BECHER AND STAHL.

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vanced in 1682 by Becher, a German chemist then living in England, and was worked out into a complete system some years later by Stahl. According to this theory, the principle of fire is everywhere diffused throughout Nature, but enters into the composition of different bodies to a very unequal extent. Combustible substances are bodies very rich in phlogiston, and burning consists in the escape of phlogiston into the atmosphere. I have already referred to this theory, and shown that it was in variance with the great principle of the law of gravitation, that quantity of matter is proportional to weight. Still, as I said before, this principle of Newton made its way into chemistry very slowly, and the theory of Stahl was in complete accordance with the philosophy of Aristotle, which at the time held. an entire supremacy over the intellectual world. And was the theory wholly false? I believe not; and I am persuaded that every theory, which gains among thinking men such universal acceptance as did this theory of Stahl, has its element of truth. The men of the seventeenth century were not less acute thinkers than ourselves, and we must be careful not to judge of their ideas from our stand-point. The authors of the theory never attached to phlogiston the idea of weight which we necessarily associate with all matter. It was to them a principle, an undefined essence, and not matter in the sense we understand it. Vague and indefinite idea, no doubt, like many of the metaphysical ideas of the time, but not absurd. And that it was not absurd a single consideration will show. Translate the word phlogiston energy, and in Stahl's work on chemistry and physics, of 1731, put energy where he wrote phlogiston, and you will find there the germs of our great modern doctrine of conservation of energy

one of

the noblest products of human thought. It was not a mere fanciful speculation which ruled the scientific thought of Europe for a century and a half. It was a really grand generalization; but the generalization was given to the world clothed in such a material garb that it has required two centuries to unwrap the truth. Still, the sparkle of the gem was there, and men followed it until it led them into a clearer day. It is a great error to suppose that the theory of Lavoisier superseded that of Stahl. It merely added to it. Stahl clearly saw that the chief characteristic of burning was the development of energy, and, although he called energy phlogiston, and did not comprehend its real essence, he recognized that it was a fundamental principle of Nature. He did not understand the chemical change which takes place in the process, and this Lavoisier discovered. But both Lavoisier and his followers, to a great extent, ignored the more important phenomenon in magnifying the less, and it is only within a few years that the true relations of the two have been understood. All honor to these great pioneers of science, and let their experience teach us that, in science as in religion, we see as through a glass darkly, and that we must not attach too much importance to the forms of thought which, like all things human, are subject to limitations and liable to change.

LECTURE XII.

METATHESIS AND QUANTIVALENCE-ALKALIES AND ACIDS.

IN classifying reactions we distinguished besides analysis and synthesis a third type of chemical changes which we called metathesis, and I will begin my lecture by exhibiting several experiments which illustrate processes of this kind. This white solid is familiar to the druggists under the name of sugar of lead. It is made from metallic lead and acetic acid, the acid principle of vinegar, and is called by the chemists acetate of lead. It is a crystalline salt, very soluble in water, and this clear solution has been prepared for our experiment. In the solution I now hang a strip of thick sheet-zinc. As, however, the process we have started requires several days, I have placed at the side of the jar holding the solution of acetate of lead a similar jar originally filled with the same solution, and in which a similar strip of zinc was placed soon after our last lecture; and notice that suspended from the strip are festoons of brilliant metallic spangles. These consist of pure metallic lead, and if, after the process is ended, we pour off the still clear solution which has undergone meanwhile no apparent change, we shall find, on evaporation, that it

contains no longer acetate of lead, but another white salt, equally well known as acetate of zinc.

This beautiful experiment, known to the alchemists, and called by them "arbor Saturni" (lead-tree), is as striking an example of metathesis as I can show you. Metathesis, as you remember, consists in an interchange of elements between two substances without otherwise altering their structure, and here there has been a simple interchange between the two metallic elements, lead and zinc.

For a second experiment we have prepared a solution of a well-known blue salt called blue vitriol, or sulphate of copper, and in the solution we will hang a strip of sheet-iron. This reaction, like the other, being a slow process, we were provident enough to start the same. experiment in another jar in time to show you the result. As you see, large spongiform masses of metallic copper are suspended to what remains of the strip of iron, still more of the copper sponge has fallen to the bottom of the jar, and the blue color has wholly disappeared from the solution. If, now, we pour off the solution and evaporate it, we shall obtain green crystals of sulphate of iron, the green vitriol of commerce. Here, therefore, there has been an interchange between copper and iron. Let us now write these reactions with symbols:

(1.) (Pb C4 Hε O4 + Aq.) + Zn = (Zn C1 H6 O1 + Aq.) + Pb 63.3

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(2.) (Cu SO4 + Aq.) + Fe = (Fe SO4 + Aq.) + Cu

These formulæ not only show that the general order of the two processes is that described above, but they also indicate that in the replacements which have taken place definite proportions by weight have been pre

served. The symbols, as you remember, stand for the relative weights of the atoms or molecules represented, and the equations express the fact that in the first reaction 65.2 parts of zinc took the place of 206.9 parts of lead, and that in the second reaction 56 parts of iron took the place of 63.3 parts of copper. Now as is true here so, in general, metathesis consists in the interchange of atoms, or groups of atoms, between two molecules, and implies that the structure of these molecules is not otherwise altered. Such an interchange, of course, involves the breaking up of one set of molecules and the regrouping of the atoms to form another set, and from this general point of view all reactions are essentially alike; but the cases are so very common of chemical processes in which one atom, or a group of atoms, is simply substituted for another, without otherwise altering the structure of the molecules concerned, that it is convenient to study these reactions by themselves.. Moreover, they have served to elucidate in a most wonderful way the manner in which the atoms and the molecules are grouped together. Before, however, I attempt to directly illustrate this point, let me ask your attention to a few other examples of metathetical reactions in an order which will help to gradually open up the problem of molecular structure.

I have here a perfectly colorless solution of a wellknown compound of silver called nitrate of silver, or lunar caustic. In this solution I place a strip of metallic copper, and at the side is a jar in which the same experiment has gone on to completion. Notice that the solution has acquired a decided blue color, which, to every one who knows that this is the characteristic color of the salts of copper, is of itself a proof that this metal must have been taken up from the copper strip.