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14.] ATOMS AND MOLECULES. 29

Hydrogen gas and gaseous chlorine unite in the ratio of one volume of each to form hydrochloric acid, or one part by weight of hydrogen to 35*5 parts by weight of chlorine, the two gases after their combination still occupying two volumes, or the same volume which they did when separate, though their united weight is of course 36'$. But when hydrogen gas combines with oxygen gas in the formation of water, union takes place in the ratio of two volumes of hydrogen to one volume of oxygen. This, therefore, corresponds by weight to 2 parts of hydrogen and 16 of oxygen. Supposing that the two gases before their union were heated to a temperature above the boiling-point of water, say to 1200 C, and the product of the combination after union is still kept at the same temperature, the steam produced, instead of occupying three volumes, becomes condensed into two; but the weight of the steam formed is equal to that of the united weights of the oxygen and hydrogen which have entered into its composition. Compound gases and vapours, in combining, follow the same regularity and simplicity in the ratios by volume in which they unite, as is observed to prevail among elementary bodies; and the compounds resulting from such union, when gaseous, or convertible into vapour, exhibit the same equally simple ratio in volume to that of their components.

In order to give precision to our language, although it would be premature at this point to enter into the reasons in detail, it will be convenient here to draw a distinction between two magnitudes of the component particles of all elementary bodies—viz.: 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 elements, but which is not known in a separate form except in the cases of some elements such as mercury and cadmium; and 2, the Molecule or the smallest quantity of any elementary substance which is capable of existing in a separate form. H, for instance, represents the atom of hydrogen, whilst HH, or H„ indicates its molecule. Each molecule of chlorine and of the other allied elements, when in the gaseous state, appears to consist of 2 atoms. Assuming that the molecule of any element in vapour always occupies 2 volumes, the atom of hydrogen being 1 volume | |, or (H= 1), the molecules of oxygen, sulphur, selenium, and tellurium, each contain 2 atoms; but certain metals, such as mercury and cadmium, yield a vapour the molecule of which contains only a single atom of the element; and if it were possible to make the experiment with the other analogous metals—magnesium, copper, &c.—it is

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probable that the molecule of each member of this class of elements would be found to contain but a single atom.

The molecule of nitrogen contains 2 atoms ; but the molecules of phosphorus and of arsenicum each contain 4 atoms. Analogy leads to the conclusion that the molecules of antimony and of bismuth also each contain 4 atoms.

Some of these relations are indicated in the following table:—

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The application of the terms atom and molecule may be extended to compound substances, the atom of a compound being the smallest quantity of any compound substance which is capable of existing in combination with other particles of matter; and the molecule of a compound is, as before, the smallest quantity of that substance which can exist in an isolated or separate condition: if, for instance, C.H. represent the compound atom of ethyl (the radicle of ether), (C2H6, CgHB), or (C2II.)S, would indicate its molecule.

Since the terms atom and compound are usually employed to indicate precisely opposite things (a compound being divisible and an atom indivisible), it will be necessary to guard against a misunderstanding of the expression atom of a compound. For this purpose some chemists call the quantity of ethyl represented by C2H6 the semi-molecule; in the same way that H represents the semi-molecule of hydrogen. The semi-molecule, which in this case corresponds to the atom of an element such as hydrogen or chlorine, is not always the quantity of a compound which exists in combination: for just as the atom and molecule of

15.] MOLECULAR WEIGHTS OF COMPOUNDS. 31

cadmium aud mercury are of the same weight, so some compounds, such as ammonia, NH3, and ethylene, CgH^ unite by molecules and not by semi-molecules.

It will be shown hereafter that equal volumes of every gas and vapour, whether simple or compound, expand equally for equal increments of heat, if both are compared at the same temperatures (134). The same is true when equal volumes of the different gases are submitted to au equal increase or diminution of pressure (27). Hence it appears to be a legitimate and necessary conclusion that equal volumes of all gases, whether simple or compound, contain an equal number of molecules of their constituents.

The volume of the molecule of a compound body, in the aeriform state, is exactly double the volume of the atom of hydrogen. To this rule, indeed, there are some exceptions, real or apparent; but the number of these is gradually diminishing, under the explanations afforded from time to time by the progress of science.

When the complicated bodies met with in organic chemistry admit of being vaporized without undergoing decomposition, they obey this law of vapour volume as strictly as the simplest combinations of inorganic nature, however numerous the atoms which enter into the formation of their molecule. A molecule of alcohol (C2HflO), or a molecule of the still more complex body aniline (CgH.N), for instance, yields the same volume of vapour as a molecule of hydrochloric acid (HC1). Consequently, if the weight of a given bulk of hydrogen, the lightest substance known, be taken as the unit of comparison, the vapour density of a compound body is represented by half its molecular weight. This number will be designated constantly hereafter, in speaking of vapour densities as the relative weight of the vapour. For example:—

Equal Molecular Relative

vols. weight. weight.

