effect all transmutations. The pursuit of this One Thing became the central quest of alchemy. "There abides in nature" we read in an alchemical treatise "a certain pure matter, which, being discovered and brought by art to perfection, converts to itself proportionately all imperfect bodies that it touches."

Alchemy was a fascinating dream; but chemistry is a more satisfying reality.

Let us return to experiment. Let a small weighed quantity 17 of magnesium be burnt in air under conditions such that the whole of the magnesia produced in the burning remains in the vessel in which the burning proceeds. The apparatus shewn in fig. 5 is a simple one for the purpose. Some magnesium

ribbon is placed on a piece of wiregauze and covered with an inverted funnel, the stem of which is connected by caoutchouc tubing with another funnel; the upper funnel is covered with filter-paper. The whole apparatus is counterpoised; the funnels are removed; the magnesium is ignited by allowing a Bunsen-flame to play on to it from above; the funnels are then replaced. When the burning is complete the apparatus is allowed to cool and is then counterpoised. It is found that the magnesia produced weighs more than the magnesium before burning. We therefore conclude that the magnesia is produced by adding to, or combining with, the magnesium, some other kind of matter. As the change from magnesium to magnesia proceeded in air, it is probable that the new kind of matter, which, by our hypothesis, has combined with magnesium and so produced magnesia, is derived from the air. To find whether this conclusion is correct or not, it would be necessary, (1) to burn a known weight of magnesium in a known quantity of air; (2) to determine the weight of magnesia produced, and the diminution in the quantity of air which accompanied the production of this weight of magnesia; (3) to change the magnesia back to magnesium and air, and to determine the weight of each of these obtained. If the difference between the weight of the magnesia and that of

Fig. 5.

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the magnesium, by burning which the magnesia was formed, was equal to the weight of air which disappeared during the burning; and if the magnesium obtained from the magnesia weighed the same as the magnesium originally burnt; and if the weight of air obtained from the magnesia was the same as the weight of air which disappeared during burning; then we should be justified in concluding that the chemical change which occurs when magnesium is burnt consists in the addition to, or combination with, magnesium, of a portion of the surrounding air, and that the new kind of matter produced is composed of two kinds of matter, viz. magnesium and air. We should further have learned that although the magnesium has disappeared it has not been destroyed; and that although magnesia has been produced it has not been produced from previously non-existent matter. We should have given a definite meaning to the terms 'produced' and 'disappeared', as regards the change of magnesium to magnesia at any rate; and we should, to some extent at least, understand what is meant by saying the magnesia has taken the place of the magnesium which was burnt.

It is not easy to arrange quantitative experiments by which the chemical change of magnesium to magnesia may thus be examined. But if we use mercury in place of magnesium, we can arrange an experiment which will enable us to find an answer to each of the three questions stated in par. 15.

The change of mercury to burnt mercury, or as we now call it oxide of mercury, was examined quantitatively by Lavoisier. A sketch of the essential parts of his apparatus is shewn in fig. 6. Lavoisier placed 4 oz. of mercury in a glass balloon, the neck of which, drawn out and bent, passed under mercury and then into the air contained in a bell-jar; the bell-jar contained 50 cub. inches of air. The mercury was heated nearly to its boiling point by means of a furnace; red specks appeared on the surface of the mercury and the volume of air in the bell-jar slowly decreased. After some days the production of red solid matter on the surface of the mercury seemed to have ceased; heating was continued for a few days more (12 days in all), and was then stopped. The air in the bell-jar now measured between 42 and 43 cub. inches, the diminution in volume was therefore between 7 and 8 cub. inches; the red solid was collected, and was found to weigh 45 grains. These 45 grains of the red solid produced by slowly burning mercury in air were placed in a glass tube

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 41 grains; the gas measured

Fig. 6.

between 7 and 8 cub. inches. The gas was proved to be, not air, but oxygen: now 7 cub. inches of oxygen measured at the temperature and pressure of Lavoisier's experiment, weigh 3 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, 411⁄2 grains of mercury and 3 grains of oxygen; and these 3 grains (or 7 cub. inches) of oxygen were originally present in the 50 cub. inches of air contained in the bell-jar. When the mercury was burnt 41 grains of it disappeared, and at the same time 3 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 41 grains of mercury and the 3 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.

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Now if all the experiments already described were repeated so that the weights of the different kinds of matter taking part in each chemical change were determined, and the weights 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 occur

rences.

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.