A HISTORY OF SCIENCE 



served to do, were the particles of water absolutely 

 continuous. ' ' The latter illustration is one the validity 

 of which appeals as forcibly to the physicists of to-day 

 as it did to Hero. The same is true of the argument 

 drawn from the compressibility of gases. Hero has 

 evidently made a careful study of this subject He 

 knows that an inverted tube full of air may be im- 

 mersed in water without becoming wet on the inside, 

 proving that air is a physical substance ; but he knows 

 also that this same air may be caused to expand to a 

 much greater bulk by the application of heat, or may, 

 on the other hand, be condensed by pressure, in which 

 case, as he is well aware, the air exerts force in the at- 

 tempt to regain its normal bulk. But, he argues, 

 surely we are not to believe that the particles of air 

 expand to fill all the space when the bulk of air as a 

 whole expands under the influence of heat ; nor can we 

 conceive that the particles of normal air are in actual 

 contact, else we should not be able to compress the air. 

 Hence his conclusion, which, as we have seen, he makes 

 general in its application to all matter, that there are 

 spaces, or, as he calls them, vacua, between the par- 

 ticles that go to make up all substances, whether 

 liquid, solid, or gaseous. 



Here, clearly enough, was the idea of the "atomic" 

 nature of matter accepted as a fundamental notion. 

 The argumentative attitude assumed by Hero shows 

 that the doctrine could not be expected to go un- 

 challenged. But, on the other hand, there is nothing 

 in his phrasing to suggest an intention to claim orig- 

 inality for any phase of the doctrine. We may infer 

 that in the three hundred years that had elapsed since 



246 



