WHAT IS ELECTRICITY? 195 



found which allows the water to pass, while blocking the 

 way for sugar. It is as if gravel of two sizes were being 

 shaken on a sieve; the stones which pass through the 

 meshes do not press on the sieve, while those which are 

 stopped by the sieve may be recognised by their 

 pressure. 



Substances other than sugar, too, can be stopped by the 

 same screen ; for example, tartaric acid can. And it has 

 been found that the pressure produced by equal numbers 

 of molecules or particles of sugar and of tartaric acid, 

 contained in equal volumes of water, is equal. 



Common salt is a compound of a metal named sodium 

 and a yellow-green gas called chlorine. Each molecule or 

 particle of salt must therefore contain these two elements ; 

 that is, each particle must be made up of at least two 

 smaller particles, and these smaller particles are called 

 ' atoms/ If a spoonful of salt be placed at the bottom of 

 a glass of water, like the sugar, its particles will wander 

 through the water, so that, after some time, the water will 

 become salt all through. 



Just as with sugar, it is possible to find a membrane 

 which will allow water to pass through it, while it stops 

 the passage of salt; and it is possible to measure the 

 pressure of molecules of salt on the membrane. 



Now here a very curious thing has been found ; mole- 

 cules of salt give twice as great a pressure as an equal 

 number of particles of sugar, spread through the same 

 volume of water ; it looks as if there were twice as many 

 particles of salt present. And it is supposed that there 

 really are twice as many. To account for this, it is 

 believed that each molecule of salt splits up into two 

 atoms, one of sodium and one of chlorine, and that each 

 atom plays the part of a molecule, in so far as it is able to 

 raise pressure. Owing to the habit which such minute 

 particles as the atoms of sodium and chlorine have of 



