The properties of rooftop air
(Book)

"In the slum known as the St. Giles rookery in 19th century London, the beggar guild run by Horrabin the Clown is the last resort of the down-and-out. Horrabin is rumored to maim his people to make them more effective mendicants, and when dimwitted beggar Isaac Fairchild is summoned by the clown, he fears the worst. But in the subterranean chamber known as the Nursery, Fairchild learns that Horrabin's purpose is to greatly increase his intelligence, by grafting his rudimentary mind into the group mind shared by Horrabin's gang of Spoonsize Boys--alchemically-hatched homunculi, two-inch-tall men employed by the clown for subtle thefts and assassinations. Fairchild yearns to be able at last to think clearly, understand conversations--read books!--but there's a cost." -- Publisher summary.