1. there are degrees of hardness in the hardest metal
2. there are degrees of blackness in black itself
3. there are degrees of asperity in Mrs Clennam's demeanour towards people.
Just as 1. and 2. are true, so is 3.
So, even given Mrs Clennam's asperity, her demeanour towards Little Dorrit had degrees/fine gradations.
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