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compound is definite and unchangeable; that elements combine, or interact, in the ratios of their combining weights, and compounds in the ratios of their reacting weights, or in ratios bearing a simple relation to these ; and that the volumes of gaseous elements and compounds which combine, or interact, are simply related to each other and to the gaseous products of the reactions. We have also gained some notion of the meanings of the terms combining weight, and reacting weight, as applied to elements and compounds respectively; and we have seen how. difficult it is to determine the values of these quantities by purely chemical considerations.

The study of the composition of compounds has necessitated some study of the properties of elements and compounds. The properties we have found it incumbent on us to examine have not been those exhibited by elements or compounds considered apart from each other, but rather those exhibited in the mutual interactions of elements and compounds. To arrive at any just generalisations regarding the connexions between changes of composition and changes of propertiesthe study of which connexions is the business of chemistrywe have found it necessary to study the relations between classes or groups of chemical events. The study of isolated occurrences, or the study of isolated elements or compounds, cannot lead to far-reaching conclusions concerning chemical change. We have repeatedly found that chemical changes are accompanied by physical changes : we have tried to keep our attention fixed on the chemical parts of the phenomena; but we should always remember that nothing in nature is “defined into absolute independent singleness."

CHAPTER VII.

CHEMICAL STUDY OF WATER AND AIR.

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That we may become better acquainted with the kind of phenomena which chemistry studies, and that we may apply the principles gained in our study of chemical changes, so far as that study has gone, let us examine some of the phenomena presented to the chemist by the two kinds of matter, air and water.

Water. To which of the classes, Element or Not-Element, does water belong ?

We have already had an answēr to this question. In par. 27 we separated a specified mass of water into two kinds of matter different from, and each weighing less than, itself.

This separation, or analysis, was effected (1) by passing an electric current through water; (2) by the interaction between water and sodium. In the first process, the gases into which water was decomposed, hydrogen and oxygen, were collected separately and examined. In the second process, a portion of one of the gases, hydrogen, was obtained, but the other gas,

and the rest of the hydrogen, combined with the sodium to form caustic soda. These proofs that water is a not-element were supplemented by the synthesis of water (1) by passing electric sparks through a mixture of 1 part by weight of hydrogen with 8 of oxygen; and (2) by the interaction of hydrogen with hot copper oxide, whereby water and copper were produced

Having proved water to belong to the class Not-Element, the next question to be answered is; is water a compound or a mixture? The experiments whereby water is proved not to be an element afford an answer to this question. Water is so

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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

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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.

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. He 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. “It seems evident then,” says Davy, “that water, chemically pure, is decomposed by electricity into gaseous matter alone, into oxygen and hydrogen.”

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, electrified.

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

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