A Reagent is defined as a "substance or compound that is added to a system in order to bring about a chemical reaction or is added to see if a reaction occurs". Such a reaction is used to confirm the presence of another substance. Examples of such analytical reagents include Fehling's reagent, Millon's reagent and Tollens' reagent. In organic chemistry, reagents are compounds or mixtures, usually composed of inorganic or small organic molecules that are used to affect a transformation on an organic substrate. Examples of organic reagents include the Collins reagent, Fenton's reagent, and Grignard reagent.