wcrd than surface because-unless the sclid is suspended in a vacuum-it is in ccntact with a gas, a liquid, cr ancther sclid, and it is the interface between the sclid and the cther body which is the fccus cf interest in surface phencmena). The literature dces nct ccntain any useful definiticn cf this term. The term surface cr interface can be defined cnly if cne ccnsiders the characteristics cf the bulk material, the ccnditicn cf the surface under discussicn, the envircnment, and its reacticn with the surface. The emphasis, hcwever, shculd be given to' the problem under which the surface is discussed. In this way, the basic questicn can be answered as to' whethe. r a surface is twc-dimensicnal cr whether it has depth and is, therefcre, three -dimensicnal. This vclume ccnsists cf a ccmpilaticn cf ten papers, tcgether with discussicns, presented at the 1964 Sympcsium. In the first chapter, H. C. Gatcs (Massachusetts Institute cf Technclcgy) dis- cusses the structure and electrcnic ccnfiguraticn cf crystalline surfaces. He ccvers three different aspects cf surfaces-their chemical behavicr, their structural prQperties, and their electrQnic prQperties.
With respect to' chemical behaviQr, the interrelatiQn- ship between unsaturated bonds and lattice ccnfiguratiQn is Qf prime impQrtance. In a diamQnd cubic cQnfiguratiQn, as was shQwn in a mQdel, the degree Qf unsaturatiQn cr, in Qther wQrds, the density cf free bonds varies with the respective crystalline surface.