194 ESSAYS BIOGRAPHICAL AND CHEMICAL 



The colour will then be blue, for iodine and starch give a 

 blue colour. Now why does the current pass ? 



To explain this, let us consider what happens to a 

 lump of sugar lying at the bottom of a cup of water. 

 After a few minutes the sugar will melt, or, more cor- 

 rectly, dissolve in the water. But the water at the top 

 will not be sweet for a long time ; the sugar takes a good 

 many minutes before it spreads up into the water. Why ? 

 It is believed that sugar consists of minute invisible 

 particles called molecules ; and they are in motion. 



Although we cannot see molecules move, we may 

 nevertheless make an experiment which will prove to us 

 that particles of matter, easily visible under a fairly 

 powerful microscope, are always in rapid motion. 



An ordinary water-colour paint, rubbed with water, 

 gives particles of a convenient size ; gamboge is perhaps 

 the best colour to take. These particles are always 

 ' jigging' to and fro; their motion is not regular, but 

 spasmodic; and they spread, in virtue of that motion; 

 so that they move from one part of the water to 

 another. 



So it is with the sugar molecules ; that they do spread 

 is proved by the water becoming sweet, even at the 

 surface. In fact the sugar particles try to move from 

 where they are to where they are not. If one felt inclined 

 to moralise on the subject, one might ask, Is not that 

 what we all try to do ? Is not an attempt at motion what 

 makes for progress of all kinds in the world ? 



If such motion could be hindered, say by a screen which 

 would block the passage of the sugar molecules, while 

 allowing the water molecules to pass, the sugar molecules 

 would bombard the screen, giving it innumerable blows, 

 and these blows would make themselves evident as a kind 

 of pressure on the screen. 



This pressure has been measured ; a partition has been 



