The problems of sealing are adhesion and strain. The simplest, but perhaps not the only means ofadhesionis a firm layer of metal oxide.Strainscan be avoided if, and only if thethermal expansions of metal and glass are the same.This postulate is thoroughly tested. Equations are derived for the stresses in cylindrical seals, in terms of the difference in expansion of metal and glass, and the stresses thus calculated are compared with those observed photoelastically in carefully annealed test seals. Such tests are carried out on all the important sealing materials. These tests include accuratecontraction coefficientmeasurements of glass and metals, (Fig. 8); graphical exhibition of axial stresses by photographs through crossed Nicols and a quartz wedge, (Figs. 9 and 10); and numerical evaluation of all the stresses, both theoretical and experimental, (Table VII). Two new sealing alloys are described:fernichrome(Fe 37 percent, Ni 30 percent, Co 25 percent, Cr 8 percent) which matches standard lead glass; and fernico (Fe 54 percent, Ni 28 percent, Co 18 percent) which matches Corning glasses G‐71 and G‐705 A O. The unique feature of these alloys is that theirexpansion characteristics are curvedin such a manner that they match those of the glasses mentioned with considerable fidelity. The seals made with these combinations are found to be practically free from strain.