------------------------------------------------------------------------------ Arachidic | C_{19}H_{39}COOH | 75 | Arachis or earth-nut oil, | | | rape and mustard-seed oils.

------------------------------------------------------------------------------ Behenic | C_{21}H_{43}COOH | ... | Ben oil, black mustard-seed | | | oil, rape oil.

------------------------------------------------------------------------------ Lignoceric | C_{23}H_{47}COOH | 80.5 | Arachis oil.

------------------------------------------------------------------------------ Carnaubic | C_{23}H_{47}COOH | ... | Carnauba wax.

------------------------------------------------------------------------------ Pisangcerylic | C_{23}H_{47}COOH | ... | Pisang wax.

------------------------------------------------------------------------------ Hyaenic | C_{24}H_{49}COOH | ... | Hyaena fat.

------------------------------------------------------------------------------ Cerotic | C_{25}H_{51}COOH | 78 | Beeswax, China wax, spermaceti.

------------------------------------------------------------------------------ Melissic | C_{29}H_{59}COOH | 89 | Beeswax.

------------------------------------------------------------------------------ Psyllostearylic| C_{32}H_{65}COOH | ... | Psylla wax.

------------------------------------------------------------------------------ Theobromic | C_{63}H_{127}COOH | ... | Cacao b.u.t.ter ------------------------------------------------------------------------------

Medullic and margaric acids, which were formerly included in this series, have now been shown to consist of mixtures of stearic and palmitic, and stearic palmitic and oleic acids respectively.

The acids of this group are saturated compounds, and will not combine directly with iodine or bromine. The two first are liquid at ordinary temperatures, distil without decomposition, and are miscible with water in all proportions; the next four are more or less soluble in water and distil unchanged in the presence of water, as does also lauric acid, which is almost insoluble in cold water, and only slightly dissolved by boiling water. The higher acids of the series are solid, and are completely insoluble in water. All these acids are soluble in warm alcohol, and on being heated with solid caustic alkali undergo no change.

II. _Oleic Series:_--

-------------------------------------------------------------------------- Acid. | Formula. | Melting | Found in | | Point, | | | C. | -------------------------------------------------------------------------- Tiglic | C_{4}H_{7}COOH | 64.5 | Croton oil.

-------------------------------------------------------------------------- Moringic | C_{14}H_{27}COOH | 0 | Ben oil.

-------------------------------------------------------------------------- Physetoleic | C_{15}H_{29}COOH | 30 | Sperm oil.

-------------------------------------------------------------------------- Hypogaeic | C_{15}H_{29}COOH | 33 | Arachis and maize oils.

-------------------------------------------------------------------------- Oleic | C_{17}H_{33}COOH | 14 | Most oils and fats.

-------------------------------------------------------------------------- Rapic | C_{17}H_{33}COOH | ... | Rape oil.

-------------------------------------------------------------------------- Doeglic | C_{18}H_{35}COOH | ... | Bottle-nose oil.

-------------------------------------------------------------------------- Erucic | C_{21}H_{41}COOH | 34 | Mustard oils, marine animal | | | oils, rape oil.

The unsaturated nature of these acids renders their behaviour with various reagents entirely different from that of the preceding series.

Thus, they readily combine with bromine or iodine to form addition compounds, and the lower members of the series are at once reduced, on treatment with sodium amalgam in alkaline solution, to the corresponding saturated acids of Series I. Unfortunately, this reaction does not apply to the higher acids such as oleic acid, but as the conversion of the latter into solid acids is a matter of some technical importance from the point of view of the candle-maker, a number of attempts have been made to effect this by other methods.

De Wilde and Reychler have shown that by heating oleic acid with 1 per cent. of iodine in autoclaves up to 270-280 C., about 70 per cent. is converted into stearic acid, and Zurer has devised (German Patent 62,407) a process whereby the oleic acid is first converted by the action of chlorine into the dichloride, which is then reduced with nascent hydrogen. More recently Norman has secured a patent (English Patent 1,515, 1903) for the conversion of unsaturated fatty acids of Series II. into the saturated compounds of Series I., by reduction with hydrogen or water-gas in the presence of finely divided nickel, cobalt or iron. It is claimed that by this method oleic acid is completely transformed into stearic acid, and that the melting point of tallow fatty acids is raised thereby about 12 C.

