198 PHYSIOLOGICAL BOTANY. 



cannot be derived from external forces. These are de- 

 nominated vital forces. They are not in the least more 

 incomprehensible than the force of attraction ; in fact 

 they are less so. Newton found, with regard to the force 

 of attraction, that the intensity with which it acts upon a 

 body is inversely proportional to the square of the distance 

 of the body from the centre of attraction. But has this 

 law been shown to exist in the case of other forces ? Does 

 it apply to cohesion, elasticity, magnetism, &c. ? 



Liebig says, in his ' Chemical Letters ' (Heidelberg, 

 1844, p. 18), "Medical writers and kindred minds are 

 annoyed at the great simplicity of the truth, although 

 they have not succeeded in practically applying it, not- 

 withstanding all their pains ; they therefore give us the 

 most improbable views, and by the term vital force they 

 understand a wonderful thing, by means of which they 

 explain everything which they do not understand ; by a 

 thoroughly incomprehensible and indefinite something, 

 everything which is incomprehensible is explained ! " 



That bodies combine chemically in definite proportions 

 only is undoubtedly a law of combination, but I should 

 like to know according to what law chemical decom- 

 positions take place. Is the chemistry of decomposition 

 anything more than a specification of the results of ex- 

 periments made upon individual bodies, each individual 

 experiment being calculated according to a fixed form ? 

 Is the word affinity anything more than a word ? Nothing 

 is explained in chemistry; everything in chemistry is 

 incomprehensible. The vital force, on the other hand, 

 acknowledges general laws. There is the law of pe- 

 riodicity, which is the direct antithesis of the law of 

 inertia, according to which motion increases to a certain 

 extent, and then again diminishes ; there is the law of 

 habit, according to which reaction is not always equal to 

 action, but diminishes in proportion to the frequency of 

 the repetition of reaction. However, I am not writing 

 upon general physiology. 



Liebig's work, mentioned above (Organic Chemistry 



