8 FRAGMENTS OF SCIENCE. 



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 aether, and diffused by it throughout space. 



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

 midst of sethereal 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 aether. The earth also is warm, and sends 

 its heat-pulses incessantly 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 con- 

 ception of space being filled, we must therefore add 

 the conception of its being in a state of incessant 

 tremor. 



The sources of this vibration are the ponderable 

 masses of the universe. Let us take a sample of these 

 and examine it in detail. When we look to our planet, 

 we find it to be an aggregate of solids, liquids, and 

 gases. Subjected 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 elementary parts. We learn, for example, 

 that the water of our rivers is formed by the union, in 

 definite proportions, of two gases, oxygen and hydrogen. 

 We know how to bring these constituents together, so 

 as to form water : we also know how to analyse the water, 

 and recover from it its two constituents. So, likewise, 

 as regards the solid portions of the earth. Our chalk 

 hills, for example, are formed by a combination of car- 

 bon, oxygen, and calcium. These are the so-called 

 elements the union of which, in definite proportions, has 



