44

THEORY OF COMPOUND RADICLES.

tained may be rendered evident by comparing the empirical formulæ of these compounds with those in which the organic radicle is assumed to be present :—

It is, moreover, to be observed, that many substances corresponding in composition to certain of the organic radicles may be isolated. The existence of cyanogen as an electro-negative organic radicle, and the remarkable analogy in its mode of combination to that of chlorine and the halogens, were known long before Liebig had attempted to generalize the theory of organic radicles. Bunsen subsequently isolated kakodyl (Є,H,As),, which is an excellent representative of the electro-positive class of radicles, and other bodies of analogous composition have since been obtained; whilst several compounds having the composition of such radicles as ethyl and methyl, which are supposed to form the bases of the alcohols and ethers, have been obtained in a separate form by the valuable researches of Frankland and others.

(1061) Theory of Isolated Radicles-Atoms and Molecules.The recent progress of research has rendered it very probable that the bodies thus isolated are not really the radicles that they were once supposed to be. Later writers indeed usually follow Gerhardt, who expressly rejects isolability as constituting any part of his idea of a radicle (Traité, t. iv. p. 569). He says: "Let it be clearly understood that, in speaking of a radicle, I do not indicate any substance under the form and with the properties which it would have in an isolated condition; but I distinguish by it simply the proportion in which certain elements, or groups of elements, may be substituted for others, or may be transferred from one body to another in the process of double decomposition."

A radicle may consist of a single elementary atom, and it then forms a simple radicle; or it may consist of a group of atoms, in which case it constitutes a compound radicle.

Bodies when in combination present characters and properties very different from those which they exhibit in what is usually regarded as their free or isolated condition. The chemical attractions of bodies in their nascent state,' or at the moment of their liberation from combination with others, are by all chemists ad

THEORY OF ISOLATED RADICLES-ATOMS AND MOLECULES. 45

mitted to be much more active than those which they possess in their free state.'

In fact, it appears to be highly probable that at least the monad elementary substances in their uncombined condition are in reality unknown to us,-such so-called elementary bodies being really compounds of at least two atoms of the true element with each other.

2

H

HS'

Η

or

Hydrogen gas, for example, is not simply H, but is H, or hydride of hydrogen; metallic potassium is not simply K, but K,

or

CI)

Cl

K

K

,

or potasside of potassium; in like manner chlorine gas is

or chloride of chlorine. This view has also been extended

to the monatomic so-called compound organic radicles; cyanogen EN

gas being not EN, but ENS, or cyanide of cyanogen; and liquid kakodyl is Kd or kakodylide of kakodyl. Indeed, it admits of experimental proof in many cases, as in that of methyl (1209), that the formula which represents the molecule of the compound must be thus doubled, the molecule of methyl being truly H

methylide of methyl; that of ethyl being

ен

or

or ethylide of

The important influence which this view has exerted and is still exerting upon the progress of the science, renders it necessary to develop it somewhat further.

A distinction has already been drawn (note, p. 29, part I.) between, 1. the Atom, or smallest and chemically indivisible particle of each element which can exist in a compound united with other particles either of the same or of different kinds of matter, but which is not known in a separate form; and 2. the Molecule, or the smallest quantity of any elementary substance which is capable of existing in a separate form. H, for instance, indicates the atom

of hydrogen, whilst represents its molecule.

The molecule of a compound must, it is obvious, consist of at least two atoms, since it cannot consist of less than one atom of at least two different kinds of matter; and in like manner it is probable that the molecules of the bodies hitherto described as

46

ISOLATED RADICLES-ATOMS AND MOLECULES.

isolated elements are, at least in the case of the monads, and triads, compounds of the element with itself. If this view be correct, it must happen that when such bodies unite with other so-called elements of the same group, the act of combination is in most cases attended by double decomposition, and not by direct union.

When, for example, hydrogen and chlorine unite to form hydrochloric acid, the change should be thus represented :—

the equation indicating that one of the atoms of hydrogen in the molecule of this gas changes places with one of the atoms of chlorine in the molecule of chlorine gas: and the result is, the formation of two molecules of hydrochloric acid gas.

