3i8 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 

 The atomic theory afforded an obvious explanation of 



(1) The Law of Fixed Proportions. " The elements combine 

 together in fixed proportions by weight." 



(2 ) The Law of Multiple Proportions. " When two compounds 

 are formed, these proportions are in a simple ratio." 



(3) The Law pf Reciprocal Proportions. " The proportions 

 in which two elements combine with a third are in a simple 

 ratio to those in which they combine with a fourth element or 

 with one another." 



The Law of Reciprocal Proportions as applied to acids and 

 alkalis was tacitly assumed by Cavendish, when in 1766 he 

 referred to certain quantities of marble and of potash as 

 equivalent to one another in their power of neutralising acids. 

 It was formulated more definitely about 1792 by J, B. Bichter 

 (1762 1807), who showed that neutral salts remain neutral 

 after double decomposition because the acids and alkalis 

 unite reciprocally in equivalent quantities, e.g. the quantities 

 of lime and potash which neutralise a fixed weight of sulphuric 

 acid are identical with those required to neutralise a (different) 

 fixed weight of nitric acid. A complete table of equivalents of 

 acids and alkalis was drawn up by Fischer in 1802. 



Berzelius in 1807 recognised the practical value of the Law 

 of Reciprocal Proportions as consisting in the fact that, after 

 analysing carefully the compounds of all the bases with (e.g.] 

 sulphuric acid and of all the acids with (e.g.} baryta, one could 

 calculate the composition of all other neutral salts. He applied 

 the same idea to the elements and set to work to determine 

 their combining weights by the exact analysis of a few typical 

 compounds of each. This work was continued by J. S. Stas 

 (1813 1891), who determined with extraordinary care the equiv- 

 alents of carbon, nitrogen, sulphur, chlorine, bromine, 

 iodine, lithium, sodium, potassium, lead and silver. In recent 

 years similar work has been carried on by Prof. Guye at Geneva 

 and by Prof. T. W. Richards at Harvard University. 



Lists showing the chief analyses used in determining the 

 equivalents or Combining weights of the elements have been 

 given by Ostwald (Outlines of General Chemistry, tr. 1912, pp. 

 126 to 150), and by Freund (Study of Chemical Composition, 

 1904, pp. 220 to 224). 



