chemical processes, which will bring back our material to the condition of lime and carbonic dioxide, the substances from which we started. The first is a reaction, identical with the one I have just written. Since the beginning of the lecture, I have been strongly heating some lumps of chalk in this platinum crucible. The process is a slow one; and it was necessary to begin the experiment early, in order that I might show you the result. The chemical change is identical, however, with that which may be observed in any lime-kiln, where lime is made by burning limestone. Each molecule of chalk, CaCO,, looses a molecule of carbonic dioxide, CO2, and we have left a molecule of lime, CaO. But the change in the appearance of the white mass produced by burning is so slight that I must bring in the aid of experiment to prove that any change has taken place; and, first of all, I must show you the test I am going to use. In the first of these two jars I have an emulsion of chalk, and in the second milk-of-lime. Notice that this piece of paper, colored by a vegetable dye called turmeric, remains unchanged when dipped in the emulsion of chalk, but turns red in the milk-of-lime. Let us test, now, the contents of our crucible. We will first empty it into some water. The white lumps almost instantly become slaked, and render the water milky. We will now dip in a sheet of turmeric-paper, and you see that, although we began with inactive chalk, we have obtained a material which acts on the turmeric-paper like caustic lime. Thus, then, we have regenerated the lime. Let us next see if we can regenerate the carbonie dioxide: In the last experiment, carbonic dioxide was pro DECOMPOSITION OF CHALK. 167 duced, but it escaped so slowly, and in such small quantities, as entirely to escape notice. Where, however, limestone is burned on a large scale, the current of gas from the kiln is frequently very perceptible; and more than one poor vagrant, who has sought a night's lodging under the shelter of the stack, has been suffocated by the stream. But we can make evident the production of carbonic dioxide from chalk without the aid of such a sad illustration. FIG. 22.-Pneumatic Trough, with Two-necked Gas-bottle. One of In this bottle we have some bits of chalk. the two necks of the bottle is closed by a cork, through which passes tightly an exit-tube, to conduct away any gas that may be formed. The other is also corked, and through the cork passes a funnel-tube, by which I can introduce any liquid reagent into the bottle (Fig. 22). On pouring in some muriatic acid, a violent effervescence ensues, and a gas is formed which, flowing from the exit-tube, displaces the water in this glass bell. The bell stands in what we call a pneumatic trough, and this simple apparatus for collecting gases must, I think, be familiar to all of my audience. The open mouth of the bell rests on the shelf of the trough under water, and the liquid is sustained in it by the pressure of the air. Let me, while the experiment is going on, write out the reaction: CaCO3 + (2HCl + Aq.) = (CaCl, + Aq.) + CO2. Chalk. Hydrochloric Acid. Calcic Chloride. We already know the symbols of all the factors, and we may, therefore, confine our attention to the products. The products are, first, carbonic-dioxide gas; and, secondly, a solution in water of a compound whose molecule consists of calcium and chlorine, and which we call calcic chloride. And, now that the jar is filled, I can easily show that we have regenerated carbonic dioxide. Removing the jar from the trough, we will first lower into it this lighted candle, and then pour into it some lime-water. The candle is instantly extinguished, and the lime-water rendered turbid. Thus we end the torture of these molecules. You have seen how easily we have formed them, and how readily we have broken them up. We began with lime and carbonic dioxide, which we united to form chalk. We dissolved the chalk in a solution of CO2, and learned how, in Nature, various forms of limestone could be crystallized from this solution. Lastly, we have recovered from the chalk the lime and carbonic dioxide with which we begun. I hope you have been able to follow these changes, and to understand the language in which they are expressed. If so, we have taken another step in advance, and, at the next lecture, shall be able to go on and classify these reactions, and thus prepare the way by which we may reach still further truth in regard to this wonderful microcosm of molecules and atoms. LECTURE VIII. CHEMICAL CHANGES CLASSIFIED. AMONG chemical reactions we may distinguish three classes: 1. Those in which the molecules are broken up into atoms; 2. Those in which atoms are united to form molecules; and, 3. Those in which the atoms of one molecule change places with those of another. Reactions of the first kind are called analysis, those of the second synthesis, and those of the third metathesis -terms derived from the Greek, and signifying respectively to tear apart, to bind together, and to interchange. This classification is one of great theoretical importance. But it must be further stated that a simple analytical or synthetical reaction, as here defined, is seldom if ever realized in Nature. Almost every chemical process is attended both with the breaking up of molecules into atoms and the regrouping of these atoms to form new molecules, that is, it involves both analysis and synthesis; and this is true even in the many cases where the products or factors of the chemical reaction are elementary substances; for, when the molecules of the elementary substances consist of two or more atoms, the breaking apart or coalescing of these atoms, although they are atoms of the same ele ment, constitutes analysis or synthesis, as here defined. Thus, when, in the burning of hydrogen gas, this elementary substance unites with the oxygen of the air to form water, the molecules of oxygen must be divided into atoms before the synthesis of the water molecule is possible; and so, on the other hand, when water is decomposed, the resulting atoms of oxygen unite by twos to form molecules of oxygen gas; and this pairing is, according to our definition, a process of synthesis. The chemical reactions, which express these changes, illustrate very clearly the point here made: Burning of Hydrogen Gas. 2H-H+0=0 = 2H2O. Hydrogen Oxygen Gas. Gas. Water. Decomposition of Water. The first states that from one molecule of oxygen are formed two molecules of water, and this, of course, necessitates a division of the oxygen molecules; while the second states that from two molecules of water only one molecule of oxygen gas results, a process which involves the union of the two oxygen atoms, previously separated in the two molecules of water. Indeed, a purely analytical or a purely synthetical reaction would only be possible theoretically in those cases where elementary substances were involved, whose molecules consist of a single atom, that is, where the molecule and the atom are identical, and we can recall no well-defined reactions of this kind. But, although we should be obliged to seek among the unfamiliar facts of chemistry for examples of pure analysis or pure synthesis, yet processes, in which one or the other is the predominant feature, and which illustrate the special characteristics of each, are close at hand. Some of these I now propose to bring before you, beginning with the analytical processes, and I |