completely unlike either of its constituents, hydrogen and oxygen, that we must consider it a compound, not a mixture, of these. Of course it might be urged that a compound may be formed by the union of these two gases, but that water may be a mixture of this compound with some other substance or substances.

In describing the experiments whereby the composition of water has been demonstrated it was assumed (the assumption was noted at the time) that the hydrogen and oxygen used for the synthesis of water were perfectly pure, and that every precaution was taken in the experiments.

The statement that water is a compound of hydrogen and oxygen, and that these gases combine to form water in the ratio 1:8 by weight, implies, that the whole of the hydrogen and the whole of the oxygen disappear, that water is the only substance produced, and that the mass of the water thus produced is exactly equal to the sum of the masses of the hydrogen and oxygen. Details of the experimental methods by which each of these statements is proved to be correct were not given. Nor need these details be given here. But it will be well briefly to recapitulate the stages in Davy's proof of the fact that when pure water is decomposed by an electric current, hydrogen and oxygen are the only kinds of matter produced.

Priestley had proved that when a mixture of air and 93 hydrogen was exploded in a closed vessel, water was found in the vessel after the explosion. Knowing that air contained oxygen, Cavendish thought it probable that the water noticed by Priestley was a product of the union of hydrogen with oxygen in the air. Cavendish proved that this was really the case; he exploded mixtures of hydrogen and oxygen in various proportions; the loudest explosion was obtained when two volumes of hydrogen were used to one volume of oxygen, and in this case no trace of either gas remained in the vessel after the explosion. Cavendish found that the water produced by exploding air with hydrogen always contained a little acid; the production of this acid he traced to a constituent of the air other than oxygen; when he used pure oxygen in place of air, the water produced contained no acid.

Davy decomposed water which had been purified by distillation, in glass vessels, by passing an electric current through it; in every case a little acid was produced at the positive electrode, and a little alkali at the negative electrode. He re-distilled the water and again electrolysed it, but the

result was the same. He noticed that the glass vessels in which the water was decomposed were slightly corroded, so he placed the re-distilled water in agate vessels and passed the electric current through it. But the result was as before; traces of acid and alkali were produced. He used electrodes made of different materials; the results were the same. He distilled the water again; there was rather less alkali, but as much acid as before. After another distillation the alkali had further diminished.

Davy concluded that the source of the alkali was some substance in the water itself. He placed the water in gold vessels, and electrolysed it; a very little alkali appeared at the negative electrode; after the current had passed for some minutes the production of alkali nearly ceased; but the acid was still produced, and the quantity of it slowly increased as the process of electrolysis continued. By evaporating some of the water used to dryness in a silver dish, Davy obtained a small quantity of a white solid substance, which, after being heated, was distinctly alkaline. A small quantity of the same water was now electrolysed in a gold vessel; after a few minutes, when the production of alkali had nearly ceased, a little of the white solid obtained by evaporating the water was placed in the water being electrolysed; the alkali was at once produced in some quantity at the negative electrode. Davy then distilled some of the water he had been using in a silver retort, and electrolysed a portion of the distillate; no alkali appeared; he placed a little bit of glass in the water, alkali began to be formed. He had thus traced the production of alkali to the action of the water and the current on the glass or agate vessels used to contain the water; and he had conclusively proved that water is not changed into alkali by the action of the electric current.

But it still remained to account for the production of acid at the positive electrode. The acid he found to be nitric acid. He knew that this acid is a compound of three elements; hydrogen, oxygen, and nitrogen. Hydrogen and oxygen Davy could confidently affirm to be constituents of water; he knew that nitrogen is present in air. On these facts Davy framed an hypothesis.

As the decomposition of the water proceeds in every experiment in contact with air, and as hydrogen and oxygen are produced in this decomposition, the conditions for the production of nitric acid are realised; the hydrogen and

oxygen combine with the nitrogen of the air to produce nitric acid, and this dissolves in the water. If this hypothesis is correct, removal of nitrogen from contact with the decomposing water should be attended with cessation of the production of nitric acid; re-introduction of nitrogen should be accompanied by re appearance of nitric acid.

