XVI INTRODUCTION. 



periods were ripe for them, and we cannot doubt that if those who 

 made them had never lived, the labors of others would have speed- 

 ily attained the same results. The discoverer is, therefore, in a 

 great degree, but the mouthpiece of his time. Some discern clear! y 

 what is dimly shadowed forth to many; some work out the results 

 more completely than others, and some seize the coming thought 

 so long before it is developed in the general consciousness, that 

 their announcements are unappreciated and unheeded. This view 

 by no means robs the discoverer of his honors, but it enables us to 

 place upon them a juster estimate, and to pass a more enlightened 

 judgment upon the rival claims which are constantly arising in the 

 history of science. 



Probably the most important event in the general progress of 

 science was the transition from the speculative to the experimental 

 period. The ancients were prevented from creating science by a 

 false intellectual procedure. They believed they could solve all tho 

 problems of the universe by thought alone. The moderns have 

 found that for this purpose meditation is futile unless accompanied 

 by observation and experiment. Modern science, therefore, took its 

 rise in a change of method, and the adoption of the principle that 

 the discovery of physical truth consists not in its mere logical but 

 in its experimental establishment. It is now an axiom that not ho 

 who guetses, though he guess aright, is to be adjudged the true dis- 

 coverer, but he who demonstrate* the new truth, and thus compels 

 its acceptance into the body of valid knowledge. 



Now the later doctrines of the constancy and relations of forces, 

 and that heat is a kind of motion among the minuter parts of mat- 

 ter, have had their twofold phases of history, corresponding to tho 

 two methods of inquiry. They had an early and vague recognition 

 among many philosophers, and may be traced in the writings of 

 Galileo, Bacon, Newton, Locke, Leibnitz, Des Cartes, Bernoulli, 

 Laplace, and others ; but they were held by these thinkers as un- 

 verified and fruitless speculations, and the subject awaited the gen- 

 ius that could deal with it according to the more effective methods 

 of modern science. 



