Ah! here it is.

Unmoved, unchanged!' This, by the way, was a very remarkable circumstance, miniatures being liable to so many changes of expression--'Oh, Pluck! Pluck!' Mr Pluck made no other reply than kissing Mrs Nickleby's hand with a great show of feeling and attachment; Mr Pyke having done the same, both gentlemen hastily withdrew.
Mrs Nickleby was commonly in the habit of giving herself credit for a pretty tolerable share of penetration and acuteness, but she had never felt so satisfied with her own sharp-sightedness as she did that day.
She had found it all out the night before.

She had never seen Sir Mulberry and Kate together--never even heard Sir Mulberry's name--and yet hadn't she said to herself from the very first, that she saw how the case stood?
and what a triumph it was, for there was now no doubt about it.

If these flattering attentions to herself were not sufficient proofs, Sir Mulberry's confidential friend had suffered the secret to escape him in so many words.

'I am quite in love with that dear Mr Pluck, I declare I am,' said Mrs Nickleby.
There was one great source of uneasiness in the midst of this good fortune, and that was the having nobody by, to whom she could confide it.