Section 21. When interest on a debt or legacy is, before the expiration of the prescribed period, paid as such b

21. When interest on a debt or legacy is, before the expiration of the prescribed period, paid as such by the person liable to pay the debt or legacy, or by his agent generally or specially authorized in this behalf, Effect of part-payment of principal. or when part of the principal of a debt is, before the expiration of the prescribed period, paid by the debtor or by his agent generally or specially authorized in this behalf, a new period of limitation, according to the nature of the original liability, shall be computed from the time when the payment was made: Provided that, in the case of part-payment of principal, the debt has arisen from a contract in writing and the fact of the payment appears in the handwriting of the person making the same, on the instrument, or in his own books, or in the books of the creditor. Effect of substituting or adding new plaintiff or defendant.