Page:Proceedings of the Royal Society of London Vol 60.djvu/164

Rh the product of the elasticity E, when multiplied by a fractional power of the atomic volume is a constant for all metals, b / —V = 181 x 104. w

The divergences shown by several metals from this mean value arise, from the fact that the presence of small amounts of impuiity 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 important.

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, Trans.’ 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,

f ‘ Chem. News,’ vol. 66, 1892, p. 20
 * ‘ Phil. Mag.,’ vol. 32, 1891, p. 41.