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 probably 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 illustration 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 knowledge. 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 continuous 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 established 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. Nine tenths of them are just as revolutionary 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,