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SCIENCE 



[N. S. Vol. XL. No. 1041 



ance with all known phenomena; they enable 

 us to predict events; they suggest to us new 

 experimental investigations to carry out. We 

 shall not undertake to say that these laws are 

 true in any absolute sense. Furthermore, it 

 will not cause us any uneasiness if we find a 

 new phenomenon which contradicts one or 

 more of them. That is a thing to be expected 

 if we are making progress. It will be no sur- 

 prise if a principle which was developed to 

 relate past experiences should turn out to be 

 insufficient to deal with future experiences. 



The experimentalist is thus continually 

 finding things which run counter to his pre- 

 conceived opinions, whether they are based on 

 unreasoned intuition or on large collections 

 of facts. It is important to us to analyze the 

 way in which men have heretofore met such 

 situations. They will continually arise in our 

 experience as long as we are making progress. 

 From the most superficial examination we 

 may see that they have often stood in the 

 way of advancement. 



When an opinion has gained a strong hold 

 on our imagination it may obstinately refuse 

 to be removed although it causes us grave 

 trouble to keep it in agreement with facts or 

 even leads us into contradictions from which 

 we can find no escape. The early history of 

 astronomy furnishes us with a good illustra- 

 tion of this matter. The Pythagoreans under- 

 took to make precise the central problem of 

 this science. Plato followed with other work 

 along the same line. By means of a consider- 

 able range of speculation and reasoning, 

 which would have little weight with us to- 

 day and therefore need not be repeated now, 

 these philosophers came to the conclusion that 

 uniform motion in a circle is the most per- 

 fect of all motions, and. therefore must be 

 that of celestial bodies. But it was obvious 

 that a simple motion of this kind for each of 

 these bodies was insufficient to explain their 

 positions at various times. Thus from the 

 outset it was apparent that it would be neces- 

 sary to consider the compounding of various 

 circular motions in order to account for ob- 

 served facts. Therefore those early thinkers 

 confidently proposed as the fundamental in- 



quiry of theoretical astronomy the following 

 questions: How can we explain astronomical 

 movements by means of uniform circular mo- 

 tions ? 



It was well to have this problem proposed, 

 although it turned out to be incapable of so- 

 lution. Directly or indirectly it has exerted 

 a profound influence on the progress of every 

 science. As long as the body of observation 

 was sufficiently meager men could labor with 

 some hope of answering the question as pro- 

 posed. At first it was sufficient to compound 

 two or three motions. After observations be- 

 came more exact it was necessary to put to- 

 gether four or five circular motions for one 

 body and to introduce numerous hypothetical 

 spheres in order to have something to move 

 along the requisite circular arcs. This thing 

 continued till the explanations bewildered one 

 with their complexity. Still men held to their 

 preconceived idea of circular motion for 

 many centuries until Kepler finally broke the 

 spell by the discovery of the three laws on 

 which modern theoretical astronomy is 

 based. It is instructive to all scientific work- 

 ers, I believe, to ponder the experience of 

 men in dealing with this old problem. 



As another example of the influence of pre- 

 conceived opinion consider the old belief of 

 chemists that the formation of organic com- 

 pounds was conditioned by a so-called vital 

 force. In accordance with this theory it 

 should be impossible to synthesize organic 

 compounds from dead matter. But in 1828 

 Wohler succeeded with the synthesis of urea. 

 But the belief in the necessity of a vital force 

 died hard. Men tried to get around the new 

 fact by supposing that urea stands midway 

 between organic and inorganic substances. 

 But the accumulation of other cases in which 

 organic compounds had been synthesized 

 finally led to the rejection of vital force as a 

 factor in purely chemical relations. 



A very curious case which was obviously in 

 disagreement with facts is afforded by the old 

 phlogiston theory of combustion. According 

 to this theory combustibility is due to a prin- 

 ciple called phlogiston, which is present in all 

 combustible bodies in an amount proportional 



