32 ORGANIC EVOLUTION CONSIDERED 



heat, ana gravity? This question involves many diffi- 

 culties, some of which are probably unanswerable. 



Sound is known to be the vibration of air or other 

 material media, and the pitch depends on the fre- 

 quency of the vibrations. The lowest audible note is 

 produced by 20 to 30 vibrations per second, and it is 

 said that some ears can appreciate sounds made by 

 50,000 vibrations per second. The length of waves 

 produced by 16 vibrations per second is about 70 feet, 

 and the wave length due to 50,000 vibrations is about 

 3 /^ of an inch. The production of waves of sound 

 requires an elastic medium. The medium, air for 

 example, propagates sound by a series of longitudinal 

 vibrations of the molecules of air, resulting in a 

 series of condensations followed by rarefactions. 

 Sound is one form of mechanical energy which is 

 transmitted by means of waves through elastic mate- 

 rial media, — at the rate of about 1,100 feet per second 

 through air, at the ordinary temperature, but more 

 rapidly than this through liquids and solids. 



How can we explain the transmission of other 

 forms of energy? of heat and light and gravitation 

 through interstellar space? the motion of heat and 

 electricity through material media? the attraction of 

 magnets, and the drawing of atom to atom in chemical 

 action? 



It should be remembered that all matter is por- 

 ous — that between the molecules of all gases, liquids 

 and solids are spaces which are not occupied by any 

 known form of matter. If these intermolecular spaces 

 are an absolute vacuum, then we are left to account 

 for the transmission of energy through a vacuum. 



If we heat one end of an iron rod, the heat slowly 

 creeps along until the whole rod becomes hot. In 

 this case we may assume that the molecules of the 

 end first heated are thrown into more rapid vibration, 



