ON FORCE. 283 



double your velocity, other things remaining equal, and 

 you quadruple your amount of heat. Here then we have 

 common mechanical motion destroyed and heat produced. 

 When a violin bow is drawn across a string, the sound 

 produced is due to motion imparted to the air, and to 

 produce that motion muscular force has been expended. 

 We may here correctly say, that the mechanical force of 

 the arm is converted into music. In a similar way we say 

 that the arrested motion of our descending weight, or of 

 the cannon ball, is converted into heat. The mode of 

 motion changes, but motion still continues; the motion of 

 the mass is converted into a motion of the atoms of the 

 mass; and these small motions, communicated to the 

 nerves, produce the sensation we call heat. 



We know the amount of heat which a given amount of 

 mechanical force can develop. Our lead ball, for ex- 

 ample, in falling to the earth generated a quantity of heat 

 sufficient to raise its own temperature three-fifths of a 

 Fahrenheit degree. It reached the earth with a velocity 

 of 32 feet a second, and forty times this velocity would be 

 small for a rifle bullet; multiplying three-fifths by the 

 square of 40, we find that the amount of heat developed by 

 collision with the target would, if wholly concentrated in 

 the lead, raise its temperature 960 degrees. This would be 

 more than sufficient to fuse the lead. In reality, however, 

 the heat developed is divided between the lead and the 

 body against which it strikes; nevertheless, it would be 

 worth while to pay attention to this point, and to ascertain 

 whether rifle bullets do not, under some circumstances, 

 show signs of fusion.* 



From the motion of sensible masses, by gravity and 

 other means, we now pass to the motion of atoms toward 

 each other by chemical affinity. A collodion balloon filled 

 with a mixture of chlorine and hydrogen being hung in 

 the focus of a parabolic mirror, in the focus of a second 

 mirror 20 feet distant a strong electric light was sud- 

 denly generated; the instant the concentrated light fell 

 upon the balloon, the gases within it exploded, hydro- 

 chloric acid being the result. Here the atoms virtually fell 

 together, the amount of heat produced showing the 



* Eight years subsequently "this surmise was proved correct. In 

 tin; Franco German War signs of fusion were observed in the case of 

 bullets impinging' on bones. 



