Tollen's Reagent, better known in scientific circles as the test for aldehydes (a carbon atom bonded with a hydrogen atom and double bonded with an oxygen atom; with CHO as a chemical formula). Tollen's reagent is usually the chemical, ammonia cal silver nitrate. Even in other variations it always consists of an aqueous diaminesilver (I) complex. This diaminesilver (I) complex acts serves as an agent. This, when placed in a glass reaction vessel forms something called a 'silver mirror'. This is essentially the diaminesilver complex brought down to the silver metal. In cases where the compound is not an aldehyde, it will form a yellowish-creamish colored substance instead of the silver mirror. The above is the more commonly used test for aldehydes. Usually when one is utilizing this test to check if a given compound is a ketone or an aldehyde, it is advised that the test tube be placed in a warm water bath.
