THE STRUCTURE OF THE UNIVERSE 397 



of ultimate units ultimate so far as our present knowledge 

 extends and there seems reason to suppose that these ultimate 

 units comprise more or less of a planetary system. It seems 

 even as if we should be able to compute the times of revolution 

 of these ultra- atomic satellites, though it is not yet clear as to 

 whether there exists a central and attracting mass. 



The atoms themselves may constitute minute systems ; they 

 may in their turn be arranged into what we might term mole- 

 cular systems. A noteworthy attempt in this direction was 

 made some years ago by the distinguished Russian chemist, 

 Mendeleeff. He endeavoured to represent the arrangement of 

 the atoms within the molecule as following a Newtonian order, 

 and perhaps ruled by Newtonian law. Neither of these concep- 

 tions represents as yet anything more than ingenious conjecture. 

 But even if they should be confirmed, it is noteworthy that in 

 the next step higher we may be certain that any planetary scheme 

 of arrangement distinctly ceases. 



The motions of the molecules in a free or vaporous state are 

 known with a high degree of probability. For definite tempera- 

 tures the motions of the molecules of the air and of other gases 

 may be computed with considerable accuracy, at least as to 

 their average speeds. This constitutes our modern, or, as it is 

 termed " kinetic," theory of gases. It regards the pressure 

 which a gas exerts upon the walls of a bounding vessel as de- 

 termined by the number and force of the molecular particles 

 bombarding the walls of the vessel. But within the gas itself 

 no order or systematic arrangement obtains. The molecules 

 are conceived as flying about in every direction at a tremen- 

 dous rate of speed, coming into collision one with another, and 

 rebounding without loss of energy. 



So far as our present knowledge goes, the myriads of the 

 stars present, in their motions, rather the image of the mole- 

 cules of a gas than any arrangement of suns and planets and 

 satellites such as was pictured in the dreams of Lambert. To 

 the eye of the universal mind, the suns may be as the ultra- 

 microscopic particles of the air sporting hither and thither in 

 confused and incessant flight. The dance of the motes in a sun- 

 beam penetrating the shutters of a dark room may be, for aught 

 we know, the mirror of cosmos. Perhaps the picture will be 

 more vivid if we were to recur to the simile already employed, 

 that of a school of dancing fireflies, flashing out their light 



