Another fundamental law is the Law of Multiple Proportion, which states that: "When one substance unites with another in more than one proportion, those 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 multiples 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 any
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.
An examination of these three laws led Dalton to formulate the Atomic Theory of Matter, although other chemists be fore him had put forward from time to time a similar hypothesis. Expressed briefly, the theory is that matter is made up of minute particles called atoms, and that chemical combination takes place be tween these atoms. Atoms of the same element are similar to one another and equal in weight, and compounds are formed by the union of atoms of differ ent elements in simple numerical propor tion-1 : 1, 1 : 2, 2 : 3, 5 : 1, and so on. This theory forms the basis of quan titative chemical work, and modern chem istry offers an overwhelming mass of evi dence that the theory is true.
The Status of Chemistry. The im portance of chemistry in daily life can scarcely be over-estimated. Most of our industries are largely dependent upon it, including the manufacture of iron and steel, paper, glass, photographic mate rials, oils, drugs, dyes, explosives, soaps, concrete, paint, perfumes, and many others. The manufacture and application of fertilizers forms the connecting link between this science and the farm, and the preparation of foodstuffs, flavorings, and essences brings chemistry into the kitchen. Some knowledge of chemistry, moreover, is a necessity to the physician, the engineer, or the electrician. It is, therefore, clear, that modern existence is more dependent upon chemistry than upon any other science.