what用来补充前面的 in his pocket.把句子分开来就是：
He went off immediately to visit him.He carried remained of a bottle of medicine formerly prescribed for an indisposition of Mrs.Carlyle's with him in his pocket.
但是如果这样写的话太长和复杂,缺乏停顿.所以用what来补充之前比较清晰易懂.不对