regards the ferric compounds as containing two atoms of tetratomic iron united by the interchange of two units of saturation. In the couple (Fe-Fe), ferricum, there remain, therefore, only six free or disposable units of saturation. The violet chromic chloride, and perhaps the compounds which are called sesquichloride of osmium and ruthenium, have the same molecular constitution as the preceding chlorides.

These trioxides must not be confounded with the sesquioxides properly so called, which contain trivalent. elements, such as arsenic, antimony, bismuth, and gold. These sesquioxides correspond to trichlorides, and the two atoms of metal which they contain are united, not directly with each other, but through an intermediary atom of oxygen.

of chlorine upon the protochloride a tetrachloride is not formed, because the atunity of iron for iron is greater than that of four atoms of chlorine for iron.

We must add that important researches made by Scheurer-Kestner upon the ferric salt have confirmed the existence of sexvalent iron,

Fe,

Iridium and rhodium also form well-characterised trichlorides and sesquioxides, which seem to belong to the preceding series; but they also form dichlorides, or rather tetrachlorides, in which we may admit the existence of couples (Ir-Ir) and (Rh-Rh) formed by the union of two atoms of iridium or two atoms of rhodium, which, having exchanged one unit of saturation, now possess only four atomicities.

As a final example of these unions which the atoms of the same element may form, by the partial exchange of their atomicities or units of saturation, we may mention the cuprous and mercurous compounds, of which the first contain two atoms of copper, the second two atoms of mercury, united together.

The formula which is here attributed to mercurous chloride has been amply justified (p. 115), whence it seems allowable to attribute an analogous composition to cuprous chloride, though here there is some uncertainty.

CHAPTER. II.

I.

Affinity and Atomicity, two Distinct Properties of

Atoms.

WE have in the preceding pages defined atomicity by regarding it as the saturating capacity of atoms, or as their valency in combinations. It is, then, a property inherent in the nature of atoms. We must proceed to show how it differs from affinity.

Affinity is the force of combination, chemical energy. It determines the intensity and the direction of chemical reactions, and is estimated by the thermal effects which these reactions produce. It varies essentially with different atoms. In combining with atoms of hydrogen, atoms of chlorine, iodine, and bromine. liberate very different quantities of heat; their affinity for hydrogen is very different, and is proportional to the quantities of heat liberated. But if we consider the combinations of the same elements with oxygen we shall find the order of affinities reversed. Chlorine is the element which possesses the weakest affinity for this

body. The compounds of chlorine and oxygen are very unstable; some decompose with explosion-that is to say, are formed with absorption of heat. The affinity or chemical energy of a given body must therefore be considered as a relative property. It depends upon the nature of the element with which the one in question combines.

It depends also upon the conditions under which the bodies are placed. Berthollet long ago showed the influence which is exercised upon affinity by physical conditions, such as the degree of cohesion and the insolubility of bodies. This fact is too well known to require further remark (see p. 4); but we must remember how physical agents, such as heat, light, or electricity, can augment or diminish chemical energy, stimulate or retard the exercise of affinity. If mercury is heated to a certain temperature its atoms are in a condition capable of attracting atoms of oxygen. If the heat is increased the atoms of mercury and oxygen will be separated again. The affinity of mercury for oxygen is therefore subordinate to the temperature. It is a relative and not an absolute property, like the atomic weight. In the same manner a stream of electric sparks or the silent electric discharge can determine combinations between atoms which would have no action upon each other under ordinary conditions. Inversely, the same influences can produce decomposition, as is the case with the battery current. Here, again, the conditions in which the atoms are placed exercise a visible influence upon their affinities.

Atomicity is the capacity of saturation, or the value

of substitution possessed by atoms, and this valency is an essentially different thing from the force of combination or the energy which resides in them. It governs the form of combinations, which varies with each atom. Thus the hydrogen combinations of chlorine, oxygen, nitrogen, and carbon have a different form (p. 211), and the atoms of carbon are so constituted that they can attract four atoms of hydrogen, whilst nitrogen cau only attract three, &c. We should, moreover, observe that the force with which the hydrogen atoms are attracted by these different simple bodies is independent of the number of atoms fixed in each case. we know that while hydrogen is united to chlorine with extreme energy, oxygen combines with less force, carbon with difficulty and only when excited by most powerful influences, and nitrogen not at all directly.

Thus

These two notions, affinity and atomicity, which form the very foundation of the science, are therefore essentially different.

II.

Atomicity a Relative Property of Atoms.

Let us pursue this parallel. Is the atomicity or capacity of saturation of every kind of atom immutably fixed, whatever the combinations may be into which they enter? By no means. The action of atoms must be regarded as reciprocal, so that in a compound formed of two heterogeneous atoms the properties of the one are influenced