6 FRAGMENTS OF SCIENCE. 
of the attracting matter, regardless of its quality. But in 
the molecular world which we have now entered matters 
<J f 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 
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 with 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 hydrogen* atoms to form 
water. 
