ATOMIC THEORY, in chemistry, some times termed the doctrine of definite propor tions. This very important theory, founded on well ascertained facts, has bestowed on modern chemistry an almost mathematical degree of precision. The theory, which is to be distinguished from the experimental part of the subject, supposes that chemical com pounds result from the combination of the ultimate atoms of their constituent parts. It has been determined by experiment, and the fact serves as the basis of the theory, that a compound body, when pure, always contains the same proportion of its constituents : thus calcareous spar, and the pure part of marble, chalk, and limestone, consist of carbonate of limo, composed of uniform proportions of carbonic acid and lime ; the carbonic acid always contains uniform quantities of carbon and oxygen, and the lime uniform proportions of calcium and oxygen. The same law also exists with regard to all similarly-constituted oxides, sulphurets, and salts, and indeed to all chemical compounds whatever, whether presented to us by nature or formed by art. Wenzel, Bryan Higgins, Richter, Proust, and other chemists, made many discoveries which tended towards a law of definite propor tions in chemistry ; but Dalton reduced it to a system by his atomic theory, promulgated in 1803. According to his view, the atomic num ber, or atomic weight, or atomic equivalent, of any substance, represents the relative quan• tity or weight of each which will combine with other substances ; and the labours of later chemists have been directed to the exact determination of these numbers. Dalton thought that if hydrogen were reckoned 1, all other substances might be represented by in tegers, and this is still frequently done; hut these integers are not strictly accurate.
The following diagram will illustrate the mode in which the atomic numbers or com bining weights retain their character through out all the compositions or decompositions which may take place. An atom of nitric
acid (hydrogen being taken as 1, and oxygen as 8)weighs 54; and one of barytes 76; forming when combined 130 of neutral nitrate of ba rytes. An atom of neutral sulphate of potash 88, is composed of an atom of sulphuric acid =40, and an atom of potash = 48. Now when an atom of nitrate of barytes = 130, dissolved in water, is mixed with an atom o: sulphate of potash = 88, in solution, doubh decomposition ensues, and two new and per fectly neutral salts are formed, viz. 1 atom of nitrate of potash =102, consisting of an atom of nitric acid = 54, and an atom of potash 48. This remains in solution ; and there ie precipitated an atom of neutral sulphate of barytes = 116, composed of 1 atom of sul phuric acid = 40, and 1 atom of barytes = 76.
Tables of these combining numbers are now published for the use of chemists.
In 1809 Gay-Lussac published an important memoir on the Theory of Volumes,' in which the definite composition of compound gases was placed in a new and instructive light. Suspecting, from the previously ascertained fact that 100 volumes of oxygen gas combine with 200 volumes of hydrogen gas to form water, that other gaseous bodies would be found to unite in simple proportions, he made further experiments, and found that gaseous substances unite in the simple ratio of 1 to 1, 1 to 2, 1 to 3, See, Thus, for example,— The same rule was found also to apply to the combination of vapours with gases, thus,— Volumes. Volumes.
100 hydrogen gas with 100 vapour of sulphur. 100 oxygen gas with 100 100 hydrogen gas with 100 vapour of iodine.
As Chemistryis treated in this work only in respect to some of its practical applications, this brief notice of the atomic theory will' suffice.