Answer:
Bullion is money, specifically gold or silver that has been freshly mined but not cleaned and made into money.
Clues to the meaning include the fact that the kids need "3 dollars", the speaker doesn't know how they wanted all that "money" at one time, and they had to "raise" it, as in "raising money". When he says, "I hadn't any idea where to turn to raise all that bullion..." he means the 3 dollars.
Both poems deal with the theme of ideas in the air that are related to the conflicts between belief systems related to romanticism. More or less it is the idea of common sense between spiritual and while Ode to a nightingale focuses on this, A slant of light speaks of oppression that's in the air and a fleeting sense of spiritual.
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