Glycerol is, however, a trihydric alcohol, i.e., it contains three replaceable (OH) groups. Its formula is C_{3}H_{5}(OH)_{3}, or CH_{2}OH·CHOH·CH_{2}OH. Hence, three molecules of a monobasic acid are required to replace all of its (OH) groups. For example, C_{2}H_{5}OH + HOOC·C_{17}H_{35} = CH_{2}OOC·C_{17}H_{35} | | CHOH + HOOC·C_{17}H_{35} = CHOOC·C_{17}H_{35} + 3H_{2}O | | C_{2}H_{5}OH + HOOC·C_{17}H_{35} = CH_{2}OOC·C_{17}H_{35} It is theoretically possible, of course, to replace either one, two, or three of the (OH) groups in the glycerol with acid radicals, thus producing either mono-, di-, or triglycerides. If the primary alcohol groups in the glycerine molecule are designated by (1) and the secondary one by (2), thus, CH_{2}OH(1)·CHOH(2)·CH_{2}OH(1), it is conceivable that there may be either (1) or (2) monoglycerides, either (1, 1) or (1, 2) diglycerides, or a triglyceride, depending upon which of the (OH) groups are replaced. Compounds of all of these types have been produced by combinations of glycerol with varying proportions of organic acids under carefully controlled conditions; and all of them found to possess fat-like properties. All natural fats are triglycerides, however. Most natural fats are mixtures of several different triglycerides in each of which the three (OH) groups of the glycerol has been replaced by the same organic acid radical, as in the example of stearin shown above. But recent investigations have shown that some of the common animal fats, and perhaps some plant oils, may be made up of mixed glycerides, i.e., those in which the different (OH) groups have been replaced by different acid groups, as oleo-stearin, oleo-stearo-palmitin, etc. THE ACIDS WHICH OCCUR IN NATURAL FATS