Remarks. This chemical is made by passing hydrocyanic gas through a solution of silver nitrate. It can also be made by treating potassium cyanide with silver nitrate.

It is official only because it affords a simple method of extemporaneous preparation of diluted hydrocyanic acid. This acid is quite unstable, and should be fresh when dispensed. This rule is easily followed if the acid is made from silver cyanide and hydrochloric acid, the hydrocyanic acid that is formed remaining in solution, while the silver chloride separates out and can be filtered off.

Argenti Iodidum (U. S. P. 1890).—Silver iodide. This salt is made by combining a solution of potassium iodide and silver nitrate and collecting the precipitated silver iodide, as shown in the following equation:

This is a heavy yellow powder, and is used as an alterative in syphilis, in doses of 0.03 to 0.12 Gm. (4 to 4 grains).

It should contain 99.8 per cent. of pure Silver Oxide, corresponding to not less than 92.9 per cent. of pure metallic silver, and should be kept in dark amber-colored vials. Silver Oxide should not be triturated with readily oxidizable or combustible substances, and should not be brought in contact with ammonia.

A heavy, dark brownish-black powder, liable to reduction by exposure to light, odorless, and having a metallic taste.

Very slightly soluble in water, to which it imparts an alkaline reaction; insoluble in alcohol; it is readily and completely soluble in nitric acid without effervescence (absence of carbonate or chloride).

When heated in a porcelain crucible to about 250° to 300° C. (482° to 572° F.), it is rapidly decomposed, with the evolution of oxygen, leaving a residue of metallic silver.

The solution of the Oxide in nitric acid should be colorless, and should respond to the reactions and tests stated under Argenti Nitras.

If 0.5 Gm. of the Oxide be ignited in a porcelain crucible, it should yield not less than 0.464 Gm. of pure metallic silver.

Impurities.-Carbonate, chloride, see above. Copper, lead, foreign salts. See Silver Nitrate.

Assay. See above.

Remarks. Silver oxide is prepared by precipitating silver nitrate solution with potassium hydroxide solution, as shown in the following equation:

2AgNO, + 2KOH

=

g0+ 2KNO, + H2O.

Silver oxide must be carefully handled, owing to its energetic oxidizing action, hence the pharmacopoeial warning given above.

The only way safely to make pills of silver oxide is by using petroleum mass or kaolin and vaselin as excipient. (See p. 315.) Also beware of bringing it in contact with ammonia, for the two will form the dangerous explosive, fulminating silver.

Silver oxide is one of the most valuable compounds of silver for internal use, it being far less caustic than any of the other silver compounds.

Dose.-65 milligrammes (1 grain).

PART IV

ORGANIC CHEMISTRY

CHAPTER XXXV

INTRODUCTION

ORGANIC chemistry is the study of the carbon compounds, the adjective "organic" being a survival of the earlier days of chemical thought, when it was supposed that all substances peculiar to animal and vegetable economy-such bodies as those about to be discussedwere produced by the action of a mysterious influence called the force of life-"vis vita." Although this vis vitæ theory was exploded in 1828 by Woehler's synthesis of urea (p. 749), the name "organic chemistry" is still applied to the carbon compounds.

Essential to an organic compound are the carbon atoms it contains, and it is interesting to note that in all the numberless derivatives of carbon (some 112,000 now known to the practice) these carbon atoms always show the valence IV, just as do the carbon atoms in the carbonates discussed in previous chapters.

The reason why carbon forms so many compounds is because it possesses, to a far greater extent than any other element, the property of using some of its bonds for linking itself to adjoining carbon atoms. That one atom of an element is capable of attaching itself to another atom of the same element has been already shown, for example, in the formula Fe,Cl (p. 548), the graphic formula showing that two atoms of iron are directly attached to each other. Likewise in Hg,Cl, the two mercury atoms are attached to each other by their own bonds. Returning to the question of ferric chloride, it will be remembered that its formula was explained by the statement that we had iron with the valence IV, and with a bond of each iron atom used for attaching itself to the other, as shown in graphic formula:

Exactly analogous to ferric chloride is the formula of ethane, that carbon compound containing two atoms, and, as a matter of fact, there is known a body C2Cl, exactly analogous to Fe,Cl:

CI
CI

626

These two carbon atoms, however, can link on a third carbon atom, making a chain of three carbon atoms, which can be graphically represented as:

And in this way there can be devised chains containing four, five, or six carbon atoms, making the following series:

This series can be continued almost indefinitely, similar chains containing 60 carbon atoms being known.

These compounds are composed only of carbon and hydrogen, and are called hydrocarbons. They all belong to a similar set or series, and the members of such a series are called homologues. In this series of homologues the carbon atoms are linked into a long string or chain, and the series is called the chain series of hydrocarbons. Because these hydrocarbons and their derivatives are found in fats, the series is sometimes called the fatty series.

