Whether the so-called elements be compounds or not, it is certain that several of them have the power of uniting with themselves and with others in such a way as to form bodies called compound radicals which resemble elements in many respects. These groups of atoms may enter into and again pass out of combination with other substances, just as elements do.

For example, when compounds of the elements unite, an interchange of elements takes place. Thus when calcium oxide (CaO) and hydrochloric acid (HC1) combine, the oxygen leaves the calcium to combine with the hydrogen and form water, while the chlorine leaves the hydrogen and combines with the calcium to form calcium chloride.

CaO + 2 HCl = CaCl2 + H2O.

1 Prevost, Revue Medicate de la Suisse Romande, p. 553, Nov. 15; p. 605, Dec. 15, 1882; p. 5, Jan. 15, 1883.

But when ethylic alcohol (C2H6O) is treated with hydrochloric acid (HC1), it is not oxygen which leaves the alcohol and is replaced by chlorine. The alcohol does not split up into the group C2H6 and the element oxygen, but into the two groups OH and C2H5.

C2H6O + HC1 = C2H5C1 + H2O; or, as it may also be represented -

To the group OH the name of hydroxyl has been given

To the group OH the name of hydroxyl has been given, and to the group C2H5 that of ethyl.

Similarly, when acetic acid (C2H4O2) is treated with phosphorus trichloride (PC13) the three atoms of chlorine leave the phosphorus, and are replaced by three hydroxyl (OH) groups.

3 C2H4O2 + PC13 = 3 C2H3OC1 + PO3H3; or, as it might be represented -

This mode of representation is awkward and cumbrous

This mode of representation is awkward and cumbrous, although it is clear, and the same reactions may be represented more shortly thus :

3 C2H3O. OH + PC13 = 3 C2H3O. Cl + P. (OH)3.

Here again it is not oxygen, but hydroxyl (OH), which breaks off from the acetic acid, just as it did from alcohol; but instead of the group C2H5 (ethyl) being left behind, we have another group, C2H3O (acetyl).

It is evident that such groups of atoms or radicals, as they are termed, as hydroxyl, ethyl, acetyl, etc, behave in combination just like elements. They are not known in a free state.

In order to exhibit the valency and probable relationships of radicals, they are sometimes expressed by graphic formulas, in which the affinities are shown by a - , as well as in the ways already shown.

As the position of the radicals in some compounds, e.g. in the organic alkaloids, is probably of great importance in regard to their action, although the subject is not well understood at present, the most important radicals are given below, with their graphic as well as their ordinary formula.

Hydroxyl, OH, or - O - H. This is a monad radical, consisting of one atom of dyad oxygen, - O - , with one of its two affinities saturated by an atom of hydrogen, and the other affinity free. It was at one time called water-residue, as it is the residue left after the removal of one atom of hydrogen from water, which may be regarded as the hydride of the radical.

Hydroxyl is an important constituent of alcohols, regarding the chemical constitution of which two views may be taken. They may either be looked on as water in which one atom of hydrogen is replaced by organic radicals, or as compounds of the radicals with hydroxyl. The constitution of water and alcohol may be represented graphically, R standing for a monad organic radical H - O - H water :

R- O - H alcohol, e.g.,

Ethyl alcohol.

Ethyl-alcohol.

-0-H;

Phenyl alcohol, or Phenol

Phenyl-alcohol, or Phenol.

-O-H

The presence of the hydroxyl group in certain substances, and also its position in them,1 appear to be of great importance in regard to their physiological action.2

By replacing the hydrogen in one atom of hydroxyl by a monad, or in two atoms by a dyad element, other radicals are formed, e.g.

Potassoxyl, KO, or - O - K.

Zincoxyl, OZnO, or Carbonyl, CO or is a dyad radical consisting of tetrad carbon, in which two affinities are saturated by dyad oxygen, and two left free. It exists in aldehydes, ketones, and acids, although in aldehydes it is combined with hydrogen, and in acids with hydroxyl, to form other radicals. In ketones both its free affinities are saturated by organic radicals, which may either be of the same kind or of different kinds.

When united to carbonyl

When united to carbonyl, hydroxyl forms a very important radical carboxyl.

Organic Radicals 11Aldehyde Group

Aldehyde Group, CHO, or of this group is saturated by a monatomic radical, we get aldehydes; thus

When the free affinity

When the free affinity

1 Efron, Pfliiger's Archiv, xxxvi. p. 467.

2 Stolnikow, Zeitschr. f. physiol. Chem., 1884, viii. pp. 235 and 271.

Ethyl aldehyde

Ethyl-aldehyde.

Benzoic aldehyde

Benzoic aldehyde (oil of bitter almonds).

Carboxyl, CO.OH, or

This is a monad radical

This is a monad radical. When its free affinity is saturated by an organic radical, it forms monad organic acids, in which the hydrogen of the hydroxyl is readily replaced by a basic element.

Carbon forms an immense number of radicals by union with itself and with hydrogenCarbon forms an immense number

Carbon forms an immense number of radicals by union with itself and with hydrogen, e.g.

Methyl

Methyl.

Ethyl

Ethyl.

Phenyl

Phenyl.

Nitrogen gives origin also to a number of most important radicals.

Nitroxyl, NO2.

Amidogen, NH2, or

Imidogen

Imidogen, NH or Phosphorus, arsenic, and antimony give origin also to a number of radicals similar to those of nitrogen.

Organic Radicals 23

PH2, or -

Organic Radicals 24

SbH2, or -

Organic Radicals 25

AsH2, or -

Organic Radicals 26

PH, or -

Organic Radicals 27

SbH, or -

Organic Radicals 28

AsH, or -

Organic Radicals 29

Sulphur also gives origin to some important radicals.

.Sulphuryl (sulphon), SO2, or

Organic Radicals 30

Sulphin, SO, or -

Organic Radicals 31