1. I wish in the first place to express my warmest thanks to the Council of the Royal Aeronautical Society for the honour they have done me in inviting me to read this paper on alloy structures. I have no claim whatever to be considered a metallurgist or an expert in any way on the technical uses to which alloys are put. I only venture to give this lecture because nowadays a physicist has a finger in most pies. The discoveries of physics concern all sciences interested in the structure and properties of matter. Since ultimately the properties of matter are dependent upon its atomic structure, and this is the field in which physics has made such rapid recent advances, its findings have become of especial importance.