Argonaut Group said its board of directors has instructed the firm's management and its financial adviser, Goldman Sachs & Co., to conduct negotiations with Clarendon Group and other parties that have expressed an interest in buying the company . . . Santa Monica-based Wickes Cos. said its previously reported one-for-five reverse split of its common stock will become effective at the close of business on August 11 . . . National Sanitary Supply Co. of Los Angeles has acquired the janitorial supply unit of Calgon Corp., a competitor with operations in Phoenix, Los Angeles and St. Louis, for an undisclosed price . . . Texas Air Corp. said its System One Corp. subsidiary has been selected by a consortium of European airlines to provide applications software and technical support for a major computerized travel distribution network . . . Imperial Chemical Industries of Great Britain said it has completed its $1.69-billion purchase of Stauffer Chemical from Unilever.