TBANSACTIONS OF SECTION A. 245 



powers is generally less, often of trifling amount. The question is put to Nature, 

 and it is Nature usually that gives the bulk of the answer. The most striking 

 monument of splendid achievements by the experimental method of investigation 

 unaided by the deductive method is to be found in the science of chemistry. 



An equally typical instance of the power of the deductive method is the science 

 of mechanics. This science, which has sunk deeper into the secrets of Nature than 

 any other science, and which is the science towards whom all other physical 

 sciences are at present more or less gravitating, is essentially deductive. There is 

 little or no difficulty about its fundamental data. They are facts of Nature so 

 patent to all men, and so indelibly implanted in human conception, that some 

 persons have supposed that we have an intuitive perception of them. But, while 

 the materials from which the mind is to work are thus easily obtained, it has taxed 

 to the utmost the reasoning powers of understandings like Newton's to evolve the 

 few consequences of them which are already known, and the investigator has to 

 call to his assistance every aid to prolonged consecutive thought which mathe- 

 maticians can devise. 



In grappling with the problems of Nature we are seldom allowed the choice of 

 the method of investigation we shall employ. This is commonly settled for us and 

 not by us. Where we cannot advance without further information, we must make 

 further observations, i.e., we must employ the experimental method, the appeal ad 

 experientiam : where we cannot advance without understanding better what the 

 information we possess really amounts to, we must employ the deductive method. 



No reach of intellect applied to the materials in existence before 1860 could 

 have elicited the fact that iron exists upon the sun. This great discovery was 

 made by Professor Kirchhoff, a scientific man who was equally versed in both 

 methods of investigation. On the present occasion it was the experimental method 

 he employed. He applied to the scrutiny of the sun's spectrum four prisms of the 

 most homogeneous glass that could be procured, figured with the greatest accuracy 

 that the eminent artist Steinheil could attain. He expended far more pains 

 on their adjustment for each successive part of the spectrum than any of his 

 predecessors had done, and he was rewarded by a more perfect vision of the sun's 

 glorious spectrum than had met the human eye before. In a collateral inquiry, 

 suggested by an observation made by Foucault, he and Bunsen placed a metallic 

 vapour emitting bright rays in front of a still brighter incandescent body, so that 

 the light from the brighter background had to pass through this vapour, and they 

 found that the vapour now caused dark lines in the spectrum occupying the 

 positions which its own bright lines had before filled. Professor Kirchhoff there- 

 upon added an appliance to his spectroscope which enabled him to bring a metallic 

 spectrum and the solar spectrum together into the field of view, alongside of one 

 another. On accomplishing this he saw sixty of the brightest of the iron rays as 

 continuations of sixty of the strongest of the dark lines in the sun's spectrum ; 

 and, by an elaborate scrutiny, he satisfied himself that the observations had been 

 pushed to a sufficient degree of exactness to make sure that a deviation would have 

 been detected in any one of these sixty cases if it had amounted to as much as one- 

 fourth of the average interval between consecutive lines of the solar spectrum. 

 From this it was obvious that the sixty coincidences are not due to chance, but 

 indicate that there is really iron vapour in the path of the rays. It will be observed 

 that Kirchhoff's great merit and the real difficulty of his work lay in the scientific 

 foresight and the industry which were required to frame hypotheses that were 

 worth testing, to guide the investigation by these hypotheses, to contrive, construct, 

 and adjust adequate apparatus, and to make with it the elaborate observations and 

 the exact observations and maps which were necessary. But when by these means 

 the new facts had been brought to light, the inference from them that there is 

 iron in the atmosphere of the sun was an easy one. This example will better 

 convey than a definition what are the characteristic features of an experimental 

 inquiry. 



On the other hand, no series of observations or experiments, however skilfully 

 arranged, could have enabled anyone to understand the cause of that familiar but 

 truly surprising phenomenon that a top stands upon its peg while it is spinning. 



