_Influence of Mechanical Pressure._—Mechanical pressure is also capable of causing one metal to pass into another. We need not recall the well-known experiment of Cailletet, who, by employing considerable pressure, caused mercury to sweat through a block of iron. In a more simple manner W. Spring showed that a disc of copper could be welded to a disc of tin by pressing them strongly one against the other. Up to a certain distance from the surfaces of contact a real alloy is formed; a layer of bronze of a certain thickness unites the two metals, and this could not take place did not the particles of both metals mutually interpenetrate. § 4. INTERNAL ACTIVITY OF ALLOYS. _Structure of Alloys._—Metallic alloys have a remarkable structure, which is essentially mobile, and which we have only now begun to understand by the aid of the microscope. Microscopical examination justifies to a certain degree Coulomb’s conjecture. That illustrious physicist explained the physical properties of metals by imagining them to be formed of two kinds of elements—integral particles, to which the metal owes its elastic properties, and a _cement_ which binds the particles, and to which it owes its coherence. M. Brillouin has also taken up this hypothesis of duality of structure. The metal is supposed to be formed of very small, isolated, crystalline grains, embedded in an almost continuous network of viscous matter. A more or less compact mass surrounding more or less distinct crystals is the conception which may be formed of an alloy. _Changes of Structure produced by Deforming Agencies._—It has been shown that profound changes of crystalline structure can be produced by various mechanical means, such as hammering, and the stretching of metallic bars carried to the point of rupture. Some of these changes are very slow, and it is only after months and years that they are completed, and the metal attains the definite equilibrium corresponding to the conditions to which it is exposed. Though there may be discussions concerning the extent of the transformations to which it is subjected, though some believe they affect the chemical condition of the alloy, while others limit its power to physical effects, it is nevertheless true—and this brings us back to our subject—that the mass of these metals is at work, and that it only slowly attains the phase of complete repose. _The Slow Re-establishment of Equilibrium. Residual Effect._—These operations by which the physical characters of metals are changed, and by which they are adapted to a variety of industrial needs—compression, hammering, rolling, stretching, and torsion—have an immediate, very apparent effect; but they have also a consecutive effect, slowly produced, much less marked and less evident. This is the “residual effect,” or “Nachwirkung” of the Germans. It is not without importance, even in practical applications.