Among the members of the series there exists a mathematic relationship between the relative number of carbon and hydrogen

atoms.

This relationship can be expressed in the mathematic formula CnHan + 2 which means twice the number of carbon atoms, plus two, gives the number of hydrogen atoms.

Let us prove this:

Take methane, CH,. There is one carbon-Cn means C.

H20 means 2 X 1
+2

2; 2 + 2 = 4, H2n means H.

So with ethane, C2H.. Cn means C2.

H2n

+ 2

means 2 X 2

H20

means 2 X 3 +2

=

=

4; 4 + 2

=

=

+ 2

6, H2n+2 means H..

Likewise with propane, C,Hg. In this case Cn means C,.
6; 6+2 8, H2n +2 means Hg.
Lastly the hydrocarbon, CH122- In this case Cn means Co.
H2 + 2 means 2 x 60 120; 120 + 2

In methane, ethane, propane, and the other hydrocarbons, possessing the general formula CnH2n+2, the carbon atoms are arranged

in a chain, that is, one linked after another, like beads on a string. Another variety of chain hydrocarbons are those having the formula CnH2n, of which ethylene and propylene are the types. These bodies are unsaturated, that is, at least two of the carbon atoms of the compound are linked together by means of double bonds, as outlined in the graphic formula:

C=

C=

These are said to be unsaturated, because the double bonds split whenever opportunity is presented for them to be used for attaching other elements to the carbon atom. Thus, if ethylene is treated with iodine, the double bond is immediately severed and an iodine atom is attached to each bond, making ethyl iodide, as shown in the following graphic formula:

Another series of chain hydrocarbons are the acetylene series, in which adjoining carbon atoms are connected by a triple bond, as shown in acetylene and allylene, graphic formulas of which are given:

Besides these chain series of hydrocarbons, we have a series of hydrocarbons of which benzene, C,H,, is the type, in which the carbon atoms form a ring, as explained on p. 757. This series is called the aromatic series, in contradistinction to the fatty series of chain hydrocarbons just cited.

Radicles are atomic groups, either organic or inorganic, not existing in a free state, but assigned names, and temporarily considered as entities, merely for convenience.

Chemists run across scores of chemical compounds containing the group, say, C2H, (examples: C,H,OH, or alcohol; (C2H),O, or ether, etc.), and as many others containing, say, COOH. Neither C2H, nor COOH exist in a free state, but since we meet them so often in compounds, we give them names. Thus, we call C,H, "ethyl," simply because it is much easier to say "ethyl" than to call the compound "two atoms of carbon and five atoms hydrogen," and for similar reason we call COOH "carboxyl." Such atomic groupings are called radicles.

In methane, CH,, we have a body in which all the bonds of the

carbon are taken up by hydrogen. In all known bodies the employment of all the bonds of the carbon atoms, in some way or other, is distinctly noticeable, and carbon compounds having free bonds do not exist.

As already mentioned, free atoms do not exist; yet free atoms enter into chemical combinations. Such is the case with what are called radicles, and of these, we may take as example the group CH,, which we call methyl. Note that

C

has a free bond, hence it does not exist in a free. state. But as it enters into chemical reactions, we may consider it just as we do atoms.

For instance, just as sodium (Na) and chlorine (Cl) join to form sodium chloride (NaCl), so is obtained CH,Cl, which is called methyl chloride.

Just as the radicle CH, is considered as a derivative of CH,, so from C2H, is derived the radicle ethyl, C2H, and from C,H, the radicle C,H,, which is called propyl.

Another similarity between atoms of elements and radicles.

Elements are divided into positive and negative, according as their action resembles metals or non-metals.

The radicles mentioned thus far are all typical of a character somewhat similar to metals, since each (CH,, for example) combines with chlorine, just as sodium or iron does. Others there are that act like the negative part of the molecule; such groups as (OH), (COOH), and (CHO) resembling negative elements.

But too much trust must not be placed on this idea of radicles, as the following facts show that they should be considered more as a convenient figure of speech than as definite chemical entities.

Na, the atom of sodium, does not exist. Na2, the molecule, does (graphically expressed, Na-Na), the atoms linked "hand in hand.' Methyl, the radicle CH,, does not exist; (CH,), does, but as a body entirely different from the hypothetic methyl, from which it is derived.

which, when brought in contact with chlorine, does not form CH,Cl (as Na, treated with chlorine yields NaCl), but the product is C,H,Cl.

Another difference is that the atom of an element is unchangeable; methyl, the radicle, can be modified from (CH3) to (CH2Cl)1, (CHCl,), or (CCI)', and exactly so with all other radicles.

The Roman numeral outside the brackets in each formula means that the radicles have as many free bonds as the numeral indicates. Further discussion of radicles is beyond the limits of this book, and in conclusion there will only be added the statement that the name of each radicle is given the termination "yl.