Another method which has been proposed is to run the liquid olein over a series of electrically charged plates, which effects its reduction to stearin.

Stearic acid is also formed by treating oleic acid with fuming hydriodic acid in the presence of phosphorus, while other solid acids are obtained by the action of sulphuric acid or zinc chloride on oleic acid.

Acids of Series II. may also be converted into saturated acids by heating to 300C. with solid caustic potash, which decomposes them into acids of the stearic series with liberation of hydrogen. This reaction, with oleic acid, for example, is generally represented by the equation--

C_{18}H_{34}O_{2} + 2KOH = KC_{2}H_{3}O_{2} + KC_{16}H_{31}O_{2} + H_{2},

though it must be really more complex than this indicates, for, as Edmed has pointed out, oxalic acid is also formed in considerable quant.i.ty.

The process on a commercial scale has now been abandoned.

One of the most important properties of this group of acids is the formation of isomeric acids of higher melting point on treatment with nitrous acid, generally termed the _elaidin reaction_. Oleic acid, for example, acted upon by nitrous acid, yields elaidic acid, melting at 45, and erucic acid gives bra.s.sic acid, melting at 60C. This reaction also occurs with the neutral glycerides of these acids, olein being converted into elaidin, which melts at 32C.

The lead salts of the acids of this series are much more soluble in ether, and the lithium salts more soluble in alcohol than those of the stearic series, upon both of which properties processes have been based for the separation of the solid from the liquid fatty acids.

III. _Linolic Series:_--

-------------------------------------------------------------------------- Acid. | Formula. | Melting | Found in | | Point, | | | C. | -------------------------------------------------------------------------- Elaeomargaric | C_{16}H_{29}COOH | ... | Chinese-wood oil.

-------------------------------------------------------------------------- Elaeostearic | C_{16}H_{29}COOH | 71 | Chinese-wood oil.

-------------------------------------------------------------------------- Linolic | C_{17}H_{31}COOH | Fluid | Linseed, cotton-seed and | | | maize oils.

-------------------------------------------------------------------------- Tariric | C_{17}H_{31}COOH | 50.5 | Tariri-seed oil.

-------------------------------------------------------------------------- Telfairic | C_{17}H_{31}COOH | Fluid | Telfairia oil.

These acids readily combine with bromine, iodine, or oxygen. They are unaffected by nitrous acid, and their lead salts are soluble in ether.

IV. _Linolenic Series:_--

-------------------------------------------------------------------- Acid. | Formula. | Found in -------------------------------------------------------------------- Linolenic | C_{17}H_{29}COOH | Linseed oil.

-------------------------------------------------------------------- Isolinolenic | C_{17}H_{29}COOH | Linseed oil.

-------------------------------------------------------------------- Jecoric | C_{17}H_{29}COOH | Cod-liver and marine animal oils.

These acids are similar in properties to those of Cla.s.s III., but combine with six atoms of bromine or iodine, whereas the latter combine with only four atoms.

V. _Ricinoleic Series:_--

----------------------------------------------------------- | | | | | | Acid. | Formula. | Melting | Found in | | | | Point, | | | | | C. | | |------------|----------------------|---------|-------------| | | | | | | Ricinoleic | C_{17}H_{22}(OH)COOH | 4-5 | Castor oil. | -----------------------------------------------------------

This acid combines with two atoms of bromine or iodine, and is converted by nitrous acid into the isomeric ricinelaidic acid, which melts at 52-53 C. Pure ricinoleic acid, obtained from castor oil, is optically active, its rotation being [alpha]_{d} +6 25'.

_Hydrolysis or Saponification of Oils and Fats._--The decomposition of a triglyceride, brought about by caustic alkalies in the formation of soap, though generally represented by the equation already given (pp. 6 and 7)--

C_{3}H_{5}(OR) + 3NaOH = C_{3}H_{5}(OH)_{3} + 3RONa,

is not by any means such a simple reaction.

