Question

What does Sinclair mean when he says “They were regular alchemists at Durham’s” (lines 25-26)

Answer 1

In the novel, Durham's had created new and kind of extraordinary foods from ordinary products, like alchemists creating gold from regular metals.

Answer 2

In "The Jungle" when Sinclair refers to "regular alchemists at Durham's" means that they were creating meats from a variety of sources, later in the text you found this comment “They advertised "potted chicken,"--and it was like the boardinghouse soup of the comic papers, through which a chicken had walked with rubbers on. Perhaps they had a secret process for making chickens chemically--who knows?” here you can infer that the chicken announced actually does not look like chicken.