6 f&AQtflSJft'S OF SOtKNCE. 



of the attracting matter, regardless of its quality. But in 

 the molecular world which we have now entered matters 

 are otherwise arranged. Here we have atoms between 

 which a strong attraction is exercised, and also atoms be- 

 tween which a weak attraction is exercised. One atom 

 can jostle another out of its place, in virtue of a superior 

 force of attraction. But, though the amount of force ex- 

 erted varies thus from atom to atom, it is still an attraction 

 of the same mechanical quality, if I may use the term, 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 

 32; 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 amount- 

 ing 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 chemical 

 affinity; we have then what is called a chemical combina- 

 tion. The effect of the union in this case also is the de- 

 velopment of heat, and from the amount of heat generated 

 we can infer the intensity of the atomic pull. Measured 

 by ordinary mechanical standards, this is enormous. Mix 

 eight pounds of oxygen witli one of hydrogen, and pass a 

 spark through the mixture; the gases instantly combine, 

 their atoms rushing over the little distances which sepa- 

 rate them. Take a weight of 47,000 pounds to an eleva- 

 tion 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 l^drogen atoms to form 

 water. 



