14 FRAGMENTS OF SCIENCE. 



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

 of its 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 incessant tremor. 

 The sources of 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. 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 pro- 

 portions, of two gases, oxygen and hydrogen. We know 

 how to bring these constituents together, and to cause them 

 to form water : we also know how to analyze the water, 

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

 regards the solid proportions of the earth. Our chalk-hills, 

 for example, are formed by a combination of carbon, oxy- 

 gen, and calcium. These are elements the union of which, 

 in definite proportions, has resulted in the formation of 

 chalk. The flints within the chalk we know to be a com- 

 pound of oxygen and silicium, called silica ; and our or- 

 dinary clay is, for the most part, formed by the union of 

 silicium, oxygen, and the well-known light metal, alumin- 

 ium. By far the greater portion of the earth's crust is 

 compounded of the elementary substances mentioned in 

 these few lines. 



The principle of gravitation has been already described 

 as an attraction which every particle of matter, however 

 small, has for every other particle. With gravity there is 

 no selection ; no particular atoms choose, by preference, 

 other particular atoms as objects of attraction ; the attrac- 

 tion of gravitation is proportional to the quantity of the 

 attracting matter, regardless of its quality. But in the 

 molecular world which we have now entered matters are 



