114 PRACTICAL LESSONS IN SCIENCE. 



be saturated, and any amount added beyond this limit gradually 

 settles to the bottom of the solution. 



Oxygen and hydrogen, whether free or combined as water, are 

 the most important and interesting substances within the do- 

 main of chemistry. They are alike in most of their physical 

 qualities, but differ widely in their chemical properties. Oxygen 

 combines with many substances with which hydrogen does not 

 combine, while hydrogen combines with some substances which 

 do not readily combine with oxygen, but under favorable condi- 

 tions they combine with each other. In fact, in this case and in 

 many others, unlike elements combine more freely than those 

 which have many properties in common. 



