HOW TO CULTIVATE THE POWER OF COMPARISON. 331 



of scientific truths exist in physical and chemical series 

 and tables of constants ; for instance, the order of sub- 

 stances with regard to their atomic weights and numbers, 

 degrees of tenacity, compressibility, elasticity, specific 

 gravity, transparency, and refractive and dispersive power 

 for rays of light, heat, and chemical force ; conducting 

 power for sound, heat, and electricity ; degrees of specific 

 and latent heat, positive and negative thermo-electric 

 energy, positive and negative statical electric power, 

 and of para-magnetic and dia-magnetic capacity ; their 

 position in chemical, chemico-thermic and chemico-elec- 

 tric series, degrees of acid and basic power, &c. Exten- 

 sive and familiar acquaintance with the properties of 

 forces and substances, and great familiarity in the use of 

 such classified truths and practice in comparing them, 

 enable us most readily to detect the class to which new 

 actions or substances belong, or which they most resemble. 

 Tables of physical and chemical constants are contained in 

 nearly every book on physics and chemistry; and some 

 books are entirely devoted to them ; for instance, ' Con- 

 stants of Nature,' Parts I. and II., by F. W. Clarke, 

 Smithsonian Collection, Washington ; ' Livres des Poids 

 Specifiques,' par Walter Warnotte, Paris, 1867. 



All these remarks show that, in addition to a natural 

 genius for science, and extensive scientific knowledge, a 

 discoverer needs large and varied experience in experi- 

 ment and observation, and to be familiar with the real 

 analogies of different substances, forces, and actions, and 

 the relative degrees of their similarity or difference, in 

 order not to be misled by false analogies or be too much 

 influenced by feeble ones. 



Mr. Alfred Smee published in the year 1851 a book 

 entitled ' The Processes of Thought, Adapted to Words 

 and Languages,' in which he proposed a ' differential 



