16 FRAGMENTS OF SCIENCE. 



thousand pounds to an elevation of one thousand feet above 

 the earth's surface, and let it fall ; the energy with which 

 it would strike the earth would not exceed that of the eight 

 pounds of oxygen atoms as they dash against one pound 

 of hydrogen atoms to form water. 



It is sometimes stated that the force of gravity is dis- 

 tinguished from all other forces by the fact of its resisting 

 conversion into any other. Chemical affinity, it is said, 

 can be converted into heat and light, and these again into 

 magnetism and electricity. But gravity refuses to be so 

 converted ; it is a force which maintains itself under all 

 circumstances, and is not capable of disappearing to give 

 place to another. If by this is meant that a particle of 

 matter can never be deprived of its weight, the assertion 

 is correct ; but the law which affirms the convertibility of 

 natural forces was never meant, in the minds of those who 

 understood it, to affirm that such a conversion as that here 

 implied occurs in any case whatever. As regards converti- 

 bility into heat, gravity and chemical affinity stand on pre- 

 cisely the same footing. The attraction in the one case is 

 as indestructible as in the other. Nobody affirms that 

 when a stone rests upon the surface of the earth the mutual 

 attraction of the earth and stone is abolished ; nobody 

 means to affirm that the mutual attraction oi oxygen for 

 hydrogen ceases after the atoms have combined to form 

 water. What is meant in the case of chemical affinity is, 

 that the pull of that affinity, acting through a certain space, 

 imparts a motion of translation of the one atom toward the 

 other. This motion of translation is not heat, nor is the 

 force that produces it heat. But when the atoms strike and 

 recoil, the motion of translation is converted into a motion 

 of vibration, and this latter motion is heat. But the vibra- 

 tion, so far from causing the extinction of the original at- 

 traction, is in part carried on by that attraction. The atoms 

 recoil in virtue of the elastic force which opposes actual 



