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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 115o, 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, 29 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 11 parts of oxygen.

CHAPTER IV.

CONSERVATION OF MATTER.

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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 experiments.

Before the time of Lavoisier there was much vague 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.

He gave

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Lavoisier carried on the work begun by Black. the true interpretation of very many chemical changes, on the superficial qualitative examination of which the structure of alchemy had been raised.

That water had been repeatedly changed into earth was granted by all the alchemists. Water was boiled for a long time in a glass vessel; the water disappeared, and a considerable quantity of a white earth-like solid remained in the vessel. Lavoisier placed some water in a weighed glass vessel; he closed the vessel and weighed it with its contents; he kept the water hot for 101 days, and then poured out the water into another vessel and boiled it until the whole of it had disappeared; there remained 201 grains of solid earthy matter; he then dried and weighed the glass vessel in which the water had been heated, it weighed 173 grains less than it had weighed before the water was heated in it. Lavoisier concluded that the earthy matter was produced by the action of the water on the glass ; that is to say, that the alleged transmutation of water into earth did not occur, but that the earth was a part of the material of the vessel in which the water was heated. The small difference between 204 and 171 grains was due, according to Lavoisier, to experimental errors: this conclusion was fully confirmed when more accurate methods of weighing became possible. From quantitative experiments such as these, Lavoisier drew the all-important conclusion, that the total quantity of matter which is concerned in any chemical change is the same at the end of the change as at the beginning.

Every accurate investigation conducted since the time of Lavoisier has confirmed this generalisation. Under the name of the principle of the conservation of matter, or sometimes conservation of mass, it is now one of the foundations of all modern science. Experimental proofs of this generalisation have been given in preceding paragraphs.

However we may change the form of matter, whatever transmutation we may succeed in accomplishing, there is one thing we cannot change, and that is the quantity, or mass, of matter taking part in each of these transmutations.

The statement of this principle, or law, sometimes takes such a form as this; we cannot create or destroy a single particle of matter, we can only change its form. It is important to notice that the test of creation, or destruction, is here, increase, or decrease, of the total mass of matter.

In place of the indefinite and indefinable elementary.

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principles of the alchemists we have the 70, or so, elements of chemistry. Each element is a definite kind of matter characterised by its own properties which can be accurately stated frequently in terms having a quantitative signification. By bringing these elements into contact with each other under various conditions, we can accomplish stranger changes than those which alchemists dreamt of; but we know that the new kinds of matter thus produced are formed by the combinations of the elements; we have learned that no particle of any of the interacting elements is destroyed, but that the quantity of matter in the products is always exactly equal to the quantity of matter in the interacting elements.

CHAPTER V.

LAWS OF CHEMICAL COMBINATION.

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We have seen that a mixture may be made of two elements or compounds in different proportions, that the properties of the resulting mixture are the sum, or nearly the sum, of the properties of its constituents, and that the greater the proportion of one of the constituents the more nearly do the properties of the mixture resemble those of that constituent. A chemical compound, on the other hand, is wholly unlike the elements or simpler compounds from which it is formed ; its properties are perfectly definite and fixed, and are different from those of any of its constituents. Does the compound differ also from the mixture in having a fixed composition? Do the constituents of the compound combine in definite quantities? It is evident that we must quantitatively examine the composition of compounds if we desire to discover the laws of their formation.

Let us return to the first experiment by which we gained a rough notion of the difference between chemical and physical change. Let us again burn the element magnesium in air; but let the magnesium be weighed before it is burnt, and let the magnesia which is produced be collected and weighed. The result of this experiment is ;

1 gram of magnesium when completely burnt in air, or in oxygen, produces 1.66 grams of magnesium oxide or magnesia : we already know that the substance produced is a compound of magnesium and oxygen. This result may also be stated thus ;

100 parts by weight of magnesium oxide are formed by the combination of

60 parts by weight of magnesium, and
40 parts by weight of oxygen.

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