ALEXANDRIAN OR HELLENISTIC PERIOD 



the time of Anaxagoras, that philosopher's idea of the 

 molecular nature of matter had gained fairly wide 

 currency. As to the expansive power of gas, which 

 Hero describes at some length without giving us a clew 

 to his authorities, we may assume that Ctesibius was 

 an original worker, yet the general facts involved were 

 doubtless much older than his day. Hero, for ex- 

 ample, tells us of the cupping-glass used by physicians, 

 which he says is made into a vacuum by burning up 

 the air in it ; but this apparatus had probably been long 

 in use, and Hero mentions it not in order to describe 

 the ordinary cupping-glass which is referred to, but a 

 modification of it. He refers to the old form as if it 

 were something familiar to all. 



Again, we know that Empedocles studied the press- 

 ure of the air in the fifth century B.C., and discovered 

 that it would support a column of water in a closed 

 tube, so this phase of the subject is not new. But there 

 is no hint anywhere before this work of Hero of a clear 

 understanding that the expansive properties of the air 

 when compressed, or when heated, may be made avail- 

 able as a motor power. Hero, however, has the clearest 

 notions on the subject and puts them to the practical 

 test of experiment. Thus he constructs numerous 

 mechanisms in which the expansive power of air un- 

 der pressure is made to do work, and others in which 

 the same end is accomplished through the expansive 

 power of heated air. For example, the doors of a tem- 

 ple are made to swing open automatically when a fire 

 is lighted on a distant altar, closing again when the fire 

 dies out effects which must have filled the minds of 

 the pious observers with bewilderment and wonder, 



247 



