﻿Electrical Changes Induced hy Ultraviolet Light, 399 



the following way : — The zinc plate consists of atoms of zinc; 

 each of these consists of u zinc stuff" combined with two 

 electrons. The hydrochloric acid, in its turn, consists of ions 

 of " hydrogen stuff," i. e. of atoms of hydrogen, each of which 

 has lost an electron; and of ions of chlorine, generally termed 

 chlorions : these may be viewed as ''chlorine stuff" (what 

 we generally term chlorine) each atom of which is in com- 

 bination with an electron. It may also be postulated that 

 when zinc goes into solution each atom loses two electrons ; 

 and also that if electrons be delivered to one end of a wire, 

 a displacement of electrons occurs throughout the wire, and 

 unless electrons escape at the other end. the wire will be 

 charged, or, more exactly expressed, overcharged with 

 electrons. Now a zinc plate and a copper wire are in a 

 sense semi-permeable membranes, that is, they are per- 

 meable to electrons, but impervious to "matter": — to 

 hvdrogen, chlorine, or zinc. Hence the solution of zinc 

 in HC1 involves each zinc atom on the exterior of the 

 plate parting with two electrons ; these enter the zinc plate, 

 displace others, which in their turn displace those in the 

 copper wire, and finally in the copper plate, until at last, 

 the external atoms of the copper plate are over-charged 

 with two electrons. Each atom of the hydrogen in the 

 ionic state present in the solution of hydrochloric acid, 

 however, is capable of combining with one electron ; and 

 when thus combined it becomes ordinary hydrogen, and is 

 evolved as gas. Thus the flow of electricity through a wire, 

 and the electric pressure thus caused, may be regarded as 

 analogous to the flow of water through a tube, if that water 

 enter a solution containing a compound which is prevented 

 from escape by a semipermeable membrane. It may be re- 

 marked that there remains a difficulty, namely, the union of 

 two atoms of hydrogen to form a molecule ; that fact should 

 not be disregarded, and the M explanation " given above does 

 not cover it. 



Atoms of ' ; matter ? ' and electrons may be associated in 

 another manner : and here, too. an analogy may be brought 

 forward, although a somewhat imperfect one. An object 

 may be " charged with electricity." What does this mean ? 

 On the corpuscular hypothesis we must consider it as covered 

 with a layer of electrons greater or fewer in number. The 

 quasi-analogy consists in the wetting of a solid object with a 

 film of liquid. Here the liquid distributes itself over the 

 surface and is attached to the solid by virtue of the fact, as 

 usually stated, that the attraction of the liquid for the solid 

 is greater than that of the liquid for itself. The phenomenon 



