4i8 THE POPULAR SCIENCE MONTHLY 



philosophical sense, but in the common every-day intelligible sense. As 

 an illustration of theories which are in this category we may take the 

 germ theory of disease, meaning by that the theory that some diseases at 

 least are due to definite micro-organisms which invade the system. 

 In 1850, before Lister's and Pasteur's discoveries this was a pure 

 hypothesis. To-day it is an hypothesis which has been definitely proved 

 to be correct, and which henceforth we may indeed extend and modify 

 but which we need never expect to see abandoned. 



Again, the hypothesis that the earth is a round ball, rotating daily 

 on its axis, and swinging annually around the sun was a few hundred 

 years ago a mere assertion which almost nobody believed ; to-day it is an 

 established doctrine which we may count upon to endure, though of 

 course the earth may not always 'keep on doing what it is doing to-day. 



In the second category are a large group of theories which are prob- 

 ably correct, but which may at any time be proved to be false ; while in 

 the third category are theories which are very uncertain, some of them 

 being little more than " pipe dreams," the best perhaps which we can do 

 in the present state of our ignorance, but the ignorance upon which they 

 are based is after all abysmal. The nebular hypothesis was a fine illus- 

 tration of one of these dreams. It was not a part of our knowledge, 

 because, as Kelvin so well says, there is no knowledge until we have been 

 able to apply exact quantitative tests to our hypotheses. In other words, 

 there is no science without exact measurement. There may be many 

 good guesses without it, many plausible explanations, but no real knowl- 

 edge. Such exact quantitative tests the nebular hypothesis has never 

 been able to call to its support. 



Now the progress of science consists simply in the slow but continu- 

 ous sweep of these two broad lines of division in the direction of the last 

 category, that is, it consists in nothing else save the continual transfer of 

 theories from category 3 over to 2 and from 2 over into 1, and it is my 

 purpose herein to trace the most fascinating history of the gradual 

 transfer of two of these theories, from the outermost edge of 3 across 

 .the two boundaries over into 1, where they now rest so securely estab- 

 lished that it is not too much to say that there is as much likelihood 

 that man will some day cease to believe in the rotation of the earth as 

 that the kinetic theory of matter and the atomic theory of electricity 

 will ever cease to be the corner stones of all physical science. 



But first, just a word about these revolutionary discoveries which are 

 continually being announced. Mne tenths of them are just as revolu- 

 tionary as was the discovery of the seven-year-old boy who came home 

 from school one day altogether disgusted, saying that for a week his 

 teacher had been telling him that 3 and 4 made seven, and he had just 

 got it well learned when she told him that 5 and 2 make seven. So it is 

 with our discoveries in science. We do indeed discover new relations, 



