ALEXANDRIAN OR HELLENISTIC PERIOD 



we are about to describe was Ctesibius, the master. 

 Yet we owe a debt of gratitude to Hero, the pupil, for 

 having given wider vogue to these discoveries, and in 

 particular for the discussion of the principles of hy- 

 drostatics and pneumatics contained in the introduc- 

 tion to his book. This discussion furnishes us almost 

 our only knowledge as to the progress of Greek philoso- 

 phers in the field of mechanics since the time of Archi- 

 medes. 



The main purpose of Hero in his preliminary thesis 

 has to do with the nature of matter, and recalls, there- 

 fore, the studies of Anaxagoras and Democritus. 

 Hero, however, approaches his subject from a purely 

 material or practical stand-point. He is an explicit 

 champion of what we nowadays call the molecular 

 theory of matter. " Every body," he tells us, " is com- 

 posed of minute particles, between which are empty 

 spaces less than these particles of the body. It is, 

 therefore, erroneous to say that there is no vacuum 

 except by the application of force, and that every space 

 is full either of air or water or some other substance. 

 But in proportion as any one of these particles recedes, 

 some other follows it and fills the vacant space ; there- 

 fore there is no continuous vacuum, except by the 

 application of some force [like suction] that is to 

 say, an absolute vacuum is never found, except as it is 

 produced artificially." Hero brings forward some thor- 

 oughly convincing proofs of the thesis he is maintaining. 

 " If there were no void places between the particles of 

 water," he says, " the rays of light could not penetrate 

 the water; moreover, another liquid, such as wine, 

 could not spread itself through the water, as it is ob- 



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