i4o THE POPULAR SCIENCE MONTHLY 



and profound meditation he was convinced of its truth, and gave his 

 reasons for his changed opinions in an essay which received the prize 

 and was read to the academy in 1819, but which for some reason was 

 not published till 1826. The difficulty in explaining transverse vibra- 

 tions was overcome by showing that polarized rays of light have lateral- 

 ity, and by showing how a stretched string vibrates. The mechanical 

 difference between light and sound was discovered. The tremor which 

 causes sound, it was shown, is in the elastic fluid which carries the 

 movement forward. Without motion there can be neither vision nor 

 hearing. As motion is required to explain the behavior of gases and 

 that fact had been set forth clearly in the experiments of Clausius, 

 Maxwell and Joule, the conclusion did not seem to be far fetched that 

 motion exists everywhere, and must be considered in whatever explana- 

 tion one attempts to make of physical phenomena. It was also discov- 

 ered that bodies moving rapidly round an axis, when immersed in water 

 or in any movable medium, cease their rotation and flow forward as other 

 bodies flow in water. In 1857 Helmholtz and Lord Kelvin brought 

 forward the vortex theory to explain cosmic conditions and to account 

 for the formation of the planetary worlds. The studies and experi- 

 ments of Faraday in electricity and magnetism seemed to favor this 

 new theory of motion. It was shown that the medium which carries 

 light and sound can not possess the ordinary properties of a solid, a 

 liquid or a gas. Hence a demand for a medium in which everything 

 may exist, through the aid of which all movements or vibrations may 

 be conveyed. Careful studies discovered the velocity which particles of 

 hydrogen, for example, obtain, and Faraday pointed out lines of force, 

 afterwards called " tubes of force " by Hertz, along which in a mag- 

 netic field, the electric movement passes. These movements were meas- 

 ured mathematically and with such care and accuracy that not only is 

 the course which electricity takes known but its velocity also. But even 

 if electricity be displacement, and its track be accurately discovered 

 and followed, no one pretends to know just what is its innermost nature 

 or denies that future study may entirely change present opinions as 

 to its nature and its laws. 



It has been proved, scientists declare, that all forces of matter may 

 be measured and expressed in terms of energy or motion. During the 

 last twenty years the conception has been formed that light is an elec- 

 tric or a magnetic phenomenon. Luminous waves are short rapidly 

 moving waves and difference in color is caused by difference in length 

 and frequency. But what is most important in this theory of motion 

 is that it is not necessary, as in the gravitation theory or the atomic 

 theory, to believe in the theory of action at a distance. Bodies 

 move because other bodies press upon them. We see and hear because 

 some sort of motion brings the ether into connection with eye and ear. 

 The fact of gravity is not denied. Nor is it denied that Newton's dis- 



