328 PERSONAL PREPARATION FOR RESEARCH. 



distinguish them. In detecting resemblances and differ- 

 ences, we must disregard all other circumstances, and fix 

 our whole attention upon the two things we wish to com- 

 pare ; these, in research, usually consist of some unex- 

 plained phenomenon and a known one with which we wish 

 to compare it. The essential similarity or difference of 

 two phenomena, one of which is a newly-found one which 

 we wish to explain, is often not perceptible at first sight, 

 even to a scientific mind well educated in the subject, and 

 frequently requires a number of experiments to be made 

 before it is manifest. Competency to distinguish real 

 differences and similarities has, therefore, a very wide 

 meaning, and the scientific intellect of a man may be, to 

 a large extent, measured by it. 



Our power of scientific insight is but feeble when 

 compared with the profundity of nature, because deep 

 truths require deep thought to enable us to understand 

 and value them. Essential resemblances are also fre- 

 quently disguised ; and the more fundamental the nature 

 of a phenomenon is, the more deeply hidden usually is it 

 from our view by other phenomena which more affect our 

 senses. From these causes our faculties are much more 

 impressed by superficial and unreal likeness than by 

 deeply-hidden and essential similarity or difference ; and 

 the deepest truths are the least perceived, unless we 

 deeply ponder them. 



Chemical combination frequently disguises essential 

 resemblance ; the elements of a compound and the com- 

 pound itself often appear widely different. In such a case 

 the most essential properties are the least affected ; for 

 instance, the mass and weight of a compound is always 

 precisely the same as the combined masses and weights of 

 its ingredients ; heavy metals usually produce heavy com- 

 pounds ; the same element is usually thrown down from 



