MOLECULAR FORMULE OF THE NITROXYGEN SERIES.

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equation of each set, it will be seen that they are identical expressions; a circumstance due to the fact that, in the decomposition of water by sodium, the proportion both of the metal employed and of the hydrogen set free is 1 molecule = 2 atoms (comp. p. 218).

The two following series of formulæ set in like contrast the old and new modes of symbolizing the decomposition of water and ammonia by chlorine :

In constructing molecular equations of compound bodies care must be taken to select those expressions which, while representing the true relative proportions of the elementary constituents, embody for this purpose the smallest number of atoms with which the compound or product-molecule can be built up. On the other hand, symbolic expressions must never comprise fractional parts of atoms: such fractional formulæ are of course inadmissible, implying, as they would do, the division of that which, by hypothesis, and by the name founded thereon, is recognized as indivisible. The rules here laid down are well exemplified in the following table of the molecular formulæ, representing the oxides of nitrogen :

Molecular formula of the Nitroxygen series.

162 SPECIAL APTITUDES OF ATOMIC AND MOLECULAR NOTATIONS.

The value of this conception of the molecular structure of elementary as well as compound gases will become more apparent to us in future stages of our inquiry than it is at present. But the distinctness which the atomic theory lends to our views of material phenomena can be immediately perceived. It affords us a satisfactory explanation of the definiteness and immutability of chemical composition; of the step-by-step gradations, in simple multiple ratios, by which the proportions of such elements as form more than one compound with each other are observed to vary; and it enables us readily to understand the fact that compounds in so many cases become less stable as they increase in complexity; in other words, as their molecules are built up of a larger number of atoms.

The incorporation of these views in the formulæ of simple and compound bodies, and of their reactions, evidently impresses upon our symbolic language a new significance, and adapts it to aid in theoretically interpreting the phenomena which it also depicts and records.

We have only to add on this subject that both the atomic and molecular forms of symbolic expression have their peculiar merit; the former being more succinct, the latter more comprehensive. When very complete and encyclopædic expressions are required, including the proportions, both by volume and by weight, as well of the bodies brought into action as of the resultant products, simple and compound, molecular formulæ are indispensable. When only the relative weights of the bodies in action, and of their products, require representation (as in the majority of practical problems), atomic equations are sufficiently comprehensive, and have the advantage in point of conciseness and simplicity. Many chemists indeed use this latter form only of the symbolic language; let it be our care to master both modes of expression, so that we may be able to employ each in turn for its appropriate purposes.

LECTURE X.

Molecular and atomic constitution of the typical compounds-curious relations, ponderal, numerical, and potential, of the typical elementary atoms —two sorts of chemical value or power, molecule-forming and atom-fixing— unitary standard of atom-fixing power-major and minor equivalent weights -coefficients of atom-fixing power, or quantivalence-comparative quantivalence of the typical elements and their congeners-germ of a natural system of chemical classification—volume-condensing power of atoms, how far proportionate to their quantivalence-alternative standards of quantivalence chemical value in exchange--exemplification thereof in the syntheses of hydrochloric acid and water, and in the hypothetical syntheses of ammonia and marsh-gas-also in their decomposition by chlorinein the contrasted action of chlorine and oxygen on hydriodic acid—and in the comparative structure of ammonia and nitrous acid-quantivalential equilibrium of the nitroxygen series—distinction between numerical and potential quantivalence—also between quantivalence and chemismtransitional state of the question-tabular summary.

IN the light of our new conceptions concerning the molecules and atoms of which the elementary bodies and their compounds are built up, we reviewed, at our last meeting, the diagrammatic symbols previously employed to represent the volumetric and ponderal composition of hydrochloric acid, water, ammonia, and marsh-gas; and we found that the double squares, expressing the dilitral product-volumes of these compounds, are perfectly well adapted to represent for us their respective free molecules; while the single squares, which previously served us to denote the monolitral unit-volumes of their respective constituents, answer equally well to depict the combining atoms of those elementary bodies.

In the following diagram the dilitral product-volumes, or, as we must now say, the molecules, of our four typical compounds, are placed in a column by themselves, in contrast with the monolitral unit-volumes, or, in our present view, the atoms of the elements which they respectively contain; the arrangement

164 TYPES OF ATOMIC AND MOLECULAR CONSTITUTION.

being such that the atoms of chlorine, oxygen, nitrogen, and carbon, occupy the second column of the diagram, with the hydrogen-atoms they respectively take up displayed on the righthand side, and the resulting compound molecules on the left.

MOLECULAR AND ATOMIC CONSTITUTION OF THE FOUR

In examining this diagram, we are at once struck with the fact that the four elements, displayed in the second column, stand very differently related, on the one hand, to the volume and weight of the compound molecules they respectively form, and, on the other hand, to the volume and number of the atoms that take part with them in forming those molecules.

Thus, a glance at the left and central columns shows us that, though the volumes of the compound molecules are equal, they contain very unequal weights of the four elements under consideration, viz., 35.5 of chlorine, 16 of oxygen, 14 of nitrogen, and 12 of carbon respectively.

Again, looking to the number of hydrogen-atoms depicted on the right-hand side, we see that the atoms of the four central elements, chlorine, oxygen, nitrogen, and carbon, stand related respectively to 1, 2, 3, and 4 hydrogen-atoms.

THEIR CURIOUS NUMERICAL RELATIONS.

165

It is a curious circumstance, and one which the diagram almost forces us to notice, that the heaviest of the four central atoms (Cl = 35.5) is precisely the one which engages the smallest number of hydrogen-atoms, viz., only one; while the other three (O 16, N = 14, and C = 12), as they grow successively lighter, engage increasing numbers of hydrogen-atoms, viz., 2, 3, and 4 atoms respectively.

=

In other words, it takes the whole atom-power of chlorine, 35.5, to engage 1 atom of hydrogen; whereas, the atom-power of oxygen, 16, suffices to engage 2 hydrogen-atoms; and the atompower of nitrogen and carbon suffice, respectively, to engage 3 and 4 hydrogen-atoms.

It is impossible to overlook the singular numerical relations which these unequal atom-engaging powers bring about in the product-volumes, or molecules, depicted on the left side of the diagram. While carbon, nitrogen, and oxygen have, as their atom weights, 12, 14, and 16, respectively, the molecules they form, in combining with hydrogen, weigh 16, 17, and 18 respectively; the molecular weights advancing by 1 only, while the atomic weights advance by 2, at each grade. This is made still more remarkable by the fact that from 18, the weight of the water-gas molecule, to 36.5, the weight of the hydrochloric acid molecule, the advance is by a sudden spring to about double the first-named quantity; 18 × 2 being 36, while the actual weight of the hydrochloric acid molecule is 36.5. Another element (fluorine), to which our attention has not been directed, fills up, in an equally interesting manner, an intermediate link in this numerical chain, as we shall hereafter learn. These curious relations are entirely unexplained, though they have latterly attracted much attention. They do not, however, belong to our present inquiry; from which, it must be owned, we have digressed for a moment to bestow on them this passing notice.

Returning to our immediate study, we observe that the table places before us the four centrally-disposed elements, in two perfectly distinct chemical relations; the first more especially volumetric and molecular, the second essentially numerical and