CHAPTER II. CHEMICAL COMPOSITION OF DIFFERENT KINDS OF MATTER. 22 23 We must now more fully examine the conception expressed in the phrase composition of this or that kind of matter. It has been already proved by experiment that when a specified mass of magnesium is burnt in air, a new body is produced composed of the magnesium and oxygen taken from the air, and that the mass of this product of the burning of magnesium is greater than the mass of the magnesium used. The magnesium has been changed by adding to, or combining with, it, another kind of matter, viz. oxygen. If a little very finely divided iron is weighed, and then heated to redness, the iron will glow brightly; if the source of heat is then withdrawn it will be seen that a reddish brown substance, quite unlike the original iron, has been produced; if this substance is weighed, when cold, it will be found to weigh more than the iron used. Iron, like magnesium, has been changed into a new substance, and this change has been effected by causing the iron to combine with some kind of matter different from itself. As we know that the burning of magnesium consists in combination with oxygen, and as the conditions under which the iron has been chemically changed are similar to those which prevailed during the burning of magnesium, we conclude that the change which the iron has suffered probably consists in combination with oxygen. This conclusion has been verified by experiments. A weighed quantity of finely divided copper is dissolved in moderately concentrated warm sulphuric acid; when solution is complete, the greenish blue liquid is evaporated to dryness by steam; the blue solid is kept at 100° until it is perfectly dry, when it is collected and weighed. It weighs more than the copper did. The blue solid is now dissolved in water, A little concentrated nitric acid is heated in a porcelain 24 dish; after a time the whole of the liquid has disappeared; it has been entirely gasified. A few scraps of thin tin-foil are now weighed into a porcelain dish and a little concentrated nitric acid is allowed to fall, drop by drop, on to the tin; the tin is changed to a loose white powdery solid; this is heated so long as any gas comes off, then allowed to cool, and weighed. The white powder weighs more than the tin did. But we know that nitric acid is entirely volatilised by heating in an open dish; hence we conclude that in the reaction between the two kinds of matter, tin and nitric acid, the tin has probably laid hold of some constituent or constituents of the acid, and that the white powder formed is the product of the union of the tin with this substance. This conclusion has been verified by carefully conducted experiments it has been proved that the change of tin to the white powdery solid produced in the last experiment consists in the combination of the tin with one of the constituents of nitric acid, namely, oxygen. The experiments now described have this feature in common in each a chemical change has occurred; one kind of matter has been changed into another, and the change has consisted in the combination of the original matter with another kind of matter unlike itself. Each change has been M. E. C. 2 25 26 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 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 a 29 Fig. 8. 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 minutės 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 |