1. “The Supreme Court has uniformly held that the charge of the judge to the jury must give the law of the case to the extent of covering the substantial issues made by the evidence, whether such instructions are requested or not; and this is especially true where there is but one controlling issue, and for its just determination a specific instruction should be given.” Southern Cotton Oil Co. v. Brownlee, 26 Ga. App. 782 (2) (107 S. E. 355).

2. Where in an action on account the only question for the jury to determine was whether or not the debtor directed that a particular payment, which had been credited on another account between the parties, should be credited on the account sued on, it was the duty of the court, though not requested so to do, to charge the law applicable to the application *565of payments as laid down in Civil Code (1910), § 4316, and the failure so to charge in this case was error requiring a new trial.

Judgment reversed.