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100 why he had abandoned organic chemistry, replied that "now, with the theory of substitution as a foundation, the edifice may be built up by workmen: masters are no longer needed."

Although he had his own views on most subjects, he was always open to correction. He said: "In the history of the evolution of any science, the researches of a given period of time furnish certain general relations which every moment are changed and improved upon by new discoveries. Thus an endeavour arises to arrange the new-discovered facts and to find the common bond by which they are linked together."

Liebig's definition of a compound radicle holds good to this day, although his definition of organic chemistry—as the study of the compound radicles—has been superseded by that of Schorlemmer, who defines organic chemistry as that branch of the science which treats of "the hydrocarbons and their derivatives."

Liebig predicted the organic ammonias, and discovered chloroform—the invaluable liquid which was first introduced into medicine by Sir James Simpson in 1848. He also discovered chloral and aldehyde. Liebig established the molecular formula of citric acid, and proved that it was tribasic; and in the year 1838 appeared his classical memoir on the constitution of organic acids, showing that the doctrine of basicity could be applied to them as well as to the acids of inorganic chemistry.