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a kind of matter as ammonia or hydrogen chloride; butylene bromide is marked off from other kinds of matter by properties as distinct and definite as those which characterise butylene or bromine; hydrogen chloride, so far as its physical properties indicate, is as homogeneous and as little formed of unlike parts as either hydrogen or chlorine.

Iron sulphide, or ammonium chloride, or butylene bromide, 42 or hydrogen chloride, can be separated into unlike parts; but this separation is accompanied by the disappearance of all the distinctive properties of the compound, and by the production, in each case, of two kinds of matter-iron and sulphur, ammonia and hydrogen chloride, butylene and bromine, hydrogen and chlorine—so unlike the compounds from which they have been produced that the only expression to be used regarding the occurrence is that each compound has ceased to exist and has been replaced by two new kinds of matter. Neither iron nor sulphur has yet been separated into unlike parts; the methods which succeed in separating iron sulphide into iron and sulphur fail to separate iron or sulphur into kinds of matter different from iron or sulphur. Bromine likewise refuses, at present, to reveal its composition, if composition it has in the sense in which it may be said that butylene bromide is composed of butylene and bromine. But ammonia and hydrogen chloride, which are produced by separating ammonium chloride into unlike parts, can, each, be further separated into two kinds of matter totally unlike either ammonia or hydrogen chloride. Ammonia is formed by the union of, and can be resolved into, two colourless, odourless, gases--nitrogen and hydrogen ; hydrogen chloride is formed by the union of, and can be resolved into, hydrogen, and another, yellowishgreen, badly smelling, gas, chlorine. All attempts to separate nitrogen, or hydrogen, or chlorine, into unlike parts, have hitherto failed.

A mixture is separated into its constituents by making use of some property or properties of each constituent which belong to that substance when it exists apart from other kinds of matter. Thus the mixture of iron and sulphur was separated by making use of the fact that iron is attracted by a magnet while sulphur is not attracted ; or of the fact that iron sinks in water, while sulphur floats, at least for a time; or of the fact that sulphur is soluble, while iron is not soluble, in carbon disulphide. The mixture of ammonia and charcoal was separated by taking advantage of one of the properties of ammonia viz.

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that it is a very volatile gas. The property possessed by hydrogen of diffusing four times more rapidly than oxygen through a porous plate gave us a method for approximately separating a mixture of hydrogen and oxygen into its constituents.

But if a compound is to be separated into unlike parts it is necessary either to act upon it by some natural agency, or form of energy, such as heat, light, or electricity—or in some cases mechanical energy-or, and this is the more usual method, to cause it to interact under suitable conditions with some other kind, or kinds, of matter. Thus the compound water was separated into hydrogen and oxygen by passing an electric current through the water (s. experiment in par. 9). Similarly ammonia may be separated into nitrogen and hydrogen by passing electric sparks through it.

Copper oxide was separated into copper and oxygen (s. par. 28) by causing it to interact with hydrogen at a high temperature; the results of this interaction were copper and water ; but the results of a previous experiment shewed that water is produced by the combination of hydrogen with oxygen.

As we proceed in our study we shall learn more of the methods employed for separating compounds into the different kinds of matter by the combination of which they are produced; meanwhile it is important to observe that the method does not consist in making use of the physical properties belonging to these different kinds of matter. The formation and decomposition of a compound are chemical processes.

We have already learned that the chemist puts in one class all those distinct kinds of matter which he has not been able to separate into unlike parts, and calls them Elements.

We now learn that certain Not-Elements are distinct kinds of matter, each marked by its own definite and characteristic properties, yet each capable of being separated into parts, totally unlike each other, and unlike the original. These not-elements the chemist puts in one class, and calls them Compounds. One marked characteristic, viz. the constancy of composition, of compounds will be dealt with later. (pars. 58 and 59.)

All other substances belonging to the group Not-Elements are classed together and called Mixtures. An infinite number of these exists, or may be formed, by mixing elements with elements, or compounds with compounds, or elements with

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compounds, or mixtures of any of these with other mixtures; they are all marked off from elements and compounds by the facts that their properties are, broadly, the sums of the properties of their constituents, and their constituents exist in the mixtures each with its own properties scarcely, if at all, modified by the presence of the other constituent parts.

Chemistry deals with certain parts of the phenomena 45 presented in the changes of elements into compounds, and of compounds into simpler compounds or into elements. Chemistry concerns itself but little with the formation of mixtures or the resolution of mixtures into their constituents.

