16 THE FOOD OF PLANTS CHAP. 



soluble portion of the soil, a portion of which passes 

 through the roots of the plant and forms part of 

 its food. 



We have now found out not only that the 

 roots suck up water for the use of the plant, 

 but that this water contains certain parts of 

 the soil dissolved in it which are used by the 

 plant as food. 



In the last experiment we used distilled water to 

 dissolve part of the soil. Why did we not use 

 ordinary tap water ? and what do we mean by dis- 

 tilled water ? 



Take some tap or well or river water, place it 

 in the porcelain dish and boil it away. There is, 

 you notice, a residue left. Evidently the water was 

 not pure. Some things were dissolved in it which 

 were left behind when it was boiled away. We 

 could not therefore use this water for our former 

 experiment, as it is not sufficiently pure, and must 

 therefore obtain some pure water. 



You have noticed that in boiling the water the 

 substances dissolved are left behind. If, then, we 

 could collect the steam which is passing away and 

 condense it that is, bring it back into liquid again 

 we should obtain pure water. 



We can do this quite easily by the following 

 arrangement : 



