14 FRAGMENTS OF SCIENCE 



as that of gravity itself. Its intensity might be measured 

 in the same way, namely by the amount of motion which 

 it can generate in a certain time. Thus the attraction of 

 gravity at the earth's surface is expressed by the number 

 82 ; because, when acting freely on a body for a second of 

 time, gravity imparts to the body a velocity of thirty-two 

 feet a second. In like manner the mutual attraction of 

 oxygen and hydrogen might be measured by the velocity 

 imparted to the atoms in their rushing together. Of course 

 such a unit of time as a second is not here to be thought 

 of, the whole interval required by the atoms to cross the 

 minute spaces which separate them amounting only to an 

 inconceivably small fraction of a second. 



It has been stated that when a body falls to the earth 

 it is warmed by the shock. Here, to use the terminology 

 of Mayer, we have a mechanical combination of the earth 

 and the body. Let us suffer the falling body and the 

 earth to dwindle in imagination to the size of atoms, and 

 for the attraction of gravity let us substitute that of chem- 

 ical affinity; we have then what is called a chemical com- 

 bination. The effect of the union in this case also is the 

 development of heat, and from the amount of heat gen- 

 erated we can infer the intensity of the atomic pull. Meas- 

 ured by ordinary mechanical standards, this is enormous. 

 Mix eight pounds of oxygen with one of hydrogen, and 

 pass a spark through the mixture; the gases instantly com- 

 bine, their atoms rushing over the llutle distances which 

 separate them. Take a weight of 47,000 pounds to an 

 elevation of 1,000 feet above the earth's surface and let it 

 fall; the energy with which it will strike the earth will not 

 exceed that of the eight pounds of oxygen atoms, as they 

 dash against one pound of hydrogen atoms to form water. 



