12 FRAGMENTS OF SCIENCE 



of a hot body swing with inconceivable rapidity billions 

 of times in a second but this power of vibration neces- 

 sarily implies the operation of forces between the atoms 

 themselves. It reveals to us that while they are held to- 

 gether by one force, they are kept asunder by another, 

 their position at any moment depending on the equilib- 

 rium of attraction and repulsion. The atoms behave as 

 if connected by elastic springs, which oppose at the same 

 time their approach and their retreat, but which tolerate 

 the vibration called heat. The molecular vibration once 

 set up is instantly shared with the ether, and diffused by 

 it throughout space. 



We on the earth's surface live night and day in the 

 midst of ethereal commotion. The medium is never still. 

 The cloud canopy above us may be thick enough to shut 

 out the light of the stars ; but this canopy is itself a warm 

 body, which radiates its thermal motion through the ether. 

 The earth also is warm, and sends its heat-pulses inces- 

 santly forth. It is the waste of its molecular motion in 

 space that chills the earth upon a clear night; it is the 

 return of thermal motion from the clouds which prevents 

 the earth's temperature, on a cloudy night, from falling 

 so low. To the conception of space being filled we must 

 therefore add the conception of its being in a state of in- 

 cessant tremor. 



The sources of this vibration are the ponderable masses 

 of the universe. Let us take a sample of these and exam- 

 ine it in detail. When we look to our planet, we find it 

 to be an aggregate of solids, liquids, and gases. Sub- 

 jected to a sufficiently low temperature, the two last would 

 also assume the solid form. When we look at any one of 

 these we generally find it composed of still more elemen- 



