the reaction between sodic hydrate and hydrochloric acid; and, if, as I said before, these are only examples of what is true in the case of all alkalies and all acids, we are certainly justified in deducing from our experiments the following principles: First, an alkali is a substance whose molecules have a definite structure, and differ from the molecules of water only in having a metallic atom in place of one of the hydrogenatoms of the water-molecule; secondly, an acid is a substance whose molecules contain at least one atom of hydrogen, which is readily replaced by the metallic atom of the alkali when the two substances are brought together.

As the illustrations already given indicate, the characteristic qualities of an acid depend upon the circumstance that certain hydrogen-atoms in the molecules of these substances are readily replaced by metallic atoms. In my next lecture, I shall show that this susceptibility to replacement depends upon a definite molecular structure, but I must not leave this subject without insisting on the fact that this characteristic of acids is manifested in other ways besides the special mode we have been studying. A few experiments will illustrate this point:

In this flask there are some wrought-iron nails. We pour over them some muriatic acid, and warm the vessel. At once there is a brisk evolution of gas, which we are here collecting, in the usual way, over water; and notice that, when lighted, the gas burns with the familiar flame of hydrogen. Muriatic acid is an old friend. We know all about its constitution, and it is evident that the iron-atoms have replaced the hydrogen-atoms of the acid. If we evaporate the solution left in the flask, we shall obtain a green salt con

PREPARATION OF HYDROGEN GAS.

257

sisting of chlorine and iron. The reaction is thus

As the iron-atom is bivalent, it takes the place of two atoms of hydrogen, which, when thus displaced, form a molecule of hydrogen gas.

In the second flask are some zinc-clippings, and we will pour over them some dilute sulphuric acid, one of the best known of the class of compounds we are studying. Again, notice a brisk evolution of gas (Fig. 31), which also, as you see, burns like hydrogen. Indeed, this is the process by which hydrogen gas is usually made:

H2-SO4 + Zn = Zn=SO4 + H-H. Sulphuric Acid." Zinc. Zinc Sulphate. Hydrogen Gas. In the reaction, which is here written, you notice that, as before, the metallic atom takes the place of two atoms of hydrogen; but sulphuric acid differs from

both hydrochloric acid and nitric acid in that each of its molecules has two atoms of hydrogen, which can be thus replaced.

Examples like these might be multiplied indefinitely. We will conclude, however, with one more experiment, which illustrates the same susceptibility to substitution, but under slightly different conditions. This white powder is called zinc oxide, and is a compound of zinc with oxygen. Notice that it dissolves readily in a portion of the same dilute sulphuric acid used in the last experiment. Moreover, on evaporating the solution, we should obtain zinc sulphate (ZnSO,), the same product as before. Why, then, is there no hydrogen gas evolved? Let our symbols tell us:

ZnO + H2SO4 = H2O + ZnSO4.
Zinc Sulphate.

Zinc Oxide.

Sulphuric Acid.

Water.

You see that the metathesis yields water instead of hydrogen gas, and the question is answered.

LECTURE XII.

ELECTRO-CHEMICAL THEORY.

IN our last lecture we saw that, whether an acid is brought in contact with an alkali, a metal, or a metallic oxide, one or more of the hydrogen-atoms in its molecules become replaced by metallic atoms from the molecules of the associated body, and this susceptibility to replacement was, as I stated, the distinguishing feature of that class of compounds we call acids. But I should leave you with a very imperfect notion of these important relations, if I did not proceed further to illustrate that the class of compounds we call alkalies, and which we have been accustomed to regard as the very opposite of acids, have exactly the same characteristics.

In this small glass flask there are some clippings of the metal aluminum, the metallic base of clay which has, within a few years, found many useful applications in the arts. On this metal I pour a solution of caustic potash. Notice that, on heating the flask, I obtain a brisk evolution of gas. On lighting the gas, it burns with a flame which leaves us no doubt that the gas is hydrogen. What, now, is the reaction? Somewhat more complex than those you have previously studied,

because the atom of aluminum has a quantivalence of six. Moreover, in order to satisfy certain very striking analogies, we write the symbol of this atom Al, that is, we take 27.4 m.c. of aluminum for the assumed atom, and represent that by Al, although 54.8 m.c., which we write Al,, is the smallest quantity of the element of which we have any knowledge, or which changes place with other atoms in the numerous metathetical reactions with which we are acquainted. Here

the Al, takes the place of six hydrogen-atoms, thus binding together what were before six distinct molecules of K-O-H into a single molecule of the resulting product. Evidently, then, the hydrogen-atom in the molecule of the alkali has the same facility of replacement as that in the molecule of the acid. Nor is this an isolated example, although, perhaps, the most striking we could adduce, and it illustrates a truth which was recognized long before the general adoption of the new philosophy of chemistry. Acids and alkalies belong to the same class of compounds, and caustic potash and nitric acid are simply the opposite extremes of a series of bodies in which all the intermediate gradations are fully represented. In our modern chemistry we call this class of chemical substances hydrates, and we distinguish the two extremes of the class as alkaline (or basic) and acid hydrates, respectively. The terms alkaline and basic are here used synonymously, although the first is generally restricted to the old caustic alkalies, including ammonia and the