384 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



oxides of nitrogen and his final solution of the problem of 

 the composition of nitric and nitrous acids. On the practi- 

 cal side, it may be noticed that by the year 1816 all the 

 commoner gases had been prepared and analysed, some fifty 

 different elements had been recognised clearly, water had 

 been decomposed and recomposed, the alkalis and most of 

 the earths had yielded up their metals, and the composition 

 of all the important mineral acids had at last been estab- 

 lished. On the theoretical side, it will be sufficient to 

 notice that, by the year 1816, the atomic and molecular 

 theories had been enunciated by Dalton and by Avogadro, 

 and that the Laws of Chemical Combination to which they 

 lead had been tested by Berzelius and proved to be correct. 

 The brief period of fifty years had thus sufficed, not merely 

 to lay the foundations of Inorganic Chemistry, but also to 

 reveal all the main outlines of the edifice. 



The rise and development of organic chemistry, 1815- 

 1865. It now remains to condense into a single chapter the 

 work of the succeeding fifty years, which witnessed a corres- 

 ponding development in the daughter science of Organic 

 Chemistry. For the sake of convenience the two fifty-year 

 periods may be made to overlap by a single year. ^3Phe second 

 of the periods will then extend from 1815, when Gay-Lussac 

 discovered the cyanogen radical and isolated cyanogen gas, 

 up to 1865, when Kekule first put forward his structural 

 formula for benzene. 



The period begins with some of the earliest accurate 

 analyses of organic compounds, leading at once to correct 

 formulae, based upon the theories of Dalton and of Avogadro, 

 for compounds such as alcohol and ether. Unfortunately, 

 a few obstinate exceptions threw discredit upon Avogadro's 

 hypothesis, with the result that, in the matter of formulae, 

 darkness and confusion reigned almost all through the fifty 

 years that we are now considering. The marvellous energy 

 of the many workers, coupled with the complex and un- 

 familiar formulae which they used, render this period 