He

Davy placed a gold vessel containing pure water on a plate of glass, and covered it with a strong glass jar connected with an air-pump; he exhausted the air from the jar, admitted hydrogen, again exhausted, and again filled the jar with hydrogen; he continued this treatment until he could feel sure that the whole of the air had been withdrawn from the jar. then filled the jar with hydrogen, and passed the electric current; not a trace of acid was produced; hydrogen and oxygen, and these gases only, appeared at the electrodes. He admitted air into the jar; the acid began to form at the positive electrode. But he had already proved that the production of acid was not connected with the presence of any substance in the water, nor with the nature of the vessels containing the water, nor with the material of the electrodes ; hence the production of acid always accompanied the presence of nitrogen. The latter was the cause of the former. seems evident then," says Davy, "that water, chemically pure, is decomposed by electricity into gaseous matter alone, into oxygen and hydrogen."

"It

This remarkable research is a type of all scientific inquiry. 94 Facts were noticed and verified, conclusions were drawn and tested by experiments; hypotheses were framed on the basis of the experimentally determined facts, and were used to explain these facts by suggesting fresh lines of inquiry. The result which Davy obtained was not a barren fact; it at once prompted him to further discoveries. The electric current had slowly decomposed the glass vessels; probably it would also decompose other substances more or less resembling glass in composition. Water was electrolysed in cups of gypsum; lime appeared at one electrode and sulphuric acid at the other. Other substances were employed; he generally obtained an alkaline body at the negative, and an acid at the positive, electrode. This led Davy to regard many compounds as built up of two parts, one positively, the other negatively, electri

fied.

This conception prompted him to make more experiments; these furnished him with new hypotheses; these in turn led

to further inquiry; and this reaction of experiment on theory and theory on experiment proceeded until he had framed a general conception of the composition of salts, and of the relations between salts, acids, and alkalis, which had a most important influence on the development of chemistry. The facts noticed during the electrolysis of water also led Davy to investigate the action of the current on various substances which were included in the class of elements; many of these he succeeded in decomposing; he obtained new elements, and to a large extent changIed the whole course of the chemical study of matter.

95 The synthesis of water by passing hydrogen over hot copper oxide has been already mentioned. The synthesis was carried out in the most accurate manner by Dumas. Fig. 14 represents the apparatus employed. A is a flask in which hydrogen is produced by the interaction of zinc and dilute sulphuric acid (the cylinder to the left of A contains mercury; it, and the tube dipping into it, serve as a means for allowing the hydrogen to pass away without taking the apparatus to pieces): the seven large U tubes contain materials by passing through which the hydrogen is purified and dried: the small U tube B contains phosphorus pentoxide, a substance which greedily absorbs moisture; this tube is weighed before and after each experiment, should it increase in weight after an experiment, the results of that experiment are rejected, as the increase in the weight of B tells that the hydrogen was not perfectly dry when it passed into C: C is a bulb of hard glass con

Fig. 14.

taining a weighed quantity of perfectly pure and dry copper oxide; the neck of this bulb is drawn out and passes into the next bulb D: D is a dry bulb of glass destined to contain the water produced in the reaction; it is weighed before and after each experiment: the U tubes E contain materials to absorb any traces of water which may not be retained in D: the small tube F also contains drying materials; it is weighed before and after each experiment; should it shew an increase in weight after an experiment is finished the results of that experiment are rejected, because a doubt arises as to whether the whole of the water produced has been retained in the apparatus and weighed.

Let the weight of the copper oxide before an experiment =x; the weight of the copper remaining in C after the experiment = y; the weight of water produced (that is the increase in weight of D and E) =≈: then x-y gives the weight of oxygen which has combined with hydrogen to produce the weight of water; let this weight of oxygen =a: then z— a gives the weight of hydrogen which has combined with a of oxygen to produce z of water.

Dumas' result was that 1 part by weight of hydrogen combines with 7.9804 parts by weight of oxygen, to produce 8.9804 parts by weight of water.

The volumetric synthesis of water has already been briefly 96 described: when this synthesis is conducted with all precautions the result is that one volume of hydrogen combines with half a volume of oxygen to produce water; as careful determinations have shewn that oxygen is 15.96 times heavier than hydrogen, the result of the volumetric synthesis, altogether confirms that of the gravimetric synthesis, of water.

When two volumes of hydrogen are caused to combine with one volume of oxygen in a vessel the temperature of which is above the boiling point of water, that is when the conditions are arranged so that the water produced is maintained in the state of gas, the result is that two volumes of hydrogen combine with one volume of oxygen to produce two volumes of water-gas.

Water then is a compound, not a mixture, since it has been shewn to be of constant composition, and to conform to the laws of chemical combination.

The results of experiments on the composition of water 97 are summed up in the formula H2O, and in the equation H2+0=H2O.

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