266 THE PRINCIPLES OF SCIENCE. 



yet lasts so minute a fraction of a second, that it would 

 be hopeless to attempt to detect and separate the two 

 opposite induced currents, which are nearly simultaneous 

 and exactly neutralise each other. Thus an apparent 

 failure of analogy is explained away, and we are furnished 

 with another instance of a phenomenon incapable of obser- 

 vation and yet theoretically known to exist 8 . 



Perhaps the most extraordinary and fundamental case ' 

 of the detection of unsuspected continuity is found in the 

 discovery of Cagniard de 3a Tour and Professor Andrews, 

 that the liquid and gaseous conditions of matter are only 

 remote points in a continuous course of change. Nothing 

 is at first sight more apparently distinct than the physical 

 condition of water and aqueous vapour. At the boiling- 

 point there is an entire breach of continuity, and the gas 

 produced is subject to laws incomparably more simple 

 than the liquid from which it arose. But Cagniard de la 

 Tour showed that if we maintain a liquid under sufficient 

 pressure its boiling point may be indefinitely raised, and 

 yet the liquid will ultimately assume the gaseous con- 

 dition with but a small increase of volume. Professor 

 Andrews, recently following out a similar course of in- 

 quiry, has shown that liquid carbonic acid may, at a par- 

 ticular temperature (3O*92 C.), and under the pressure of 

 74 atmosphere, be at once in a state indistinguishable 

 from that of liquid and gas. At higher pressures carbonic 

 acid may be made to pass from a palpably liquid state to 

 a truly gaseous state without any abrupt change whatever. 

 The subject is one of some complexity, because as the 

 pressure is greater the abruptness of the change from 

 liquid to gas gradually decreases, and finally vanishes. 

 As similar phenomena or an approximation to them 

 have been observed in various other liquids, there is 

 little doubt that we may make a wide generalization, 



s * Life of Faraday,' vol ii. p. 7. 



