LUCRETIUS AND THE ATOMIC THEORY 203 



motion of an all-pervading fluid endowed with comparatively 

 simple qualities. This conception of matter probably differs 

 little from the tenets of those ancient philosophers who held that 

 the universe was built of some one element, such as air, fire, or 

 water. Descartes, who has at least the merit of reviving the 

 idea, in opposition to Gassendi and others who followed Lucre- 

 tius, could devise no rational hypothesis from this assumption ; 

 but Hobbes, contemporary with Descartes, held views which 

 bear a striking resemblance to those recently broached by Sir 

 William Thomson. Hobbes thought that a moist fluid ether 

 fills the universe, so that it left no empty space at all. He 

 understood by fluidity, that which is made such by nature 

 equally in every part of the fluid body not as dust is fluid, for 

 so a house which is falling in pieces may be called fluid but in 

 such manner as water seems fluid ; he defines c a hard body to 

 be that whereof no part can be sensibly moved unless the whole 

 be moved ; ' and in explanation how a fluid can compose a hard 

 body, he says, * Whatsoever, therefore, is soft or fluid can never 

 be made hard, but by such motion as makes many of the parts 

 together stop the motion of some one part by resisting the 

 same ;' an admirable explanation of a recent discovery due to 

 Helmholtz, described below, contrasting most favourably with 

 Leibnitz's subsequent mere verbal quibble on the same point. 

 More than this, Hobbes perceived that elasticity need not be a 

 primary quality of matter, but might be conferred by motion. 

 4 If the cause of this restitution (elasticity) be asked, I say it 

 may be in this manner, namely, that the particles of the bended 

 body, whilst it is held bent, do nevertheless retain their motion, 

 and by this motion they restore it as soon as the force is re- 

 moved by which it is bent.' These are most remarkable pro- 

 positions, and, should Thomson's ideas be established, will 

 entitle Hobbes to a very high position as the precursor of the 

 true theory. Unfortunately, Hobbes did not compose an har- 

 monious system out of the above ideas. He missed the con- 

 ception of vortices of ether as atoms, and introduced particles of 

 gross matter, distinct from ether, which may after all be true. 

 He also could not get free from the old nomenclature of ele- 

 ments, and even devised those same glassy bubbles full of ether, 

 which now serve chiefly to prove that Leibnitz took (without 



