ON THE STRUCTURE OF METALS. 
429 
defective or an undesirable form of structure. If the metallurgy of steel points to a 
different conclusion, it is probably owing to the presence of gases which are often 
abundant and are apt to accumulate between the grains ; these gases are the less 
divided, and consequently are the more dangerous, when the grains are large and the 
total surface of the joints is small for a unit of volume. Our attention has been 
specially directed to this point of M. Werth, who justly attaches great importance 
to the question of the presence of gas in steel. 
The more or less advanced state of the crystallization compared with the results of 
the tests for tensile strength does not show any direct relation. 
Finally, micrographical examination only leaves us the joints themselves to account 
for the observed mechanical properties. Here the concordance is fairly good.* 
Without wishing to attribute a degree of precision which does not belong to them 
to the dimensions of joints which have been widened by etching, and perhaps 
modified to some extent by the longitudinal stress to which the bars were subjected, 
we can readily see, in a general way, that the thick and crystalline joints correspond 
to the alloys of low tenacity, while the converse is also true. But if the mechanical 
properties are in direct relation to the thickness of the joints and to the atomic 
volume of the alloyed elements, we may fairly conclude that a relation of cause to 
effect should exist between these two last variables. 
This conclusion appears rather unforeseen. If we seek its interpretation, we are 
face to face with a very important but very complex question, that of the genesis of 
the joints in the metal. 
A joint is often a surface of weakness, as we have already stated, in that it alone 
marks a sharp change of organization and constitutes the artificial border of two 
natural groups. The more it diverges from a mathematical surface and acquires 
sensible thickness the weaker it becomes. How, then, is a joint formed ? Evidently 
by an internal stress, if occluded gas, which appears negligible in the case of gold, be 
left out of the question. Such tractional stress is produced during solidification, 
cooling and consequent shrinkage (and sometimes experimental proof of it is found in 
the presence of scraps of metal attached to the edges of a fissure). In order that a joint 
may be opened it is necessary (and it is enough) that the joint should be subjected, 
at a given moment, to a greater weight than its breaking strain, and that this 
breaking strain is less than the elastic limit of the metal in the interior of the grains. 
We have then four factors at work : 
1. The stresses established by the change of volume of the metal when it solidifies. 
2. The stresses established by shrinkage, which themselves depend (the conditions 
of cooling remaining constant) on the coefficient of expansion of the metal. 
3. The elastic limit and the “ dejor mobility” of the metal which forms the body of 
the grains. 
* The alloy with indium is an exception, bnt we have verified the presence of cement which has had, 
in this case, a favourable influence on the mechanical qualities. 
