xxvn.] GENERALISATION. 613 



which is the necessary antecedent of conduction. Hence 

 Faraday inferred that whether we can measure the effect or 

 not, all substances discharge electricity more or less. 1 One 

 consequence of this doctrine must be, that every discharge 

 of electricity produces an induced current. In the case of 

 the common galvanic current we can readily detect the in- 

 duced current in any parallel wire or other neighbouring 

 conductor, and can separate the opposite currents which 

 arise at the moments when the original current begins and 

 ends. But a discharge of high tension electricity like 

 lightning, though it certainly occupies time and has a 

 beginning and an end, 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 observation and yet theoretically known to 

 exist. 2 



Perhaps the most extraordinary case of the detection of 

 unsuspected continuity is found in the discovery of Cag- 

 niard de la 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 sub- 

 ject 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 condition with but a small 

 increase of volume. Professor Andrews, recently following 

 out this course of inquiry, has shown that liquid carbonic 

 acid may, at a particular temperature (3O'92 C.), anc 

 under the pressure of 74 atmospheres, be at the same tim( 

 in a state indistinguishable from that of liquid and gas 

 At higher pressures carbonic acid may be made to past 

 from a palpably liquid state to a truly gaseous state without 



1 Experimental. Researches in J\ 

 z Life of Faraday, vol. ii. p. 7. 



