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an interaction between at least two definite kinds of matter; the product of the change has consisted of the whole of one of the interacting substances, and either the whole, or a part, of the other. The mass of the product has therefore been greater than the mass of that one of the interacting substances which was weighed before the change began.

Magnesium and iron combined with oxygen contained in the air surrounding them; copper combined with sulphur and oxygen obtained from the sulphuric acid with which it interacted; tin combined with oxygen obtained from nitric acid : the oxide of magnesium, or iron, or tin, thus produced weighed more than the magnesium, or iron, or tin, used; and the sulphate of copper produced weighed more than the copper used. But the whole of the magnesium, or iron, or tin, or copper,

formed a part of the new kind of matter into which it was changed.

Let us now turn to some chemical changes which differ from those just considered in that in each of them a specified mass of one definite substance is converted into two or more different substances the mass of each of which is less than that of the original substance.

A flask with a good fitting cork and exit tube is arranged as shewn in fig. 7; the glass cylinder is graduated, filled

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Fig. 7.

with mercury, and inverted in a vessel containing mercury. A weighed quantity of a white solid called potassium chlorate is placed in the flask : this solid is heated until it melts; the gas which comes off is collected in the graduated cylinder. The heating is continued as long as any gas is produced. The apparatus and its contents are allowed to cool, precautions being taken to prevent the mercury from rushing back into the flask. By methods which need not be described here, the whole of the gas which is in the flask and exit tube when the experiment is finished is driven into the graduated cylinder. The white solid in the flask is now weighed; the small quantity of air in the cylinder (which air was in the flask and exit tube at the beginning of the experiment) is removed by suitable methods, and the gas in the cylinder is measured. This gas is now proved to be oxygen. The properties of the white solid left in the flask are compared with those of the potassium chlorate, i.e. with those of the kind of matter used in the experiment: the two substances are easily proved to be very different. The white solid produced in the change is called potassium chloride. As the weight of a specified volume of oxygen has been accurately determined, the weight of oxygen produced in the process is easily calculated from the observed volume of the oxygen.

In this experiment, one kind of matter-potassium chlorate -has been changed, by the agency of heat, into two kinds of matter-oxygen, and potassium chloride—; the mass of each of these is less than that of the potassium chlorate, but the sum of the masses of the oxygen and the potassium chloride is equal to the mass of the potassium chlorate.

An electric current is passed through acidulated water. 27 The experiment is conducted as described in par. 9. (s. fig. 4). But the water used is weighed, and the water remaining at the close of the experiment is weighed; the volumes of hydrogen and oxygen produced are measured ; and special precautions are taken that no water is spilt or lost, and that all the hydrogen and oxygen produced are collected in the tubes. When certain corrections have been made on account of the slight solubility in water of the gases hydrogen and oxygen, the result of this experiment is, that a specified mass of water has been changed into hydrogen and oxygen, that the mass of each of these is less than that of the water used, and that the sum of the masses of the two gases is equal to the mass of the water.

A small weighed quantity of a black solid called copper 28 oxide is placed in a bulb of hard glass arranged as shewn in fig. 8. The U tubes contain calcium chloride, a substance

which greedily absorbs water; these tubes, as well as the little dry bulb a, are accurately weighed ; the bulb containing the

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copper

oxide is also weighed. Precautions are taken that the entire apparatus is quite air-tight. Pure dry hydrogen is passed slowly in as shewn by the arrow; after a few minutes the copper oxide is heated ; drops of a liquid resembling water begin to trickle down into the bulb a; heating in a slow. stream of hydrogen is continued as long as any trace of what seems to be water is produced. When the change is complete, the apparatus is allowed to cool ; and the various parts are then weighed. The liquid formed can be proved to be water, and the red solid left can be proved to be copper. Assuming that the proofs are conclusive ; and assuming that 1 part by weight of hydrogen combines with 8 parts by weight of oxygen to produce 9 parts by weight of water--the results of the last experiment shew that this is so, and the statement has been amply verified experimentally—; assuming these points, the results of the present experiment teach, (1) that the only products of the interaction of copper oxide and hydrogen are water and copper; (2) that the water is formed by the union of the hydrogen with oxygen previously combined with copper; (3) that the mass of the oxygen thus taken away from combination with copper is less than the mass of the copper oxide, and the mass of the copper thus removed from combination with oxygen is also less than the mass of the copper oxide; (4) that the sum of the masses of the copper and the oxygen is equal to the mass of the

copper

oxide. The results of the experiments described in the preceding paragraphs present certain points of similarity. In each, a specified mass of one kind of matter was changed into two

