THE SIZE OF ATOMS. 215 



condensation brings the molecules into contact. 

 Something comparable with this may be imagined 

 to be the condition of common air of ordinary 

 density, as in our atmosphere. The diagram, (Fig. 

 43), with the size of each molecule, such as, if 

 shown in it to scale, would be one millimetre (or 

 too small to be seen by you) to represent an actual 

 diameter of i/io,ooo,oooth of a centimetre, 

 represents a gas in which a condensation of I to 

 10 linear, or I to 1,000 volume would bring the 

 molecules close together. 



Now you are to imagine the particles moving 

 in all directions, each in a straight line until it 

 collides with another. The average length of 

 free path is 10 centimetres in our diagram, 

 representing i/ioo,oooth of a centimetre in 



one metre square, perforated with 100 holes in ten rows of ten 

 holes each. From each hole was suspended a cord five metres long. 

 To the lower end of each cord, in five contiguous rows, there was 

 secured a blue coloured glass ball of four centimetres diameter ; 

 and similarly to each cord of the other five rows, a red coloured ball 

 of the same size. A ball from one of the outer rows was pulled 

 aside, and, being set free, it plunged in amongst the others, causing 

 collisions throughout the whole plane in which the suspended balls 

 were situated. 



