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572

CHOCOLATE-THEOBROMINE.

some time till they become flaccid. At this stage they begin to emit a fragrant smell; they are next rolled in balls, with the hand, upon a wooden table, and a large quantity of liquid is expressed from them; after which they are shaken out, roasted for a few minutes, again rolled, and, whilst still flaccid, are exposed to the air for some hours upon shallow bamboo trays: this alternate heating and rolling is repeated three or four times; and finally, the leaves are dried slowly over charcoal fires. A species of fermentation appears to occur during the drying of the leaf, in consequence of which a development of essential oil takes place, by which the agreeable aroma is occasioned: this aroma is wanting in the fresh leaf. The change of the leaf from green to black is mainly due to chemical alterations produced by the oxygen of the air upon the constituents of the leaf, and especially upon the astringent principle; this change being prevented in green tea by the rapid process of drying to which it is subjected.

4

(1408) Theobromine (E,H,N; Glasson). This body is present in chocolate. It is extracted from the cacao-nut (Theobroma cacao) by a process similar to that employed in obtaining caffeine from tea or coffee. Theobromine is but sparingly soluble in boiling water, and still less so in alcohol and in ether; it has a slightly bitter taste. At a high temperature it may be sublimed, a part of it undergoing decomposition during the process. resembles caffeine in its property of forming crystallizable salts with some of the acids; but these compounds are decomposed by water, and its basic powers are very feeble.

It

Theobromine when dissolved in ammonia, and mixed with nitrate of silver, yields a gelatinous precipitate, which, by boiling with a solution of ammonia, becomes converted into a crystalline mass; if this be dried and heated in a sealed tube with anhydrous methyl iodide, iodide of silver is formed, and caffeine is produced (Strecker) :

Theobromide silver. Methyl iodide. Caffeine.

¤‚Í‚AgN ̧Ð1⁄2 + ¤H2I = ¤ ̧HÂN ̧Ð1⁄2 + AgI.

4 2

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2

Caffeine is, therefore, a methyl-compound of theobromine, and not a true homologue of it, as its formula might seem to indicate.

The cacao-nut is remarkable for the large proportion of fatty matter which it furnishes. This fat is not liable to become rancid, a circumstance which adds greatly to the value of the nut as an article of food. The nut, when deprived of its husk, is said to contain nearly 56 per cent. of this fat, 17 per cent. of a substance resembling gluten, from 12 to 15 of theobromine, and 22 of

GENERAL REVIEW OF THE MONATOMIC ALCOHOLS.

573

starch, gum, sugar, and ligneous tissue. The aromatic flavour of cocoa is due to the roasting to which the nut is subjected previous to grinding it. If the roasted nut be simply crushed after it has been freed from its husk, it forms the cocoa nibs of the grocer. Chocolate is prepared by grinding the roasted nut into a paste between hot rollers, and mixing it with sugar, vanilla, cinnamon, and other spices.

The number of well-defined bases of animal origin is but small, and as they are generally produced by the decomposition of azotised compounds not as yet described, it will be more convenient to defer their consideration till a later period. (1601 et seq.)

§ III. GENERAL REVIEW OF THE MONATOMIC ALCOHOLS
(Є2H2n+2).

(1409) Having now completed the description of the more important bodies connected with the alcohols of the form (ЄH2n+2), it may be serviceable to take a general review of the connexion of these different compounds with each other. For this purpose the table on the following page has been constructed; it furnishes a synoptic view of the more important classes of heterologous derivatives from these alcohols.

If space allowed us to map out the whole of the various derivatives of each alcohol in a table similar in principle to that given at pages 40 and 41, each of the general formulæ given in the second column of the following table would stand at the head of one of the columns of such a table. Each substance enumerated in this table is, therefore, to be regarded as the representative of a homologous series of compounds like that of the fatty acids, or of the alcohols, in which each of the members differs from those which immediately precede and follow it'by (EH).

It will be observed that the letters W, X, Y, and Z, are occasionally employed in the column headed "General formula:"

W indicates chlorine, or one of the halogens.

X indicates all the constituents of a monobasic acid, except the basic hydrogen.

Y indicates the corresponding portion of a dibasic acid.

Z indicates the corresponding constituents of a tribasic acid. It must be remembered that this table, extensive as it is, does not specify all the known heterologous derivatives of any single alcohol. The different forms of the phosphorus bases are

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19. Oxide of ammonium

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20. Homologues of urea

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Zinc radicle

Diarsenio-radicle

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arsenio-radicle.

