274 Wells £ Foote — One Hundred Years of Chemistry. 



of atoms, particularly to oxides, and regarded these 

 groups as positive or negative, according to the excess of 

 positive or negative electricity derived from their con- 

 stituent atoms and remaining free. He thus arrived at 

 his dualistic theory of chemical compounds, which 

 attained great prominence and prevailed for a long time 

 in chemical theory. According to this idea, each com- 

 pound was supposed to be made up of a positive and a 

 negative atom or group of atoms. For example, the for- 

 mulas for potassium nitrate, calcium carbonate, and 

 sulphuric acid corresponded to K 2 O.N 2 5 , CaO.C0 2 and 

 H 2 O.S0 3 where we now write KN0 3 , CaC0 3 and H 2 S0 4 , 

 and the theory was extended to embrace organic com- 

 pounds also. 



The eminent English chemist and physicist Faraday 

 announced the important law of electro-chemical equiva- 

 lents in 1834. This law shows that the quantities of 

 elements set free by the passage of a given quantity of 

 electricity through their solutions correspond to the 

 chemical equivalents of those elements. Faraday made a 

 table of the equivalents of a number of elements, regard- 

 ing them important in connection with atomic weights, 

 but at that time no sharp distinction was usually made 

 between equivalents and atomic weights, and it was not 

 fully realized that one atom of a given element may be 

 the electrical equivalent of several atoms of another. 



Faraday's law, which is still regarded as fundamen- 

 tally exact, has been of much practical use in the 

 measurement of electric currents and in calculations con- 

 nected with electro-chemical processes. In discussing 

 his experiments, Faraday made use of several new terms, 

 such as "electrolyte" for a substance which conducts 

 electricity when in solution, and is thus " electrolyzed, " 

 "electrode," "anode," and "cathode," terms that have 

 come into general use, and finally "ions" for the parti- 

 cles that were supposed to "wander" towards the elec- 

 trodes to be set free there. 



This term "ion" remained in comparative obscurity 

 for more than half a century, when it was brought into 

 great prominence among chemists by Arrhenius in con- 

 nection with the ionic theory. 



Cannizzaro's Ideas. — Up to about 1869 chaos reigned 

 among the formulas used by different chemists. Various 

 compound radicals and numerous type-formulas were 



