6S- SCIENCE IN THE LAST HALF CENTURY. 



three stages which, in their logical relation, are successive. The first is 

 the determination of the sensible character and order of the phenomena. 

 This is Natural History, in the original sense of the term, and here 

 nothing but observation and experiment avail us. The second is the 

 determination of the constant relations of the phenomena thus defined, 

 and their exi)ression in rules or laws. The third is the explication of 

 these particular laws by deduction from the most general laws of matter 

 and motion. The last two stages constitute Natural Philosophy in its 

 original sense. In this region, the invention of verifiable liypotheses 

 is not only permissible, but is one of the conditions of progress. 



Historically, no branch of science has followed this order of growth ; 

 but, from the dawn of exact knowledge to the present day, observation, 

 experiment, and speculation have gone hand in hand; and, whenever 

 science has halted or strayed from the right path, it has been, either 

 because its votaries have been content with mere unverified or uuveri- 

 fiable speculation (and this is the commonest case, because observation 

 and experiment are hard work, while speculation is amusing); or it has 

 been because the accumulation of details of observation has for a time 

 excluded speculation. 



The progress of physical science, since the revival of learning, is 

 largely due to the fact that men have gradually learned to lay aside 

 the consideration of unveritiable hypotheses ; to guide observation and 

 experiment by verifiable hypotheses; and to consider the latter, not as 

 ideal truths, the real entities of an intelligible world behind phenomena, 

 but as a symbolical language, by the aid of which nature can be in- 

 terpreted in terms apprehensible by our intellects. And if ])hysical 

 science, during the last fifty years, has attained dimensions beyond all 

 former precedent, and can exhibit achievements of greater importance 

 than any former such period can show, it is because able men, animated 

 by the true scientific spirit, carefully trained in the method of science, 

 and having at their disposal immensely improved appliances, have de- 

 voted themselves to the enlargement of the boundaries of natural knowl- 

 edge in greater number than during any previous half century of the 

 world's history. 



THEEE GREAT RECENT ACHIEVEMENTS. 



I have said that our epoch can produce achievements in physical 

 science of greater moment than any other has to show, advisedly ; and 

 I think that there are three great products of our time which justify the 

 assertion. One of these is that doctrine concerning the constitution of 

 matter, which, for want of a better name, I will call "molecular;" the 

 second is the doctrine of conservation of energy ; the third is the doc- 

 trine of evolution. Each of these was foreshadowed, more or less dis- 

 tinctly, in former periods of the history of science ; and, so far is either 

 from being the outcome of purely inductive reasoning, that it would be 

 hard to over-rate the influence of metaphysical, and even of theological, 



