Radical Theory. The researches of Guy Lussac on cyanogen compounds showed that in many compounds the group CN entered into reactions as if it were a single elementary atom, and other researches proved that many similar groups existed which remained unaltered during many transformations of the substancos in which they existed. In this way chemists endeavoured to explain the composition of organic compounds by supposing them built up of such atomic groups or radicals. The views upon the nature and characters of the radicals and their compounds formed the subject of much controversy during the latter part of the first half of the present contury, Berzelius and Liebig taking most prominent parts. Although many errors crept into their views, the influence of the older radical theory in the generalising and co-ordinating of chemical ideas was exceedingly great, and laid the foundation for other more advanced views afterwards promulgated.