THK CONSTITUTION OF NA TURE. 5 



We on the earth's surface live night and clay 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 re- 

 turn 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 incessant 

 tremor. 



The sources of this vibration are the ponderable masses 

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

 amine 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- 

 tary 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 analyze the water, and recover from it its 

 two constituents. So, likewise, as regards the solid por- 

 tions of the earth. Our chalk hills, for example, are 

 formed by a combination of carbon, oxygen, and calcium. 

 These are the so called elements the union of which, in def- 

 inite proportions, has resulted in the formation of chalk. 

 The flints within the chalk we know to be a compound of 

 oxygen and silicium, called silica; and our ordinary clay is, 

 for the most part, formed by the union of silicium, oxygen, 

 and the well-known light metal, aluminium. 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, exerts on 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 simply to the quantity 



