18. Frequent prayer also commends us to God. For if the prophet says, “Seven times a day have I praised Thee,” Ps. cxix. [cxviii.] 164. though he was busy with the affairs of a kingdom, what ought we to do, who read: “Watch and pray that ye enter not into temptation”? S. Matt. xxvi. 41. Certainly our customary prayers ought to be said with giving of thanks, when we rise from sleep, when we go forth, when we prepare to receive food, after receiving it, and at the hour of incense, It is doubtful whether incense was burnt as an adjunct of Christian worship so early as the time of St. Ambrose, and the reference here may be to the offering at evening in the Jewish temple. He speaks again of incense in Expos. Ev. sec. Lucam. § 28, but again there is no conclusive proof. It was certainly used as a perfume. when at last we are going to rest.