This phrase, climaxing what is perhaps the most heart-wrenching bit of writing in the novel, here occurs for the only time in the singular. There are multiple levels to it. "Material," being the raw inanimate stuff of the earth, can have no "interest" of its own, which makes the phrase a sort of indicting oxymoron pointing up the barrenness of the industrial world. But "material" also means "important to the topic at hand," which in this passage is the emotional life of the soul, whose every "material interest" in this sense is being ignored or suppressed by the industrial society. Finally, "interest" has a financial meaning too, suggesting an escalation, a continuous compounding of all this material wealth and everything pertaining to it.