NEW YORK (AP) ¿ Standard & Poor's said Friday it was replacing Cooper Industries Ltd., which is moving its home base to Ireland, with Airgas Inc. on its widely followed S&P 500 stock index. Cooper Industries, which had its administrative headquarters in Houston, is in the process of "redomesticating to Ireland, rendering it ineligible for continued inclusion in the S&P 500," the ratings agency said. Airgas, based in the Philadelphia suburb of Radnor, Pa., had been a constituent on the S&P MidCap 400. Kirby Corp. will replace Airgas on the S&P MidCap 400. Exponent Inc. will replace Kirby on the S&P SmalleCap 600 after the regular market closes on Sept. 8. Also, S&P said Encore Wire Corp. will replace Axsys Technologies Inc. in the S&P SmallCap 600 at a yet-to-be-determined date. General Dynamics is buying Axsys. In afternoon trading, shares of Airgas rose 44 cents to $46.44, while Cooper Industries fell $1.53, or 4.6 percent, to $32.10.