304 A CENTURY OF SCIENCE 



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 

 employed, dualistic and unitary formulas of several 

 kinds were in use, but the worst feature of the situation 

 was the fact that more than one system of atomic weights 

 was in vogue, so that water might be written 



HO, HO, or H,O 