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the copper oxide

(or more) kinds of matter, each different from, and each weighing less than, the original matter. Potassium chlorate was changed into potassium chloride and oxygen ; water into hydrogen and oxygen ; copper oxide into copper and oxygen : the potassium chloride, or the oxygen, weighed less than the potassium chlorate; the hydrogen, or the oxygen, weighed less than the water; the copper, or the oxygen, weighed less than

. But the sum of the masses of the products of each change was equal to the mass of the kind of matter which was changed into these products: the mass of the potassium chloride added to that of the oxygen was equal to the mass of the potassium chlorate changed; the mass of the hydrogen added to that of the oxygen was equal to that of the water; the mass of the oxygen added to that of the copper was equal to that of the copper oxide by the decomposition of which the oxygen and copper were produced.

We have now examined two classes of chemical changes. '30 One class presented to us interactions between two, or more, definite kinds of matter, resulting in the disappearance of the interacting substances, and the production in their place of one, or more, substances very unlike the original kinds of matter. We paid attention to the mass of one of the interacting substances, and to the mass of that product of the change which contained the whole of this substance (neglecting other products if other products there were); we found that the parts of the change to which we paid attention consisted in the combination of the whole of one of the interacting substances with either the whole, or a part, of the other substance. In each experiment a certain kind of matter was changed into a different kind of matter, by entering into combination with some substance different from itself.

The other class of chemical changes presented to us decompositions of one definite kind of matter into two, or more, different substances; the original kind of matter disappeared, and its place was taken by the new substances formed from it. We found that the mass of any one of the new kinds of matter was less than the mass of the matter from which it was derived, but that the sum of the masses of all the new kinds of matter was equal to the mass of the matter which had been changed into these new kinds of matter.

One of the changes considered presented features common to

1 The student should carefully follow the reasoning on which this conclusion was based. (s. par. 28.)

copper

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both classes of changes. Copper oxide interacted with hydrogen; water and copper were produced: but the water was itself shewn to be produced by the union of the reacting hydrogen with oxygen separated from the copper oxide. The mass of the water added to that of the copper was greater than that of the copper oxide; this was because the water formed was composed of the oxygen at first combined with the and also the hydrogen which was one of the constituents of the whole changing system. The change of the copper oxide into copper and oxygen was a change belonging to our second class of chemical reactions; but this was accompanied by a change belonging to the other class of reactions, viz. the production of water by the combination of the oxygen separated from the copper with the hydrogen, the presence of which hydrogen in contact with heated copper oxide was the condition under which the separation of copper oxide into copper and oxygen was accomplished.

When various kinds of matter are examined chemically, it is found that they all belong to one or other of two classes, which we may at present call the hydrogen-class and the water-class.

Those kinds of matter which are placed in the hydrogenclass are characterised by this ;—when any one of them is changed into a totally different kind of matter, the mass of this kind of matter is greater than the mass of the substance belonging to the hydrogen-class which was thus changed. These substances have never been changed by separation into unlike parts. They suffer chemical change by combining with another kind, or other kinds, of matter, the test of this combination being that the substance produced differs from, and weighs more than, the original substance.

Those kinds of matter which are placed in the water-class are characterised by this ;—any one of them may be chemically changed by separating it into unlike parts, the test of this separation being the production from a specified mass of the original substance of at least two different kinds of matter the mass of each of which is less than that of the original substance, while the sum of their masses is equal to that of the original substance. Most, if not all, of the kinds of matter placed in this class may also be chemically changed by combining with some other kind, or kinds, of matter different from themselves, and so producing a new kind of matter weighing more than the original substance.

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