



* ' Phil. Mag.,' vol. 32, 1891, p. 41. 

 t ' Chern. News/ vol. 6G, 1892, p. 206. 



