"A covenant of salt"

1. Num. 18: 19
   19 All the heave offerings of the holy things, which the children of Israel offer unto the Lord, have I given thee, and thy sons and thy daughters with thee, by a statute for ever: it is a ^acovenant of ^bsalt for ever before the Lord unto thee and to thy seed with thee.
2. 2 Chr. 13: 5
   5 Ought ye not to know that the Lord God of Israel gave the kingdom over Israel to David for ever, even to him and to his sons by a ^acovenant of ^bsalt?