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Stauno-radicle

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Alcohol hydride

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H2(enH2n+1)N H, (CnH2n+1),N (EnH2n+1)N

(CnHm+),NH@

H(CnH2n+1)N,Ce
Zn(CnH2n+1)2
As(CnH2n+1)

As(CnH2n+1),HO

HAS(CnH2n+1)22

As(CnH2n+1)s

As (CnH2n+1)

As(CnH2n+1), HO

Sn(CnH2n+1) (EnH2n+1)H

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EH

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40. Dibasic oxychloride

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Triarsenethyl oxide | As(¤‚H ̧) ̧Ð

Hydrated tetrar- Į senethylium oxide Stanno-diethyl. Ethyl hydride Olefiant gas.

Acetylene

Acetic aldehyd

Acetic acid

Acetone.

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Succinic acid Acetic anhydride. Benzo-acetic do. Succinic anhydride

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not mentioned, because they are represented by the formula of those of ammonium, from which they differ in containing an atom of phosphorus in the place of one of nitrogen. Of the homologues of urea two distinct series are known, the first of which corresponds to ethyl-urea [H ̧(¤ ̧H ̧)N ̧¤¤], and the second to diethylurea [H2(Є2H),N,CO], though the former series only is indicated in the table. With the exception of the ureas, neither amides nor polyatomic bases appear in the table. The zincradicle again represents other metallic compounds, such as those containing cadmium, which are formed upon the same type. The various arsenio-compounds are also representatives of the antimony series, which are not mentioned in the table; the arsenical compounds likewise are representatives of the compounds which contain bismuth in the place of arsenicum. Again, the stanno-radicles, themselves a numerous group, are only indicated by a single formula; whilst the compounds of lead and of mercury correspond to one or other of the different forms of the stanno-radicle.

CHAPTER VII.

ESSENTIAL OILS AND RESINS.

§ I. ESSENTIAL OILS.

(1410) THE ODORIFEROUS principle of most plants resides in certain compounds consisting chiefly of carbon and hydrogen; and although the boiling point of these bodies lies considerably above 212°, they emit at ordinary temperatures minute quantities of an intensely odorous vapour. They have a certain resemblance to the fixed oils in their inflammability, sparing solubility in water, and ready solubility in alcohol and in ether; as well as in the production of a greasy stain when dropped upon paper, though this mark is only transient; and they feel harsh instead of unctuous when rubbed upon the skin: from the points of their similarity to the oils, and from the fact of their constituting in many cases the distinctive compound of the plant which yields them, they are termed essential or volatile oils.

The production of the essential oils is not limited to any particular portion of the vegetable organism. In some natural families-as, for instance, in the Umbelliferæ-the oil is most

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abundantly contained in the seeds. The Aurantiaceae yield two different kinds of essential oil, one of which is obtained from the flower, and the other from the rind of the fruit. The Myrtacea and the Labiata supply it from the leaves, whilst the Rosacea contain it only in the petals of the flower.

Generally speaking, the oils appear to exist ready formed in the plant, being enclosed in little sacculi, which are often visible to the naked eye, as in the leaves of the Myrtacea and the peel of the Aurantiaceæ. In a few cases, such as those of the oil of bitter almonds and of mustard, they are produced by a species of fermentation operating on a principle contained in the cells of the seed; this change does not commence until the seed has been crushed and mixed with water. A peculiar azotised body present in the seed is thus brought into contact, under favourable circumstances, with the compound which yields the oil, and the peculiar aroma of the essence is speedily developed.

The oils of lemon and orange are extracted by simple pressure of the rind of the fruit; but the general process of extraction consists in placing the appropriate portions of the plant in a still, with water, and applying heat; in order to prevent the vegetable materials from being over-heated by resting against the sides of the still, which would give to the distillate a disagreeable odour, it is customary to suspend them in a net or perforated metallic vessel in the upper part of the still. A better method of obtaining the essence is to allow steam to pass over the plant, and to condense the vapour afterwards in the usual way. The presence of steam favours the volatilization of the oils at a comparatively low temperature, for it mechanically carries over the vapour: the distillate is at first milky, but the greater portion of the essence separates from the milky liquid on standing. The water which is condensed in a receiver with the oil acquires the odour and taste of the essence; such waters constitute the fragrant distilled waters of the apothecary: by a second distillation, these waters are freed from some impurities which are mechanically carried over, and which would cause the liquid to become mouldy, and destroy its fragrance. A small quantity only of the essential oil is retained in solution in the distilled water, and the greater portion of the oil which is thus retained may be separated by saturating the liquid with chloride of sodium; the essence on standing rises to the surface. It may also be removed by agitating the distilled water with ether, which dissolves the oil; and on expelling the ether from the layer which separates on standing, the pure oil is procured. In some cases where the essence becomes altered

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