Ixvi 



stituents of thehay or grain, the carbon, hydrogen, and the alkaline earths, 

 were converted into vegetable forms suitable for the wants of Rumford's 

 horse. 



Although in the manifestation of heat many millions of millions of 

 these atomic vibrations of matter occur in a second of time, science en- 

 ables us with great probability, if not with absolute certainty, to estimate 

 the rapidity of them in many substances. The phenomena attending the 

 absorption of light by these substances proves that the periods of their 

 vibrations concur or synchronize to a certain extent with the periods of 

 vibration of the waves of light. 



We see that the waves of sound will rouse into vibration sonorous bodies 

 whose periods concur with those of the sound waves. Now, a moment's 

 reflection will suffice to show that this can only be done by a loss of force 

 on the part of the sound waves. Let us imagine, for example, the 

 waves of sound of a certain rate of vibration directed against an 

 intercepting screen composed of a great mass of harp-strings, all of the 

 same pitch. If these were in accord with the sound waves they would all 

 be at once roused into vibration. The result would be that the whole force 

 of the sound waves would be expended on, or taken up by, these strings. 

 In such an event none of the waves would be transmitted through the 

 screen. Such a screen might be said then to be opaque to such a note. 

 But to sound waves of a different rate of vibration such a screen would not 

 be opaque; its strings would not be roused into activity; the force of the 

 sound impulses would not be absorbed or taken up by them, and the sound 

 would then be transmitted beyond the screen. Heat and motion are con- 

 vertible terms. If these strings be thrown into motion, they become warmer 

 than when at rest. Hence when the screen is opaque, the force of the 

 sound waves will increase its heat by rousing the strings into motion. 

 If it do not intercept the sound waves it is because its strings remain at rest ; 

 they take up none of the force of the waves, and therefore they are not 

 heated. 



Suppose the various sound waves of an orchestra were directed against 

 such a screen as I have described; a person placed beyond the screen, and 

 protected from all waves of sound except such as passed through it, would 

 be unable to hear the particular note to which the harp-strings were at- 

 tuned, whilst all the others would reach his ear. 



If such a screen were composed of three or four sets of strings, each set 

 being differently attuned, then the person so placed would fail to hear the 

 three or four notes in accord with the different sets of strings when such 

 notes were sounded by the orchestra. 



Now let us apply this illustration to the phenomena of light. 



When the waves of light fall upon a body whose periods of molecular 

 vibration concur with them, the force of the waves is expended in imrreas- 

 ing the amplitude of the vibrations of the molecules of the body, just as 

 the waves of sound would throw into vibration such harp-strings as they 

 were in accord with, and hence an increase of heat is developed. Through 

 such a body it is more difficult for those waves to pass whose periods coin- 



