Where a demand in compensation and reconvention has for its foundation or basis a letter from plaintiffs, agreeing and binding themselves to guarantee the payment of the *49debts of laborers on their plantation, and said letter has been lost, defendant cannot offer proof of its contents without advertising the lost instrument. 22 An. 210. This rule will be enforced when plaintiffs’ only objection to the proof of contents' was- that defendant had not used due diligence, for advertisement is a part of the diligence required by the law to recover the lost document.

2.Where a merchant furnishes laborers, with whom he makes a direct contract, and keeps the accounts in their names and to whom he sells without orders, a promise by the planter to guarantee the payment of such accounts is a promise to pay the debt of a third person, and must be proved by written evidence; if parol evidence of such guarantee be admitted without objection, it will be disregarded as being in contravention of a prohibitory law. Wertz vs. Labuzan, 23 An.