Complex sentence: William Rathbone VI became a partner of Rathbone Brothers and Co., general merchants, in 1842 after some time spent with Nichol, Duckworth and Co. in Liverpool, and Baring Brothers in London; and remained a partner until 1885.
Split into simpler sentences: William Rathbone VI became a partner of in the family company Rathbone Brothers and Co., general merchants, in 1842. He joined after spending some time spent with Nichol, Duckworth and Co. in Liverpool, and Baring Brothers in London; and remained a partner until 1885.