Page images

closed at one end and drawn out at the other; the open end passed, under mercury, a little way into a graduated glass vessel filled with mercury (s. fig. 6). When the red solid was heated mercury was formed and deposited on the colder parts of the tube, and a gas collected in the graduated vessel. The mercury thus formed weighed 411 grains; the gas measured

Fig. 6. between 7 and 8 cub. inches. · The gas was proved to be, not air, but oxygen : now 74 cub. inches of oxygen measured at the temperature and pressure of Lavoisier's experiment, weigh 31 grains. Therefore the 45 grains of red solid formed by slowly burning mercury in air consisted of, or were formed by the chemical combination of, 411 grains of mercury and 31 grains of oxygen; and these 34 grains (or 71 cub. inches) of oxygen were originally present in the 50 cub. inches of air contained in the bell-jar. When the mercury was burnt 411 grains of it disappeared, and at the same time 31 grains of one of the constituents of air disappeared, and 45 grains of a new kind of matter were produced; but these 45 grains of this new matter were composed of the 411 grains of mercury and the 31 grains of oxygen which had disappeared. No loss or destruction of matter occurred during the burning. The hot mercury so interacted with the oxygen contained in the air that there was produced a kind of matter altogether different from either of the interacting bodies.


Now if all the experiments already described were repeated so that the weights of the different kinds of matter taking


of each and every new kind of matter produced in each of these changes were also determined,—and this has actually been done,—we should find that the new kinds of matter formed were formed by the union or combination of the different kinds of matter which constituted the material system at the beginning of each experiment.

The terms disappeared, and was produced, do not then mean were destroyed, and were created ; they rather mean, ceased to exist under the conditions of experiment as a distinct kind of matter, and, was the product of the chemical interaction of two or more kinds of matter which previously existed each as a distinct kind of matter. Similarly the expression used in previous paragraphs, has taken the place of, is now seen to mean, has been formed by the chemical interaction of; the expression also implies that the weight of the new matter which has taken the place of that formerly present is equal to the weight of that which it has replaced. In the case of the magnesium burnt to magnesia, it would be correct to say that the magnesia has taken the place of the magnesium and a certain weight of oxygen in the air.

We now see more clearly than before what is meant by saying that this or that body has been chemically changed into certain other bodies. But a definite and accurate meaning can be given to this and similar expressions only when we have learned more about chemical occurrences.

In the preceding paragraphs the important truth has been assumed that the one fundamental property of matter is its mass or quantity. Moreover it is assumed that the student has learned the proportionality of mass and weight *; that any portions of matter whose masses are equal, however different they may be in other properties, are of equal weights. The mass of any portion of matter is the quantity of matter in that portion; the weight is the force with which that portion is attracted towards the earth's centre. But in all practical problems with which we shall have to deal, the terms mass and weight may be taken as synonymous; because the relative

* If the student is not familiar with the connection between mass and weight he ought to consult a treatise on physics.

masses of substances are determined in chemistry by weighing them against a standard mass of brass, or other material, called 1 gram, or 1 decigram, or 1 milligram, and the weights of substances as thus determined are independent of variations in the force of gravity.

We have now some notion of what is implied in saying that in a chemical change some kinds of matter cease to exist as such, and some other kinds of matter are produced by the interactions of those originally present. To some extent we see that in chemical occurrences change of properties is connected with change of composition.





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, some sulphuric acid is added, and an electric current is passed through the liquid until it is perfectly colourless. Every particle of the red solid which has formed on one of the platinum plates is washed off the plate into a small basin, where it is repeatedly washed with water, then with alcohol, then dried over oil of vitriol, and weighed. It weighs the same as the copper did at the beginning of the experiment; moreover an examination of the properties of this red solid proves that it is copper. In this series of changes, copper has been converted into a blue solid—called copper sulphate—by causing it to react with warm sulphuric acid, and this copper sulphate has been re-converted into copper by electrolysing it. The copper sulphate was produced by combining some other kind of matter with the copper : the experimentally determined fact, that the mass of the copper obtained from the copper sulphate was equal to the mass of copper dissolved in sulphuric acid, proved that the copper sulphate was produced by the union of the copper with some other substance.

A little concentrated nitric acid is heated in a porcelain 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


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 com- 25 mon: 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.

« PreviousContinue »