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Hydrogen chloride HCI. Antimony chloride SbCI.
Ferric chloride Fe Cle

Benzene HC
Acetylene HC

Methane H.C.
These formulae suggest a question, the answer to which is
of the utmost importance, but a question to which a satis-
factory answer cannot yet be given.

Why should we choose to represent hydrogen peroxide as composed of 2 combining weights of hydrogen with 2 c. ws. of oxygen? Water is represented as produced by the union of 1 c. w. of oxygen with 2 c. ws. of hydrogen; why should not the composition of peroxide of hydrogen be represented by the formula HO? The ratio H : 0 is the same

as the ratio H, : 0% Again, why should the formula of ferric chloride be Fe,ci, rather than FeCl,? Chloride of antimony, SbCl,, is represented as formed by the union of 1 c. w. of antimony with 3 c. ws. of chlorine; why should we choose a formula for ferric chloride which represents the composition of that mass of this compound which is formed by the union of 2 c. ws. of iron with 6 c. ws, of chlorine?

Similar questions are suggested by the other formulae. In some cases the formula appears to be the simplest that could be given to the compound, e.g. H,O, H,S, HCI, SbCl,; in other cases a needless and foolish complication seems to be introduced. Why not HCO, in place of H.CO,; HC in

, place of H.C.; SCI in place of S,cl,; HO in place of H,O,; FeCl, in place of Fe, Cl,? Or if the more complex formulae are to be used, why should such formulae not be always used ? Why not H,9, or Ho, in place of H.,O; H S, or H.S, or H, S. in. place of HS; $b Ci, in place of SbC1,; &c. ?

There must be some reason for these apparent inconsistencies. There are several reasons; but we are not yet in. a position fully to understand and appreciate these reasons. We

may however gain some notion of the kind of reasoning employed in determining which of several possible formulae best represents the composition and reactions of a compound. The gist of the matter, as we shall hereafter find, is in the conception expressed by the words composition and reactions. So long as we look only at the composition of compounds we cannot find answers to our questions. If we disregard the composition and look only at the reactions of compounds we cannot find answers to our questions.

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The symbol of an element represents a certain mass of 86 that element usually called its combining weight. Elements combine in the ratios of their combining weights, or in ratios bearing a simple relation to these.

The formula of a compound represents the composition of a certain mass of that compound; this mass we propose to call the reacting weight of the compound. Compounds interact in the ratios of their reacting weights, or in ratios bearing a simple relation to these.

The reacting weight of water is 18 (H, = 2 + 0 = 16).

The combining weight of sodium is 23"(this, as a matter of fact, is the mass of sodium which combines with 8 parts by weight of oxygen). Let us examine the interaction of water and sodium.

When sodium is thrown into water a reaction immediately occurs; the sodium rapidly disappears and hydrogen gas is produced. When the reaction is finished, let the solution be evaporated; water passes away as steam, and a white solid (caustic soda) remains. The composition of this solid is represented by the formula NaOH (Na = 23,0 = 16), that is to say, this compound is produced by the combination of one combining weight of sodium, one c. w. of hydrogen, and one c. w. of oxygen.

We know that water is a compound of hydrogen and oxygen, and that sodium is an element. Hence the oxygen and hydrogen which form part of the caustic soda must have come from the water. But besides caustic soda, hydrogen was produced ; this must also have come from the water. Hence when sodium and water interact, a portion of the hydrogen which was combined with oxygen is evolved as hydrogen gas, and another portion enters into combination with the sodium and the oxygen to produce caustic soda. When this experiment is made quantitative, it is found that 23 parts by weight of sodium interact with 18 parts by weight of water, and there are produced 40 parts by weight of caustic soda and 1 part by weight of hydrogen. The 40 parts of caustic soda are composed of 23 parts of sodium, 16 parts of oxygen, and 1 part of hydrogen.

The conclusion from these experiments is, that, as regards the interaction of water with sodium, 18 is the reacting weight of water, and that the decomposition of one reacting weight of water results in the production of 2 combining weights of hydrogen and 1 c. w. of oxygen. But if H= 1 and 0 = 16, the formula H,0 (18) summarises the results of this experiment. M. E. c.

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A quantitative study of the reactions of water, carried out in the way thus briefly indicated, leads to the conclusion that the mass of water which interacts with other compounds and with elements can always be represented as 18, or as a whole multiple of 18.

The composition of the hydrocarbon benzene is most simply represented as one c. w. of carbon combined with one c. w. of hydrogen ; therefore the smallest value that can be given to the reacting weight of benzene is 13 (CH; C= 12, H= 1).

Is this the best value to adopt for the reacting weight of benzene?

