Final answer:
Tocqueville's statement that 'From this filthy sewer pure gold flows' refers to the economic wealth generated from the labor of poor factory workers who endured harsh and unhealthy living conditions during the Industrial Era's rapid urbanization.
Step-by-step explanation:
When Alexis de Tocqueville described urban areas full of poor factory workers as "From this filthy sewer pure gold flows," he meant that despite the deplorable and unhygienic living conditions of the workers in the burgeoning urban areas, their labor was generating considerable wealth for factory owners and fueling the economic growth of the nation. This wealth, metaphorically referred to as 'pure gold', was extracted at the expense of the workers' well-being and health, within environments rife with pollution, poor sanitation, and overcrowded housing.
The Industrial Era led to rapid urbanization in the United States, characterized by a stark contrast between the opulence of the rich and the squalor of the working poor. Tenements were often overcrowded and unsanitary, leading to the spread of disease and contributing to high rates of mortality among the working class. This grim reality forms the background to Tocqueville's observation, highlighting the paradox of societal progress that benefitted a few while exploiting many.