8 INTRODUCTION. 



recognise them, simply homogeneous gravitating matter, 

 without specific, or, what is called, elementary diversity of 

 substance. This simplicity, however, is by no means to be 

 regarded as belonging to the actual nature and constitution 

 of those distant orbs themselves ; it is founded solely on the 

 simplicity of the conditions of which the assumption suffices 

 for the explanation and prediction of their movements in 

 celestial space, and (as I have already more than once had 

 occasion to remark : Kosmos, Bd. i. S. 56 60, and 141 j 

 Bd. iii. S. 4, 18, 2125, 594, and 626; Engl. Yol. i. 

 p. 5054 and 126 ; Yol. iii. p. 4, 18, 2125, 421, and 

 44 8 J on the exclusion of all direct and assured perception on 

 our parts of diversities of substance in the heavenly bodies. 

 We thus have presented to us for solution the great problem 

 of a system of celestial mechanics, subjecting all that is 

 variable in the uranological sphere solely to the doctrine of 

 the laws of motion. 



Periodical changes in the appearance of the light reflected 

 from the surface of Mars do indeed indicate difference of 

 seasons and meteorological phsenomena, i. e. precipitations 

 occasioned by the cooling of the atmosphere of the planet 

 at its opposite poles in the opposite portions of its year 

 (Kosmos, Bd. iii. S. 513 ; Engl. p. 371). Guided by 

 analogy and connection of ideas, we may indeed infer from 

 hence the existence of ice or snow (and therefore of oxygen 

 and hydrogen, the constituents of water) in the planet 

 Mars ; as we may also infer the existence of different kinds 

 of rock in the erupted masses and flat annular plains of the 

 Moon ; but we cannot assure ourselves of the actual state of 

 tilings by direct observation. Newton only permitted him- 

 self to entertain conjectures as to the elementary constitution, 



