Mr Jenkyns is an ill-fated friend of the Callum family, into whose guest room Dick accidentally spilled his apparatus for making sulphuretted hydrogen (an old name for hydrogen sulphide, also known as rotten egg gas). Dorothea, who had no uncles of her own, compared Captain Flint to Mr Jenkyns, hoping that the former would be more understanding toward her and Dick (WH21). * See Wikipedia: Hydrogen sulphide