In further justification of this view, it may be stated as a uniform result of observation, that in all the reactions in which chlorine gas is concerned, not one is known in which less than two atoms of chlorine is required; take, for example, the successive steps of the action of chlorine upon Dutch liquid (488) :—

And the same is true in all other cases. Hence it has been concluded on chemical grounds, independently of physical considerations equally strong, that the smallest particle of chlorine which can exist in the free state-its molecule, in short-must contain two atoms of chlorine.

A similar necessity for the presence of at least two atoms of bromine, and of iodine, has been observed in every reaction in which they take part; and the same thing is true of hydrogen and of all the metals which correspond to hydrogen in atomic character. Many of the simpler compound substances may be indeed looked upon as intermediate between the elements or the radicles which enter into their formation; as for instance,

In like manner the hydrated alkalies and the ordinary acids may be regarded as compounds intermediate between water and the anhydrous oxides. So also hydropotassic sulphide is intermediate between sulphuretted hydrogen and dipotassic sulphide, and mercaptan between sulphuretted hydrogen and diethyl sulphide :

1. Sulphuretted hydrogen HHS | Sulphuretted hydrogen HHS 2. Hydropotassic sulphide KHS Mercaptan H(Є2H2)S 3. Dipotassic sulphide . KKS Diethyl sulphide (Є2H,)(Є,H ̧)S

And upon this view an argument has been founded for regarding the formula of water as H2O, and not as HO, and that of sulphuretted hydrogen as H,S, not HS.

It is indeed frequently upon the formation of such intermediate compounds that the most probable inference respecting the magnitude of the molecule of a body, the vapour density of which cannot be ascertained, is founded; for in no case can any such intermediate compound be formed, unless two atoms of each of the original bodies concerned in the reaction take part in the metamorphoses under consideration. For example, it is hence urged that the molecule of ethylic (or ordinary) ether cannot be less than (EH) because of the existence of the compound methyl-ethylic ether, which is a body intermediate between ethylic ether and methylic ether: for we have

It must not, however, be supposed that there is no such thing as direct combination between two bodies: the molecules of compounds frequently exhibit the power of uniting in this manner, in which case the number of molecules of the new body is smaller than the sum of those of its components: in other

48

ADDITIVE COMBINATION.

2

words, condensation occurs. When, for instance, 1 molecule of carbonic oxide and 1 of chlorine unite, they form 1 molecule of oxychloride of carbon; +Cl2, yielding COCI. 1 molecule of mercury and I of chlorine yield one of corrosive sublimate; Hg+Cl2=HgCl,; and 1 molecule of ethylene and I of bromine yield of bromide of ethylene, ¤ ̧Н1+Bг2=¤ ̧Н ̧Bг. In like manner, ammonia and hydrochloric acid gases combine directly; H2N+ HCl becoming H,NCI. Nitric oxide with oxygen forms peroxide of nitrogen; NO+ becoming NO. Trichloride of antimony and chlorine yield the pentachloride of antimony; SbCl,+Cl2=SbCl. It may indeed be stated as a general rule, that whenever a body formed upon the type of ammonia combines with a body formed on the type of hydrochloric acid, that direct or additive combination occurs though these are not the only compounds which exhibit this tendency; for example, aldehyd (Є2H) combines directly with → to form acetic acid, (E,H12). Double salts belong to a different category, although nearly always formed by this additive process, as when perchloride of platinum and chloride of potassium unite, 2KCl+ PtCl yielding 2KCl,PtC14: because, so far as we know, such combination exists only so long as the compound is in the solid form; whereas the other bodies, previously cited, are known in the aëriform condition, in which state the relations by volume of the molecules of different compounds are definite, whilst we know but little of the relative volumes of solid molecules.

(1062) Arguments for doubling the Atomic Weight of Oxygen.We can now review with advantage the chemical line of argument which has induced chemists to adopt the proposal of Gerhardt to double the atomic weight of oxygen, of sulphur, and of carbon, and of several other important elements.

In support of this measure it is urged in the case of oxygen and sulphur, for example, that there is a marked distinction between the number of the compounds which these elements form with other bodies, when such compounds are compared with the compounds which chlorine and bromine are known to form with the same bodies:-so that there are numerous oxides and sulphides which have no corresponding representatives among the chlorides and bromides. This fact will be rendered evident by an examination of the following table, where a certain number of the compounds of each of these elements are enumerated :