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CHEMISTEY 367 erto been universally received. This gave rise to the first modification of any importance which Lavoisier's system suffered. Oxygen could no longer be regarded as the sole acidify- ing principle. The idea of hydrogen acids was introduced, and substances which contain no oxygen admitted to be salts. Of the many other researches conducted by Davy, that upon flame and combustion, which led to the discovery of the safety lamp, was especially valuable. His electrical theory has been since modified, but, like most of his. work, has been of immense im- portance to the science. The value of his dis- covery of the metals of the alkalies was not limited to a theoretical bearing alone. From the enormous affinity for oxygen which they possess, a means was furnished far stronger than any previously known of decomposing other bodies. They did not obtain their full importance as reagents, however, until a pro- cess of manufacture was devised by Gay-Lussac and Thenard, by which they were obtained in comparatively large quantities. By them the chemists last mentioned were led at once to the discovery of boron, of hydrofluoric acid, of fluoride of boron, &c. Since then they have been the means of many discoveries. As an investigator and diffuser of chemical knowl- edge, as well as by his systematic classifica- tions, Thenard (1777-1857) was of great ser- vice to the science. His division of the metals into groups, according to their behavior at dif- ferent temperatures in presence of water, has been quite generally followed. Among his important discoveries may be mentioned that of the peroxide and of the persulphide of hy- drogen. No one chemist since Lavoisier has exerted an influence comparable with that of Berzelius (1779-1848). In him were united all the different impulses which have advanced the science since the beginning of the present epoch. The fruit of his labors is scattered throughout the entire domain of the science. Hardly a substance exists to the knowledge of which he has not in some way contributed. A direct descendant of the school of his country- man Bergman, he was especially renowned as an analyst. No chemist has determined by direct experiment the composition of a greater num- ber of substances. No one has exerted a greater influence in extending the field of analytical chemistry. The use of hydrofluoric acid in decomposing minerals, and of chlorine in the analysis of minerals containing metals capable of forming volatile chlorides, originated with him. Of his manifold and always admirable researches may be mentioned those upon the organic acids; upon selenium, which he dis- covered ; upon the alkaline sulphides, in which the theory of sulphur salts, analogous to those of oxygen, was brought forward; upon the fluorine compounds ; upon platinum and the metals occurring with it ; upon tellurium ; upon meteorites, and upon the silicates. He first isolated several substances, as silicon, zir- con, tantalum, &c. The remarkable amal- 178 VOL. iv. 24 gam which mercury forms with what is sup- posed to be ammonium was first obtained by him in conjunction with Hisinger. One of the principal services he rendered was in developing the present theory of the science. Before Dai- ton's views had become generally known, Berze- lius had perceived the importance of Richter's tables of the combining equivalents of acids and alkalies, and had carried out an extended investigation of the composition of salts to as- certain if they were true. The results obtained convinced him at once of the correctness of Dalton's generalization, which soon afterward came to his knowledge. He continued his re- searches, and worked out most fully the details of the subject. Among other things, he thus discovered the simple relation which the oxygen of the acid bears to the oxygen of the base in neutral salts. He also endeavored to ascertain with the greatest possible exactitude the rela- tions of weight in which the different elements unite to form compounds. His acuteness in selecting the materials best suited for his exper- iments, and the precision with which their quantitative analysis was conducted, have never been surpassed. He did not limit himself to prove these equivalent weights for only a few substances. By far the greater number of the elements were investigated by him, and a large proportion of the equivalent numbers still in use are his. Corrections have been made only by those experimenters who have been able to procure purer materials ; as an accurate analyst, Berzelius has never been surpassed. He first demonstrated that organic bodies combine ac- cording to the equivalent weights of their in- gredients, and introduced the method of analyz- ing their compounds with inorganic bodies of known equivalent weight as a means of arriving at their equivalent. He maintained most per- sistently the view that organic substances form combinations analogous to those of inorganic bodies, and that they ought to be explained in accordance with what is known of the latter, having steadfastly opposed the innovations on this doctrine brought forward by other chemists. He made, however, numerous accurate analyses of organic substances, and chemistry owes to him the theories of copulate acids and of or- ganic radicals. The latter, in accordance with which a substance may be composed of two or more elements, and yet be capable of entering into combination with elementary bodies as if it were itself elementary in a word, play the part of an element has since exerted a most important influence upon the development of the science. The admirable system of chemi- cal symbols now in use also orginated with Berzelius. Following in the footsteps of Davy, Faraday (1791-1867) was most fortunate in de- veloping the relations of electricity to chemis- try. He widely extended the idea suggested by Davy that electricity and chemical affinity are only different expressions of one and the same force. All his experiments tended to support this view. Although most of his re-