What do you think Tocqueville meant when he described urban areas full of poor factory workers as; "From this filthy sewer pure gold flows." - Alexis de Tocqueville a.
People found a way to use sewage for fertilization of fields, yielding better crops.

b.
Meaning, the poor filthy areas were populated by people who worked in factories, making others wealthy.

c.
Tocqueville grew up very poor, literally homeless and in the sewers of the city; he became wealthy after creating the assembly line.