Hydrogen gas . . . (HH) ,'_ | | 2 1

Hydrochloric acid gas (HC1) 36.5 1825

Aqueous vapour . . (H20) 18 9

Alcohol vapour . . (C2H0O) [ ' 46 23

Aniline vapour . . (C,H7N) Q 93 46-5

(15) Applications of the Law of Combining Proportion or Combining Ratio.—Compound bodies unite with other compounds, just as simple bodies unite with other simple ones, and the combining proportions or molecular weights of such compounds are

32 DOUBLE COMPOSITION. [15.

represented by the sum of the atomic weights of all the elements which enter into their composition; the molecular weights of the compound can never be less than that sum, but sometimes it is a multiple of that number. For example, the molecular weights of the following compounds are thus obtained :—

Hydrochloric acid . (1 At. H =1 +1 At. CI =355) HC1 =36-5

Ammonia (3 At. H =3 +1 At. N =14) HaN =17-0

C°dTc°hloSe (!°:} (« At- Na ='3 +x At. CI =35-5) NaCl=58 5

Water (2 At. H = 2 +1 At. O =16) H30 =18-0

Anhydrous potash . (2 At, K =782 + 1 At. O =16) K30 =94*2

Potassic hydrate . . (1 At. K =39-1 + 1 At. H =1 + 1 At. 0= 16) KHO=56i

^iZate ^dr.°:) <« Mo1- H*N=I7 + ' MoL HCl=36-5) H3N, HC1 =53.5

The law of combining proportions holds good not only between the compounds formed by the union of simple substances with each other, but also between the bodies formed by the combination of compound substances with other compounds. Indeed, the reactions between compounds often exhibit very striking exemplifications both of the generality of this law and of the manner in which it may be turned to useful account. The following example of the reaction between common salt, or sodic chloride, and argentic nitrate, will afford an illustration of this kind.

Sodic chloride is a compound of i atom or 23 parts of sodium and 1 atom or 35*5 parts of chlorine: its molecular weight is therefore 585. In like manner, argentic nitrate consists of 1 atom or 108 parts of silver, 1 atom or 14 parts of nitrogen, and 3 atoms or 48 parts of oxygen, forming together 170, which we should expect to represent its molecular weight. This salt, when dissolved in water, is without action upon either red or blue litmus paper. Common salt is likewise perfectly neutral in its reactions upon coloured tests.

If we mix together a solution of 58-5 milligrammes of sodic chloride with a solution of 170 mgrms. of argentic nitrate, a very instructive result is obtained: the sodium and the silver change places; the nitrogen and oxygen unite with the sodium to form sodic nitrate; and the chlorine unites with the silver to form argentic chloride. This chloride is insoluble, and is therefore precipitated in white flocculi. But the remarkable point is, that there is neither more nor less of the nitrogen and oxygen than is required by the sodium, neither more nor less chlorine than will combine with the silver: 355 mgrms. of chlorine are chemically equivalent to the 62 mgrms. of nitrion (N03), and

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may be substituted for them in combination; and 108 mgrms. of silver are as truly equivalent to 23 of sodium.

This interchange, or double decomposition as it is often termed, is illustrated by the diagram that follows :*

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The solutions after mixture are still without action upon either blue or red litmus paper.

If instead of using exactly the equivalent quantities of the two salts, an excess of either had been employed,—say that 200 instead of 170 milligrammes of argentic nitrate had been used,— this excess of 30 mgrms. would not have influenced the result, but would have remained unchanged in the solution. One great advantage, therefore, that is derived from the employment of a table of atomic weights, is economy in the use of the materials employed in the formation of compounds, since by its means it is possible to calculate the exact proportions of the chemical agents required to obtain the full effect of their mutual reaction.

The law of combining proportions also forms the basis upon which most of the calculations in chemical analysis are founded. Suppose it were desired to ascertain the proportion of silver present in the solution of argentic nitrate. By collecting on a filter the precipitate produced on adding sodic chloride in slight excess to a given volume of the liquid, then washing, drying, and weighing the powder with suitable precautions, the quantity of silver can be at once calculated; for it is a necessary consequence of the law of combination that every 143*5 mgrms. of chloride of silver contain 108 mgrms. of silver. From this result the proportion of argentic nitrate in the solution could also be deduced

• The same changes may be represented in a single line by the use of symbols, which, if the atomic weights of the various elements be remembered, convey the ■ame information as a detailed description: e.g. :—

Sodic chloride. Argentic nitrate. Argentic chloride. Sodic nitrate.

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