In the first place, though in this equation no water appears, the presence of the latter is found to be indispensable for saponification to take place; in fact, the water must be regarded as actually decomposing the oil or fat, caustic soda or potash merely acting as a catalytic agent. Further, since in the glycerides there are three acid radicles to be separated from glycerol, their saponification can be supposed to take place in three successive stages, which are the converse of the formation of mono- and diglycerides in the synthesis of triglycerides from fatty acids and glycerine. Thus, the above equation may be regarded as a summary of the following three:--

_ _ | OR | OH (i.) C_{3}H_{5} | OR + NaOH = C_{3}H_{5} | OR + RONa |_OR |_OR _ _ | OH | OH (ii.) C_{3}H_{5} | OR + NaOH = C_{3}H_{5} | OR + RONa |_OR |_OH _ _ | OH | OH (iii.) C_{3}H_{5} | OR + NaOH = C_{3}H_{5} | OH + RONa |_OH |_OH

Geitel and Lewkowitsch, who have studied this question from the physical and chemical point of view respectively, are of opinion that when an oil or fat is saponified, these three reactions do actually occur side by side, the soap-pan containing at the same time unsaponified triglyceride, diglyceride, monoglyceride, glycerol and soap.

This theory is not accepted, however, by all investigators. Balbiano and Marcusson doubt the validity of Lewkowitsch's conclusions, and Fanto, experimenting on the saponification of olive oil with caustic potash, is unable to detect the intermediate formation of any mono- or diglyceride, and concludes that in h.o.m.ogeneous solution the saponification is practically quadrimolecular. Kreeman, on the other hand, from physico-chemical data, supports the view of Geitel and Lewkowitsch that saponification is bimolecular, and though the evidence seems to favour this theory, the matter cannot be regarded as yet definitely settled.

Hydrolysis can be brought about by water alone, if sufficient time is allowed, but as the process is extremely slow, it is customary in practice to accelerate the reaction by the use of various methods, which include (i.) the application of heat or electricity, (ii.) action of enzymes, and (iii.) treatment with chemicals; the accelerating effect of the two latter methods is due to their emulsifying power.

The most usual method adopted in the manufacture of soap is to hydrolyse the fat or oil by caustic soda or potash, the fatty acids liberated at the same time combining with the catalyst, _i.e._, soda or potash, to form soap. Hitherto the other processes of hydrolysis have been employed chiefly for the preparation of material for candles, for which purpose complete separation of the glycerol in the first hydrolysis is not essential, since the fatty matter is usually subjected to a second treatment with sulphuric acid to increase the proportion of solid fatty acids. The colour of the resulting fatty acids is also of no importance, as they are always subjected to distillation.

During the last few years, however, there has been a growing attempt to first separate the glycerol from the fatty acids, and then convert the latter into soap by treatment with the carbonates of soda or potash, which are of course considerably cheaper than the caustic alkalies, but cannot be used in the actual saponification of a neutral fat. The two processes chiefly used for this purpose are those in which the reaction is brought about by enzymes or by Twitch.e.l.l's reagent.

I. _Application of Heat or Electricity._--Up to temperatures of 150 C.

the effect of water on oils and fats is very slight, but by pa.s.sing superheated steam through fatty matter heated to 200-300 C. the neutral glycerides are completely decomposed into glycerol and fatty acids according to the equation--

C_{3}H_{5}(OR)_{3} + 3H.OH = C_{3}H_{5}(OH)_{3} + 3ROH.

The fatty acids and glycerol formed distil over with the excess of steam, and by arranging a series of condensers, the former, which condense first, are obtained almost alone in the earlier ones, and an aqueous solution of glycerine in the later ones. This method of preparation of fatty acids is extensively used in France for the production of stearine for candle-manufacture, but the resulting product is liable to be dark coloured, and to yield a dark soap. To expose the acids to heat for a minimum of time, and so prevent discoloration, Mannig has patented (Germ. Pat. 160,111) a process whereby steam under a pressure of 8 to 10 atmospheres is projected against a baffle plate mounted in a closed vessel, where it mixes with the fat or oil in the form of a spray, the rate of hydrolysis being thereby, it is claimed, much increased.