202 ESSAYS BIOGRAPHICAL AND CHEMICAL 



a bell jar; pump out all air, so that the bell jar is filled 

 only with vapour of water, and leave the whole standing 

 for a long time. The weak solution will grow stronger, 

 for it will evaporate; and the strong solution will grow 

 weaker, for the vapour of water will condense in it. Now 

 imagine that the two salt solutions are placed, not under 

 the same bell jar, but under two separate bell jars, and 

 that these bell jars are connected by a pipe. In the 

 middle of this pipe is a little engine ; the pipe from the 

 weak solution enters the steam pipe of the cylinder, and 

 the pipe leading from the cylinder, which would in an 

 ordinary engine lead to the exhaust, is connected with the 

 bell jar containing the stronger salt solution; then, if the 

 engine is delicate enough, it will be driven by the current 

 of vapour passing from the weak salt solution to the strong 

 one. 



Why? Because although steam can pass away from 

 the surface of the water, salt cannot ; the surface of the 

 water is a diaphragm which will allow steam to pass, but 

 which is impenetrable for salt. 



The analogy with a battery is this: The zinc plate is 

 like the weak solution of salt ; when it dissolves, it gives 

 up electrons at its surface ; these electrons can pass along 

 the wire, which is the analogue of the steam-pipe; if 

 required, a small magneto-electric engine could be inter- 

 posed so that it would be driven by the current passing 

 through the wire, that is, by the stream of electrons, 

 just as the steam-engine is driven by the current of 

 steam. 



On arriving at the copper plate the electrons combine 

 with hydrogen ions and escape ; and in this respect the 

 battery described resembles rather a high pressure engine. 

 But if desired the electrons may be kept in the system ; it 

 is only necessary to surround the copper plate with some 

 substance such as sulphate of copper, and the electrons 



