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 (HCl). 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.HCl. The composition of the next compound cannot be represented by a simpler formula than C.HCl,. The other compounds have compositions which cannot be expressed by formulae simpler than CH,C,, CHCI, 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 CH=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.

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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

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 :

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

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)

times

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more than 1 vol. of hydrogen.

But the combining weights of chlorine, oxygen, nitrogen, and iodine, are 355, 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 HCl, H2O, NH, COCI,, HI, CH... CH ̧Cl, CHO, 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 all 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

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 answer 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