The Effect of Thermal Agitation on Atomic Arrangement in Alloys. By W. L. Bragg, F.R.S., and E. J. Williams, D.Sc., Manchester University. (Received December 29, 1933.) Forming the subject of the Bakerian Lecture by Professor W. L. Bragg, F.R.S. (Read June 28, 1934.) Introduction. When two metals are alloyed together in various proportions, a series of solid phases is formed. A characteristic phase diagram of a binary alloy system has regions of single phase, throughout which the alloy is homogeneous, alternating with regions in which two neighbouring phases coexist. The composition of a single phase can be varied continuously over a certain range. This feature of an alloy is in contrast to the constant atomic ratio of a chemical compound, and is explained by the nature of the binding forces in an alloy which are predominantly those between the metal atoms of both kinds on the one hand and the common electronic system on the other hand, as opposed to the binding forces between atom and atom which predominate in other chemical compounds. Not only may the atomic ratio in a given phase be varied, but also an orderly space distribution of one kind of atom relative to the other, as found in typical chemical compounds, does not necessarily exist in an alloy. Although each phase is distinguished by possessing a characteristic crystalline structure which differs from that of other phases in the same alloy system, yet this structure may be merely an orderly arrangement of sites occupied by atoms. The manner in which the atoms are distributed amongst the sites of a given phase is often variable, and is, for instance, affected by the thermal treatment which the alloy has undergone. An alloy is a system in dynamic equilibrium. The atomic sites are determined by the interaction between metal atoms and the common electronic system, but atoms of different kinds are constantly being interchanged between one site and another owing to thermal agitation, without destroying the crystalline structure of the phase. This is shown by the rapid inter-diffusion of metals at temperatures far below the melting point and by changes in structure which can be followed by X-ray analysis. Although at room temperature the process of atomic interchange may have slowed down so much as to be inappreciable, at some time in its history the alloy has passed through a high temperature where the process is rapid. The character of its structure has been impressed upon it at this stage, and we must in every alloy seek for a clue to the structure in the dynamic equilibrium prevailing at such temperatures. There exist, of course, all intermediate stages between such alloys and chemical compounds. In the latter both constant atomic ratio and a permanent orderly arrangement are essential features. The place of any compound between these extremes depends upon the relative importance of the metallic structure on the one hand, and the direct interatomic forces on the other hand. The present paper is concerned with the effect of thermal conditions uponThe Effect of Thermal Agitation on Atomic Arrangement in Alloys. By W. L. Bragg, F.R.S., and E. J. Williams, D.Sc., Manchester University. (Received December 29, 1933.) Forming the subject of the Bakerian Lecture by Professor W. L. Bragg, F.R.S. (Read June 28, 1934.) Introduction. When two metals are alloyed together in various proportions, a series of solid phases is formed. A characteristic phase diagram of a binary alloy system has regions of single phase, throughout which the alloy is homogeneous, alternating with regions in which two neighbouring phases coexist. The composition of a single phase can be varied continuously over a certain range. This feature of an alloy is in contrast to the constant atomic ratio of a chemical compound, and is explained by the nature of the binding forces in an alloy which are predominantly those between the metal atoms of both kinds on the one hand and the common electronic system on the other hand, as opposed to the binding forces between atom and atom which predominate in other chemical compounds. Not only may the atomic ratio in a given phase be varied, but also an orderly space distribution of one kind of atom relative to the other, as found in typical chemical compounds, does not necessarily exist in an alloy. Although each phase is distinguished by possessing a characteristic crystalline structure which differs from that of other phases in the same alloy system, yet this structure may be merely an orderly arrangement of sites occupied by atoms. The manner in which the atoms are distributed amongst the sites of a given phase is often variable, and is, for instance, affected by the thermal treatment which the alloy has undergone. An alloy is a system in dynamic equilibrium. The atomic sites are determined by the interaction between metal atoms and the common electronic system, but atoms of different kinds are constantly being interchanged between one site and another owing to thermal agitation, without destroying the crystalline structure of the phase. This is shown by the rapid inter-diffusion of metals at temperatures far below the melting point and by changes in structure which can be followed by X-ray analysis. Although at room temperature the process of atomic interchange may have slowed down so much as to be inappreciable, at some time in its history the alloy has passed through a high temperature where the process is rapid. The character of its structure has been impressed upon it at this stage, and we must in every alloy seek for a clue to the structure in the dynamic equilibrium prevailing at such temperatures. There exist, of course, all intermediate stages between such alloys and chemical compounds. In the latter both constant atomic ratio and a permanent orderly arrangement are essential features. The place of any compound between these extremes depends upon the relative importance of the metallic structure on the one hand, and the direct interatomic forces on the other hand. The present paper is concerned with the effect of thermal conditions upon