Benzene and chlorine react to form a series of compounds, each composed of carbon, hydrogen, and chlorine ; the formation of each of these is accompanied by the formation of hydrogen chloride (HCI). The first of these compounds is composed of 35.5 parts by weight of chlorine, 72 of carbon, and 5 of hydrogen ; therefore (as C = 12, and Cl = 35.5) the simplest formula to be given to this compound is C H Cl. The composition of the next compound cannot be represented by a simpler formula than CH Cz. The other compounds have compositions which cannot be expressed by formulae simpler than CH C1, C.H.CI, CHCI, and C Cl, respectively. Now, as C = 12, and H = 1, and as carbon and hydrogen combine to form benzene in the ratio 12 :1, the simplest formula which we can use to express

the composition of the reacting weight of benzene is C.H. = 78. When we extend our quantitative study of the reactions of benzene we find that the mass of this compound which interacts with other compounds and with elements is either 78 or a whole multiple of 78.

These examples give some notion of the methods used for determining the value to be given to the reacting weight of a compound. There is no generally applicable chemical method. Each compound must be considered apart from other compounds. The object of the inquiry is to find the relative weight of the smallest mass of the compound which interacts with other compounds, or with elements, in chemical changes. The composition of this mass is then expressed in the formula of the compound.

It will be noticed that in this inquiry the combining weights of the elements are assumed to be known. But we know that great difficulties have to be overcome before the

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combining weights of the elements can be determined ; indeed it was stated that the only satisfactory principle on which a method for finding these combining weights has been based is physical rather than chemical. We shall see later on that the same physical principle gives us a means for determining the reacting weights of compounds.

In addition to the three laws of chemical combination now 87 considered—the law of fixity of composition, the law of multiple proportions, and the law of reciprocal proportionsthere is another generalised statement regarding the volumes of gaseous elements or compounds which interact and the volumes of the gaseous products of these interactions.

The law of volumes, or the law of Gay Lussac, states that the volume of a gaseous compound produced by the interaction of gaseous elements or compounds bears a simple relation to the volumes of the gases from which it is produced, and the volumes of the interacting gaseous elements or compounds bear a simple relation to each other,

All volumes are measured under the same conditions of temperature and pressure.

Thus :-
Vols. of reacting gaseous

Vols. of gaseous elements or compounds.

products. 1 vol. hydrogen and 1 vol.

2 vols. hydrogen chlorine

produce

chloride.

H + Cl = HCI. 2 vols. hydrogen and 1 vol.

2 vols. water-gas. oxygen

produce

H,+ 0 = H,O. 3 vols. hydrogen and 1 vol.

2 vols. ammonia. nitrogen

produce

3H+N =H.N. 2 vols. carbon oxide and 2 vols.

2 vols. carbonyl chlorine

produce

chloride.

CO + Cl, = COCI, 2 vols. hydrogen iodide and

2 vols. hydrogen 1 vol. chlorine

produce

chloride and 1

HI + Cl = HCl + I. vol. iodine-gas. 2 vols. ethane and 2 vols.

2 vols. chlorchlorine

produce

ethane and 2 C.H. + Cl, = C,H,Cl + HCl. vols. hydrogen

chloride.

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2 vols. alcohol-gas and 2 vols. hydrogen iodide produce 2 vols. iodo-ethane and 2 vols. water-gas.

C,H,O + H1 =C,HI+H,O. Hydrogen is taken as the standard gas to which the others are referred. Any specified volume, say 1 litre, is adopted as the standard volume, and this is called one volume.

If the weight of this one volume of hydrogen is taken as
unity, then it is found that the weight of 1 volume of
chlorine is 35.5 : that is, 1 vol. of chlorine weighs 35.5)
oxygen

1
oxygen

16
nitrogen
14:
1 nitrogen

14
iodine-gas
127 :
1

127
more than 1 vol. of hydrogen.
But the combining weights of chlorine, oxygen, nitrogen,
and iodine, are 35-5, 16, 14, and 127, respectively. Hence
the numbers which represent the combining weights of these
elements also represent the specific gravities of these elements
in the gaseous state referred to hydrogen as unity.

This statement is applicable to many of the gaseous elements.

The composition of the reacting weights of hydrogen chloride, water, ammonia, carbonyl chloride, hydrogen iodide, ethane, chlorethane, alcohol, and iodo-ethane, are represented by the formulae HCI, H,O, NH, COCI, HI, CH, CHCI, C,H,O, CHI, respectively. But these formulae also represent the composition of 2 volumes of each compound in the gaseous state; i.e. they represent the composition of that volume of each gaseous compound which is equal to twice the volume occupied by 1 part by weight of hydrogen,

This statement is applicable to all gaseous compounds.

The formula of a gaseous compound represents the composition of the reacting weight of that compound, and this is that weight which occupies twice the volume occupied by 1 part by weight of hydrogen.

These statements assume that al! volumes are measured under the same conditions of temperature and pressure.

Let us now glance back at what we have learned regarding chemical composition.

We have learned that chemical changes involve changes of composition and changes of properties; that these changes occur when elements interact with elements or compounds, or compounds with compounds; that the composition of every

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