marble weighing from 2 to 5 grams. A large Bunsen or a Teclu or Meker burner should be used. Immerse the lower end of the delivery tube in limewater, contained in a small test tube. Continue to heat for several minutes and note the continued slow evolution of gas and the effect of the gas on limewater.

To hasten the action transfer to a porcelain crucible the marble remaining in the test tube and heat in the flame of a blast lamp for 15 to 30 minutes. Allow to cool and examine the residue. Add to it as much warm water as it will absorb and allow it to stand a few minutes. Add more water to the product and test the water with litmus paper.

The material of which marble consists is known in chemistry as calcium carbonate. The gas, which it evolves on heating and which affects the limewater, is called carbon dioxide, and the residue left in the crucible after all the carbon dioxide has been driven out is quicklime or calcium oxide. Quicklime is commonly made from limestone, a less pure calcium carbonate than marble. Quicklime and water react to form slaked (or slacked) lime, and slaked lime dissolves slightly in water, yielding limewater.

The student will have observed that in each of the above experiments (Nos. 13-17) one substance is converted into two or more new substances. Chemical change of this type is called decomposition. Thus we say that the red solid, mercuric oxide, is decomposed by heating into the metallic liquid, mercury, and the colorless gas, oxygen; and that water under the influence of an electric current decomposes into the two gases, hydrogen and oxygen. A great many substances are decomposed by heating, and some of them, like the sugar in Experiment 15, give a large number of decomposition products. A great many also are decomposed by the electric current, and a special name, electrolysis, is applied to decomposition so effected. The decomposition products, we say, are simpler substances than those from which they are made. Thus, mercury and oxygen are simpler substances than mercuric oxide; and potassium chloride (the white residue in Expt. 14) and oxygen are simpler substances than potassium chlorate.

In many instances the decomposition products can be readily recombined into the original substance. For instance, the hydrogen and oxygen obtained from water in Experiment 16 can be recombined into water by simply mixing the gases and setting the mixture on fire. We are therefore justified in regarding the more complex substances as compounds of the simpler. Thus we say that mercuric oxide is composed of mercury and oxygen, or that it is a compound of the simpler substances, mercury and oxygen - although it does not in the least resemble either of these simpler substances; and that water is a compound of hydrogen and oxygen (p. 13). When water is put upon hard lumps of quicklime the two substances combine, forming the powdery substance which we call slaked lime. (See Expt. 50, p. 96.)

We commonly speak of the decay of dead animal and vegetable matter as "decomposition." The chemical changes involved in such decay are much more complex than any of those we have studied in our experiments. Every animal or vegetable organism comprises a great many different substances. Even the parts our eyes readily recognize as different such as bone, blood, muscular tissue (flesh), and fat are generally mixtures of a number of different substances. And very commonly substances outside of the organisms, particularly water and air, are involved in the process of decay. Nevertheless, since such decay does result for the most part in the production of simpler substances than those originally present in the decaying organism, the word decomposition as applied to decay has a signification closely allied to our definition of the term.

Chemical change which results in the formation of one substance from two or more is called combination. Combination has already been illustrated in the instance of hydrogen and oxygen. The following experiments furnish further illustrations of this class of chemical action.

Heat a little roll sulphur to boiling in a test tube. While the sulphur is actively boiling, drop in a piece of copper foil. Note what occurs. The great majority of chemical changes are accompanied by the evolution of more or less heat. In some, the heat is so great as to make the solid substances involved in the change give out light. Many of the chemical reactions which produce great quantities of heat belong to the class we are now considering, viz. combinations. (See Expt. 19.)

FIG. 19. Burning carbon (charcoal) in a current of oxygen. The oxygen is generated in the flask by heating potassium chlorate. By bubbling through 30 per cent potassium hydroxide in the first wash bottle, it is freed from any carbon dioxide it may contain. It then passes through the heated glass tube containing the charcoal, and finally through the second wash bottle, which contains limewater. (Experiment 19.)

Examine the product formed from the copper, comparing it with the original copper and sulphur as regards color, cohesion, etc. (Ordinarily there will be a considerable quantity of unchanged sulphur left in the test tube, but this is readily distinguished from the new substance, which takes more or less nearly the form of the piece of copper used.) In this experiment how many substances entered into action and how many new ones were formed?

Experiment 19.*

Materials:

Lumps of charcoal, thoroughly dried by heating. Apparatus:

Oxygen generator, or cylinder of compressed oxygen provided with a soda-lime tube or with a wash bottle containing a 30 per cent solution of potassium hydroxide.

Small combustion furnace (with 4-10 burners).

Hard-glass tubing.

Glass and rubber tubing for connections.

Place a few lumps of dried charcoal in a hard-glass tube in a

small combustion furnace. Provide the tubes with corks and connect one end with the oxygen generator, the other with a delivery tube. Pass oxygen through the apparatus to expel the air. Immerse the end of the delivery tube in limewater and continue to pass pure oxygen through the apparatus. Is the limewater affected by the pure oxygen?

Heat the hard-glass tube gently at first, then gradually raise the heat to redness. Pass a very slow current of oxygen over the heated carbon into the limewater. What occurs? has been formed?

What gas

If the experiment is continued long

enough, the charcoal will disappear, FIG. 20.- Oxygen generator

leaving only a small quantity of white or gray ash. Except for this ash, charcoal consists entirely of a substance called carbon. This substance, when raised to a sufficiently high temperature, combines with oxygen. What substance is the product of this combination?

using fused sodium peroxide and water. Oxygen prepared in this way contains no carbon dioxide. When this style of generator is employed limewater may be used in both the wash bottles, or the first wash bottle may be omitted entirely.

Air contains oxygen, and exactly the same chemical action takes place when charcoal burns in a draft of air as when it burns in pure oxygen. Is any heat produced in this combination? What practical use is made of this chemical change?

C

CHAPTER III

ELEMENTS

WATER, which is one of the decomposition products of sugar (see Expt. 15), may, as we have seen in Experiment 16, be itself decomposed into hydrogen and oxygen. In Experiment 14 one of the products obtained in the decomposition of potassium chlorate is potassium chloride, a white solid. If this substance is highly heated, it melts, and if an electric current is passed through the molten mass, decomposition occurs, the products being a soft metal, called potassium, and a greenish yellow gas, called chlorine.

While, therefore, water is a simpler substance than sugar, it is less simple than hydrogen and oxygen; and while potassium chloride is a simpler substance than potassium chlorate, it is evidently less simple than potassium and chlorine.

When limestone is heated in a limekiln, quicklime and carbon dioxide (carbonic acid gas) are produced. (See Expt. 17.) But carbon dioxide, as we have seen (Expt. 19), is composed of the simpler substances, carbon and oxygen. Similarly, quicklime is a compound of a metal called calcium and the gas oxygen.

We see, then, that the products of some decompositions can in their turn undergo decomposition. However, there are a number of substances which it has not been found possible to decompose. Among these are hydrogen, oxygen, potassium, chlorine, calcium, mercury, and carbon.

Substances which we cannot decompose are known as elements. This is the only sense in which the word element is used in chemistry. When we speak, as we commonly do, of wind