PROGRESS TN PHYSICS. 821 



By an inteivstino- coincidence, in the same year in which Joule 

 announced the result of his experiments the Physical Society of 

 Berlin listened to a paper almost identical with Joule's in character 

 and conclusions, but prepared quite independently, by a young German 

 physician. Herman von Helmholtz, destined to rank at the time of his 

 death (in 1893) as one of the very first mathematicians of the age, 

 dou>)tless the first physiologist of his time, and as a physicist with 

 whom not more than one other of the nineteenth century can be com- 

 pared. Helmholtz's paper was rejected by the editor of the leading 

 scientific journal of (xermany, ])ut his work was so important that he 

 must always share with Joule and Kelvin in the glory of this epoch- 

 making generalization. 



Even a brief sketch of the historj' of the doctrine of the conserva- 

 tion of energy would be incomplete if mention were not made of the 

 work of Tyndall. Although by original research he contributed in no 

 small degree to the demonstration of the theory, it is mainly through 

 his wonderful skill in popular presentation of the principles of phys- 

 ical science that he becomes related to the great movement in the middle 

 of the centiny. His masterful exposition of the new theory in a 

 course of lectures at the Royal Institution, given in 1862 and published 

 in 1863 under the title " Heat as a mode of motion," was a means 

 of making the intelligent pul)lic acquainted with its beauty and pro- 

 found significance, and the histor}^ of science aflords no more admira- 

 ble example of the possibilities and wisdom of popular scientific 

 writing than this book. As for the principle of the conservation of 

 energy itself, it is not too nuich to say that during the last half of 

 the century it has been the guiding and controlling spirit of all scien- 

 tific discovery, or of invention through the application of scientific 

 principles. 



LIGHT. 



The revival and final establishment of the undulatory or wave theory 

 of light is one of the glories of the nineteenth century, and the credit 

 for it is due to Thomas Young, an Englishman, and Fresnel, a French- 

 man. Newton had conceived, espoused, and, owing to the great 

 authority of his name, almost fixed upon the learned world the cor- 

 puscular or emission theory, which assumes that all luminous bodies 

 emit streams of minute corpuscles which are reflected, refracted, and 

 produce \'ision. Many ordinary optical phenomena were explained by 

 this hypothesis only with great difliculty, and some were quite unex- 

 plainable. The transmission of a disturbance or vibratory motion by 

 means of waves, as in the case of sound, was a well-recognized princi- 

 ple, and Young and Fresnel applied it most successfully to the phe- 

 nomena of light. Wave motion in a general way is only possible in a 

 sensibly continuous medium, such as water, air, etc., and the theory 



