ATOMIC THEORY. Analysis shows that compound bodies contain certain ele ments (see CHEMISTRY) in certain proportions. These proportions have been minutely and carefully examined by many chemists since the time when the balance was first applied to chemical investigation, and it has been proved that the respective quanti ties of each of the combining elements are not dependent entirely upon external con ditions, but are regulated by certain laws. These laws were partially observed and discussed by earlier chemists and physicists, but it was reserved for t)alton (q.v.) to systematize the somewhat incoherent labors of his predecessors, and to announce, in positive language, the four laws which regulate the union of various kinds of sub stances, and which are still acknowledged by chemists as the LAWS OF COMBINING PROPORTION, or the atomic theory. These laws regulate the combination of unlike substances by weight, and not by volume; and they are based upon the preliminary acknowledged fact, capable of experimental demonstration, that the same compound substance is always composed of the same ingredients or elements.
The first law of combination by weight comprehended under the A. T. is T11F LAW OF CONSTANT PROPORTION, which teaches that the elements or ingredients which form a chemical compound are always united in it in the same proportion by weight. Thus, water, which consists of oxygen and hydrogen, does not contain one or both of these elements in indefinite amount, but it is invariably made up of S parts by weight of oxygen to 1 part by weight of hydrogen. It makes no matter whether the total amount of either element be represented by grains, ounces, pounds, or tons, it will always be found that the proportion of 8 parts of oxygen to 1 part of hydro gen is kept up. Neither does the source of the water make any difference, for pure water obtained from rain, snow, or hail, the river or the sea, the sap of plants or the juices of animals, invariably contains the same elements in the same proportions. Again, common salt (chloride of sodium), whether it be obtained from sea-water, salt springs, rock-salt, or even the blood of animals, always consists of chlorine and sodium in the exact and never varying proportion of 314 parts of chlorine to 23 parts of Aodium. Whilst the law of constant proportion teaches us that the same compound is always built up of the same ingredients in the same proportion, it-does not neces sarily follow that the same elements or components in the same proportions will invariably form the same compound body. It is far otherwise; and many examples
can be obtained, especially from organic chemistry, the same components in the same proportions produce very different substances. Thus, starch and cotton (lignine)—very dissimilar substances—consist of carbon, hydrogen, and oxygen in the very same proportions; and gum-arabic and cane-sugar are similarly circumstanced.
See ISOMERIC BODIES. • The second law is the LAW OF RECIPROCAL PROPORTION, which tells us that the proportions in which two substances unite with a third have a simple arithmetical rela tion to that proportion in which they unite with each other. Thus oxygen and hydrogen unite in the proportion of 8 to 1 to form water. Carbon and hydrogen are present in olefiant gas in the proportion of 6 to 1, and oxygen and carbon unite in the proportion of 8 to G to form carbonic oxide. Again we have a compound of oxygen and iron containing these elements in the proportion of 8 to 28; we have also a coin pound of sulphur and iron in the proportion of 1G to 28; and sulphur and oxygen unite together to form sulphurous acid gas, which contains equal weights of the two elements—the proportion of 1 to 1 having a simple arithmetical relation to the propor. tion 8 to 16.
Numbers representing the proportions in which the elements combine (such as 1 for hydrogen, 8 for oxygen, G for carbon, 16 for sulphur, 28 for iron, etc.), are called their "combining proportions," or atomic weights (q.v.). It is obvious that analysis alone cannot enable us to fix definitely such numbers. There is nothing in time com position of their compounds to lead its to adopt the proportional numbers given above for hydrogen, oxygen, carbon, sulphur, and iron, rather than simple multiples or sub multiples of them. In fact, the numbers adopted by Berzehns, and now reintroduced,' are in the proportion—hydrogen 1, oxygen 16, carbon 12, sulphur 32, iron 50. The reasons for preferring certain particular numbers to any multiples or sub multiple of them will be found in the article CHEMISTRY.