By the composition of an element is meant the element 46 itself; so far as our knowledge goes at present, each kind of matter placed in the class element is entirely homogeneous.

By the composition of a compound is meant, primarily, a statement of the elements by the combination of which the compound is formed and into which it can be resolved, and also a statement of the mass of each element which goes to form, or can be obtained from, a specified mass of the compound. By the composition of a compound is frequently meant a statement of certain less complex compounds, and of the masses of these, which interact to produce a specified mass of the compound in question, or which can be obtained from a specified mass of this compound. Thus, experiment has shewn us (par, 35) that the compound ammonium chloride is produced by the interaction of ammonia and hydrogen chloride; experiment (par. 39) has also told us that hydrogen chloride is itself a compound of hydrogen and chlorine. It may also be proved that ammonia is a compound of nitrogen and hydrogen. The composition of ammonium chloride may be expressed by either of the following statements :

(1) 100 parts by weight of ammonium chloride are formed by the combination of 31:77 parts by weight of ammonia and 68.23 parts by weight of hydrogen chloride;

(2) 100 parts by weight of ammonium chloride are formed by the combination of 26.17 parts by weight of nitrogen, 7.48 parts by weight of hydrogen, and 66.35 parts by weight of chlorine.

We have now gained a clearer conception of chemical 47 change. We now regard such a change as, either the change of a specified mass of a compound into fixed masses of two or more compounds or elements, or the interaction of fixed masses of two or more elements or compounds to produce

definite masses of new elements or compounds. We know that in the first case the mass of each element or compound produced is less than that of the original compound before the change began. In both cases, we know that the sum of the masses of the different kinds of matter produced in the change is equal to the sum of the masses of the different kinds of matter which suffered change.

Our present conception of chemical change requires us to have clear notions of the classes of things called elements and compounds, respectively; and this, in turn, demands that we have grasped, as far as we can at this stage, the essential points of difference between chemical and physical change, and between elements and compounds, on one hand, and mixtures, on the other.

We have also learned something of the meaning of the term chemical properties of this or that kind of matter, as contrasted with the term physical properties of the same kind of matter. Sulphur, for instance, is a yellow, brittle, solid, twice as heavy as water bulk for bulk ; it crystallises in rhombic octahedra, melts at about 115°, boils at about 440°, and is a bad conductor of heat and electricity : these are some of the physical properties of sulphur, that is, the properties which are recognised as belonging to this kind of matter when it is examined apart from other kinds of matter. But when we examine the relations of sulphur to other kinds of matter, we enter on the study of its chemical properties. We find that one of the chemical properties of sulphur is its power of combining with iron; we find that when one part by weight of sulphur is heated with 13 parts of iron, 23 parts by weight of a compound of iron and sulphur (iron sulphide) are formed; we find that this compound is totally unlike either iron or sulphur, but that the whole of the iron and the whole of the sulphur have been used in its production. Further investigation would shew us that sulphur combines with oxygen to form two distinct compounds, unlike each other, and both unlike either sulphur or oxygen ; we should find that 2 parts by weight of one of these compounds are produced by the combination of one part of sulphur with one part of oxygen, and that 21 parts by weight of the other compound are produced by the union of one part of sulphur with 13 parts of oxygen.



CHEMICAL changes are evidently complex occurrences. 49 What we have learned regarding them has been learned by making quantitative experiments and by reasoning on the results of these experiments. So long as our experiments are merely qualitative we can attain to no just conceptions of those changes which it is our business, as chemists, to investigate.

Chemistry began to be a science, that is a department of exact and systematised knowledge of natural events, when quantitative investigation had superseded qualitative experimients.

Before the time of Lavoisier there was much vague 50 speculation about elementary principles. At one time the commonly accepted view was that all things were composed of the four principles, earth, air, fire, and water. A piece of green wood is burnt: smoke ascends, therefore, it was said, wood contains the element air; the flame which plays round the wood proves the presence of the element fire; the hissing sound proves that the element water is present in the wood; and the ashes which remain demonstrate that the element earth is one of the four constituents of the wood. Such reasoning, and such experiments, were possible only so long as chemists did not measure the quantities of the materials taking part in the changes which they observed.

About the middle of the eighteenth century, Black firmly established the fact that chalk and burnt lime have a definite and unalterable composition. By quantitative experiments he proved that when chalk is burnt it is changed into lime and carbon dioxide; and that when burnt lime is exposed to air, it slowly combines with carbon dioxide and chalk is re-formed.

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