General Theory of Radiation. 381 



produce an uneven sound. Suppose, for example, one vibrates at the 

 rate of 100, and the other 101 per second. At first, as the vibrations 

 run nearl} 7 in the same time, they will reinforce and create loudness. 

 But when the first is making its 50th condensed pulse, the second will 

 have made 50 and be on the return, or rarified half of its 50th, so that 

 the pulses will be antagonistic and neutralize each other. From this 

 point the antagonisms will diminish till the first has finished its 100, and 

 second its 101 vibrations, when they will again be together, and the 

 sound will be loud. This rising and falling of the sound is called a 

 beat, and it takes place whenever two discordant notes are sounded to- 

 gether. The number of beats in a second, which will occur when any 

 two notes are sounded, is always equal to the difference in the number 

 of their vibrations per second. Thus when C and D of the great oc- 

 tave are sounded together, there will be eight and one-fourth beats. 

 ( See table. ) In the combination of certain harmonic notes there do 

 not appear to be any beats. If any letter and its octave be sounded 

 together, each alternate vibration of the high one will be on the outgo- 

 ing pulse, while the low one is on an ingoing one, but the regularity and 

 evenness with which this occurs destroys the effects of beats. The phe- 

 nomena of interference and the reinforcement of vibrations by each 

 other, will be met with in discussing light and heat. In our scales from 

 the note to its octave, is a natural skip, but the filling in between these 

 two is artificial, and has been different at different times and with differ- 

 ent peoples. Some account of this is given by Blaserna in his Theory 

 of Sound. 



CHAPTER XL. 



HEAT AND LIGHT GENERAL, THEORY OF RADIATION. 



Within the last fifty years it has been demonstrated that there is no 

 such substance as caloric or heat, but that like sound, heat is our sub- 

 jective sensation of a vibratory motion of the molecules of bodies. 

 Heat in a body arises in the same way that sound does, by the transfer 

 to the body of energy from some other body. Whenever work or molar 

 motion is arrested as such it goes on as molecular motion, a part of 

 which always appears to us as heat. If the molecular motion were al- 

 ways the same in the same bodies, of course the effect on our sensations 

 would be the same. But, as we have seen, sounds vary greatly with 

 the form and structure of the bodies vibrating, and with the rapidity of 

 the vibrations in the same bod} 7 . 



So the manner in which we are affected by the forms of motion called 

 light and heat, depends on the fact that particular sorts of bodies are in 



