616 HISTORY OF SCIENCE. 



combine; in that of Berzelius the proportionate combining weights 

 are called equivalents, and the relative weights of equal volumes of 

 the elementary gases are their atomic weights. We see, then, that 

 Berzelius sought to reconcile the law discovered by Gay-Lussac with 

 the atoms of Dalton and the equivalents of Wollaston. He did this by 

 supposing that hydrogen, nitrogen, and chlorine enter into combina- 

 tion only in pairs of atoms. 



As it would be beyond the scope of this work to trace in detail the 

 successive modifications of the atomic theory, which have been made 

 in order to embrace new classes of facts as they have come into view, 

 it may be proper to notice here, for the sake of a connected survey, 

 the developments of Dalton's theory, which are of the greatest impor- 

 tance to modern chemistry, and at the same time admit of popular 

 treatment. 



Davy and Berzelius assigned to the atoms certain electrical pro- 

 perties, and proposed to explain chemical actions by the force of 

 electrical attractions and repulsions. The decompositions which the 

 then newly-invented galvanic battery effected were in many respects very 

 striking, but in none more so than in the decided manner in which 

 the results appeared to confirm Lavoisier's ideas concerning the con- 

 stitution of salts. These were, as the reader may remember, that salts 

 are the result of the direct combination of acids and bases (page 371). 

 It was found that the galvanic current always decomposed a salt into an 

 acid and an alkali. Sulphate of soda being the subject of experiment, 

 sulphuric acid invariably appeared at the positive pole, while the base 

 cr alkaline soda showed itself at the negative pole. Similarly the 

 neutral substance saltpetre was resolved into nitric acid and potash. 

 In the symbolic notation of Berzelius, saltpetre, i.e., nitrate of potash, 

 was thus represented N 2 O 5 +KO. The first part of the symbol re- 

 presents a molecule of the substance then called nitric acid ; the 

 second part shows the oxide of potassium. The notion of salts as 

 thus formed by the simple conjunction of an acid and a base reigned 

 supreme for many years. In process of time other views were adopted. 

 Davy suggested that hydrogen plays an essential part in the constitution 

 of acids, as in an acid of iodine which he examined, in which iodine 

 and hydrogen were the only constituents. The researches and views 

 of a group of distinguished French chemists, especially DUMAS (born 

 1800), LAURENT (1807 18 . .), and GERHARDT(i8i6 18. .), all men 

 of admirable genius, recast the theory of chemical constitution into new 

 forms. The numbers representing the atomic weights were in certain 

 cases changed from those used by Berzelius. It may be necessary to 

 remind the reader that those numbers are inferred from the combining 

 proportions, and where there are several of these, other circumstances 

 determine the choice. The French chemists also insisted upon regard- 

 ing the molecules of compound bodies as formed of a system of atoms 

 not necessarily arranged in the two groups of acid and base. According 



