CHAPTER IX 



INVISIBLE OBJECTS 



94. Very Small Objects. We saw in Section 84 that gases 

 have a tendency to expand, but that they can be compressed 

 by the application of force. This observation has led scien- 

 tists to suppose that substances are composed of very minute 

 particles called molecules, separated by small spaces called 

 pores ; and that when a gas is condensed, the pores become 

 smaller, and that when a gas expands, the pores become larger. 



The fact that certain substances are soluble, like sugar in 

 water, shows that the molecules of sugar find a lodging place 

 in the spaces or pores between the molecules of water, in 

 much the same way that pebbles find lodgment in the chinks 

 of the coal in a coal scuttle. An indefinite quantity of sugar 

 cannot be dissolved in a given quantity of liquid, because 

 after a certain amount of sugar has been dissolved all the 

 pores become filled, and there is no available molecular space. 

 The remainder of the sugar settles at the bottom of the ves- 

 sel, and cannot be dissolved by any amount of stirring. 



If a piece of potassium permanganate about the size of a 

 grain of sand is put into a quart of water, the solid disappears 

 and the water becomes a deep rich red. The solid evidently 

 has dissolved and has broken up into minute particles which 

 are too small to be seen, but which have scattered themselves 

 and lodged in the pores of the water, thus giving the water 

 its rich color. 



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