Since glycerol is a trihydric alcohol, _i.e._, contains three hydroxyl (OH) groups, the hydrogen atoms of which are displaceable by acid radicles, the above reaction may be supposed to take place in three stages. Thus, we may have:-- (1) C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)_{3} = C_{3}H_{5}(OH)_{2}C_{18}H_{35}O_{2} + H_{2}O monostearin (2) C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)_{2}C_{18}H_{35}O_{2} = C_{3}H_{5}(OH)(C_{18}H_{35}O_{2})_{2} + H_{2}O distearin (3) C_{18}H_{35}O_{2}H + C_{3}H_{5}(OH)(C_{18}H_{35}O_{2})_{2} = C_{3}H_{5}(C_{18}H_{35}O_{2})_{3} + H_{2}O tristearin There are two possible forms of monoglyceride and diglyceride, according to the relative position of the acid radicle, these being termed alpha and beta respectively, and represented by the following formulae, where R denotes the acid radicle:-- _Monoglyceride_:-- CH_{2}OR CH_{2}OH | | (alpha) CHOH and (beta) CHOR | | CH_{2}OH CH_{2}OH _Diglyceride_:-- CH_{2}OR CH_{2}OR | | (alpha) CHOH and (beta) CHOR | | CH_{2}OR CH_{2}OH According to the relative proportions of fatty acid and glycerol used, and the temperature to which they were heated, Berthelot succeeded in preparing mono-, di- and triglycerides of various fatty acids. Practically all the oils and fats used in soap-making consist of mixtures of these compounds of glycerol with fatty acids, which invariably occur in nature in the form of triglycerides. It was formerly considered that the three acid radicles in any naturally occurring glyceride were identical, corresponding to the formula-- CH_{2}OR | CHOR | CH_{2}OR where R denotes the acid radicle. Recent work, however, has shown the existence of several so-called _mixed glycerides_, in which the hydroxyls of the same molecule of glycerol are displaced by two or sometimes three different acid radicles.