Page:The American Cyclopædia (1879) Volume IV.djvu/374

 366 CHEMISTRY than the views of either of the chemists just mentioned. Having observed that when a de- termined quantity of any substance united with different quantities of a second substance, the quantities of the latter always bore a simple relation of weight to each other, Dalton was led to the formation of his atomic theory. He re- garded the elements as composed of homogene- ous atoms, the weights of which are different for each different element. An atom of one ele- ment can unite with one or more atoms of an- other element, the weight of the atom of the compound formed being the sum of the weights of its component atoms. He determined the relative weights of these atoms for the different elements, as expressed by the relative weights in which they unite to form compounds. Al- though the term atom used by Dalton express- es nothing more in a chemical sense than Rich- ter's word equivalent, there was a tangibility in the former which caused the view to be more readily accepted. Moreover, Dalton's discovery of the law of multiple proportions, and that the atomic weight of compounds is the sum of the atomic weights of their ingredients, made the subject so complete that it could no longer be neglected. It was at once admitted into the science, and gave rise to the views concerning the quantitative composition of bodies which now exist. Besides the atomic theory, chemis- try owes much to Dalton for his investigations of the expansion of gases, evaporation, and the relations of mixed gases, elasticity of steam, &c. Wollaston (1766-1828) did much to diffuse and extend Dalton's theory. His " scale of chemi- cal equivalents " especially aided in this result. As an exact analyst also, he was deservedly cel- ebrated. Most important in connection with Dalton's doctrine was the discovery by Gay- Lussac (1778-1850) of the law of combining volumes, in accordance with which gases unite with each other. In conjunction with Hum- boldt he first observed that one volume of oxy- gen unites with two volumesof hydrogen to form water. Extending his researches, he found that other gases unite in equally simple proportions, and that the volume of the resulting compound, if gaseous, bears a simple relation to the sum of the volumes of its ingredients. This proved conclusively that chemical compounds are formed only in a few fixed and definite propor- tions, according as it did with Proust's research- es upon the composition of solid bodies. It was soon perceived that, the specific gravity of a gas being known, its atomic weight might be read- ily calculated, whence the determination of the density of gases became at once important. Of the many valuable investigations conducted by Gay-Lussac, those upon the expansion of gases by heat, density of vapors, for the deter- mination of which he devised an apparatus, expansion of liquids, evaporation, sulphur acids, chlorine compounds, and iodine may be mentioned. His researches in organic chemis- try were also very important. He first pursued the method of research which has since exerted so great an influence upon the development of chemistry. The investigation of the cyanogen compounds and isolation of cyanogen, a com- pound of nitrogen and carbon, yet closely re- sembling in its properties the element chlorine, gave rise to the idea of organic radicals, and formed the starting point to which the present method of regarding organic bodies directly re- fers. He first devised an apparatus for the ul- timate analysis of organic substances which pro- duced useful results, infinitely superior to any previously obtained, and furnished a basis for improvement. He also introduced the system of determining the specific gravity of the vapors of substances to control analyses. Many of his applications of chemistry to the arts were of great importance ; among them are the meth- ods of assaying silver by the wet way, of alka- limetry, of chlorimetry, of assaying gunpow- der, &c., still in use. Simultaneously with the investigations upon the atomic weights and atomic volumes, which occupied chemists after the discoveries of Dalton and Gay-Lussac, an- other subject of great importance in its bear- ings upon the doctrine of affinity, and which produced most striking results when applied to the study of certain individual substances, was brought forward. This was the investiga- tion of the connection between galvanism and the phenomena of affinity. "When Berzelius and Hisinger began to study this subject in 1803, it had already been noticed that water could be decomposed by the galvanic current. They verified this experiment, and moreover showed that salts could be decomposed by the same means; while an electrical opposition between acids and bases was indicated by the fact that under all circumstances of decomposi- tion acids were set free at the positive pole, bases at the negative pole, of the voltaic pile. The subject rested somewhat obscure, how- ever, until cleared up by Sir Humphry Davy (1778-1829), most of whose labors were de- voted to electro-chemistry. He first showed by a most admirable research that pure water when decomposed by galvanism produces only hydrogen and oxygen; that the acids (nitric and muriatic) and bases (ammonia and soda) obtained by previous experimenters had been produced either from air contained in the water or from the action of the galvanic cur- rent upon the vessels used in the operation. Studying this last action more carefully, he eventually succeeded in separating metals from the fixed alkalies, potash and soda, and proved these last to be metallic oxides. Small as were the quantities of metal thus obtained, he de- scribed their properties with surprising ac- curacy. He afterward proved that the alka- line earths are similarly constituted, and in- ferred from analogy that the earths are of the same nature ; the correctness of which opinion has since been fully proved. Having studied the chlorine compounds, he was led to regard chlorine as an element, and disproved Berthol- let's view of its composition, which had hith-