CHEMISTRY 448 CHEMISTRY saw above, that the formula for ferrous sulphate was FeSO* The formula for ammonium sulphate is (NH4)2S04 and, to express the fact that the atoms of nitro- gen and hydrogen forming ammonium act as a single entity, parentheses are used, as shown. These symbols and for- mulae are valuable and necessary because they enable us to express briefly the exact composition of a substance, and because, by their use, chemical reactions can be shortly expressed in the form of an equation. Thus: CaCO, + H2SO4 = CaSOi + H2O + CO2 indicates that calcium carbonate reacts with sulphuric acid to give calcium sulphate, water and carbon dioxide. Metals. — Largely from their physical characteristics, a number of the elements are called metals. They all possess the metallic luster, are of opposite affinity to oxygen, can within certain limits for each case replace hydrogen in acids and other metals in salts. They conduct electricity and heat comparatively well, and are gen- erally solid at an ordinary temperature. Non-metals. — All the other elements are classed as non-metals. Some are solid, such as sulphur and iodine; bro- mine is liquid, and many are gaseous at ordinary temperatures, such as oxygen and chlorine. Some elements are on the border line, such as silicon and arsenic, it being hard to class such definitely as metals or non-metals. Chemical Laws. — By many years of patient research the chemists of the late 18th and early 19th century estab- lished the fact that all chemical re- actions take place in accordance with definite quantitative laws. The first and simplest chemical law is known as the Lmw of Constant Proportions. This states that: "The sam,e compound always con- tains the same elements combined to- gether in the same proportion by weight." This law establishes the difference be- tween a chemical compound and a mix- ture. A mixture may obviously be com- pounded of its ingredients m varied proportions. Putty, for instance, may contain more or less whiting, more or less oil, and still it will be putty. But cal- cium carbonate, which is the chief con- stituent of whiting, will always contain calcium, carbon, and oxygen combined in the same proportions. It may be dug from the earth as calcite, or prepared artificially by treating lime with car- bonic acid, or obtained as a precipitate by treating sodium carbonate solution with calcium chloride, but, however ob- tained, it will always contain 40 parts of calcium to 12 parts of carbon and 48 parts of oxygen. Another fundamental law is the Law of Multiple Proportion, which states that: "When one substance unites with another in more than one proportion, these differ- ent proportions bear a simple ratio to one another." This law was first estab- lished by Dalton. We have already seen that it sometimes happens that the same elements combine to form different com- pounds. Oxygen will unite with sulphur to give sulphur dioxide, or sulphur tri- oxide. In the first, 32 parts of sulphur combine with 32 parts of oxygen; in the second 32 parts of sulphur combine with 48 parts of oxygen. The ratio of the different weights of oxygen which will combine with 32 parts of sulphur is, therefore, 32 : 48 or 1 : 1.5. Similarly, nitrogen will combine with oxygen to give nitrous oxide, nitric oxide and nitro- gen trioxide. In the first compound, 14 parts of nitrogen combine with 8 parts of oxygen; in the second, 14 parts of nitrogen with 16 parts of oxygen; in the third, 14 parts of nitrogen with 24 parts of oxygen. Here, the ratio of the differ- ent weights of oxygen which combine with a definite weight of nitrogen is 8 : 16 : 24 or 1 : 2 : 3. A simple ratio, as illustrated in these two examples, is found in every similar case, thus estab- lishing the second law of chemistry. The third law is called the Law of Reciprocal Proportions, and states "The weights of different elements which combine sep- arately with one and the same weight of another element are either the same as, or simple nudtiples of, the weights of these different elements which com- bine with each other." To make this clear, let us take a concrete example. Hydrogen, oxygen, and chlorine will all combine with sulphur. In the case of hydrogen, two parts combine with 32 of sulphur, in the case of oxygen, 32 parts with 32 parts of sulphur, and in the case of chlorine, 35.5 parts with 32 parts of sulphur. Now let us see what the law of reciprocal proportions would lead us to infer from these figures. The weights of the three elements given above, which combine with the same weight of sulphur, are 2, 32 and 35.5, respectively. If ani^ two of these elements combine with each other, therefore, we should expect the weights which combine to be either the same as the above or simple multiples of them. Let us take the first two, hydrogen and oxygen. Two parts of hydrogen combine with 16 parts of oxygen, and 16 is exactly one-half of 32. Take hydrogen and chlorine. Two parts of hydrogen combine with 71 parts of chlorine, and 71 is exactly twice 35.5. We see, there- fore, that the law holds good, and the same will be found true no matter what elements are considered.
