Their business does not allow them to keep much cash unemployed.

They give interest for all the money deposited with them--an interest often nearly approaching the interest they can charge; as they can only keep a small reserve a panic tells on them more quickly than on anyone else.

They stop their discounts, or much diminish their discounts, immediately.
There is no new money to be had from them, and the only place at which they can have it is the Bank of England.
There is even a simpler case: the banker who is uncertain of his credit, and wants to increase his cash, may have money on deposit at the bill brokers.

If he wants to replenish his reserve, he may ask for it, suppose, just when the alarm is beginning.

But if a great number of persons do this very suddenly, the bill brokers will not at once be able to pay without borrowing.