204 GENERAL CONDITIONS OF SCIENTIFIC RESEARCH. 



and many other able investigators, were great qualitative 

 discoverers. 



On the other hand quantitative determinations often 

 enable us to discover simple existences, and to answer 

 qualitative questions or those of simple fact. Quantitative 

 knowledge respecting the action of gravity enabled Newton 

 to detect that force in distant heavenly bodies. If a sub- 

 stance were found possessing all the properties of potas- 

 sium, except its combining proportion, the knowledge of 

 that quantitative fact alone would disclose to us the exist- 

 ence of a new metal ; the discovery of the metal caesium 

 was nearly made by Plattner in a similar way. 1 By means 

 of quantitative knowledge of the known chemical elements, 

 Mendeljeeff has recently predicted the probable existence 

 of new elementary substances, one of which (gallium) has 

 already been found. 2 Knowledge of the atomic and mole- 

 cular weights and specific gravities of substances often 

 suggests new qualitative ideas of similarity or difference 

 in those bodies, and enables us to determine the classes to 

 which they belong. In these and other ways, qualitative 

 and quantitative knowledge act and react upon each other, 

 and aid each other's development. 



CHAPTEK XXIII. 



NECESSITY AND VALUE OF CLASSIFYING SCIENTIFIC TRUTHS. 



CLASSIFICATION of ideas is very important, because it prac- 

 tically affirms and exhibits general truths, and renders 

 knowledge more attainable and manageable. A general 

 conclusion respecting a class of truths contains and con- 

 veys to us as much information on a particular point us all 



1 See Cfomical News, vol. ix. p. 214. 



2 Ibid. No. 839, Dec. 1875. 



