Why does he later apologize to Jem after Mr. Macey's comment?

Silas apologizes because he accused Jem without a witness, and he was rash is his accusation.

"I was wrong," he said—"yes, yes—I ought to have thought. There's nothing to witness against you, Jem. Only you'd been into my house oftener than anybody else, and so you came into my head. I don't accuse you—I won't accuse anybody—only," he added, lifting up his hands to his head, and turning away with bewildered misery, "I try—I try to think where my guineas can be."

Silas apologize to jem Rodney after Mr Macey's comment as he had no witness against jem .he only suspected him.he also knew that how it feels when one gets falsely accused.