The percentage of silver and chlorine in two samples of silver chloride prepared by heating silver foil in the current of chlorine and by the interaction of silver nitrate and hydrochloric acid were found to be identical, this illustrates the law of:

According to law of constant proportion composition of elements in a given compound is same irrespective of method of preparation or source of compound.

Whether $AgCl$is prepared from $Ag$foil and $C{l}_2$or from $AgN{O}_3$and $HCl$, percentage composition of silver and chlorine will be same.