122 THE PATH OF SCIENCE 



with such precision that his determinations were not super- 

 seded for many years. 



The analytical work in w^hich Berzelius displayed such sur- 

 passing skill could, of course, determine only the combining 

 equivalents of the atoms. The assumptions made by various 

 chemists as to the number of atoms which combine to form 

 a compound resulted in different values for the atomic weight. 

 If, as it w^as easiest to believe, one atom of hydrogen com- 

 bined with one atom of oxygen to form water, the atomic 

 w^eight of oxygen was 8. The solution of the difficulty could 

 have been found in the hypothesis of Amadeo Avogadro, 

 professor of physics at Turin, who introduced the idea of 

 the molecule as the smallest part of a substance which can 

 exist free in a gas and postulated that equal volumes of gases 

 under the same conditions contain the same number of mole- 

 cules. Unfortunately, however, although this theory was 

 published by Avogadro in 1811, it was nearly fifty years be- 

 fore its importance was generally recognized and the prob- 

 lem of the atomic weights of the elements was solved in its 

 present form. 



As chemists became more and more interested in the study 

 of the innumerable compounds of carbon, they began to de- 

 vote their attention to the production of new substances, 

 that is, to synthesis. Throughout the second half of the 

 nineteenth century, the main advances in chemistry were in 

 the synthesis of new carbon compounds, in the field which is 

 now known as organic chemistry. 



In the rise of organic chemistry, the greatest influence was 

 exerted by Justus von Liebig, professor of chemistry at Gies- 

 sen, who not only contributed much to the science by his 

 own studies but also was the teacher of the great school of 

 organic chemists that flourished in Germany in the nine- 

 teenth century. In 1836, A. W. von Hofmann, for instance, 

 entered the University of Giessen with the intention of 

 studying law, but under von Liebig's influence he changed 

 his field of work to chemistry, in which he became one of 

 the great discoverers in the field of organic chemistry. In 



