How a Tree Lives 



67 



and you will find that the salt is still there although it 

 cannot be seen. If a little of this water is heated until 

 it dries up, salt is left behind, almost unaltered by 

 having been dissolved. The same thing can be done 

 with nitre, alum and with several other things. Now 

 try to do it with chalk or with fine sand. However long 

 you may leave them in the water they only settle on the 

 bottom of the glass, and Avhen you stir the water up 

 they are still there. In other words, substances like 

 these will not dissolve in water. 



Fig. 36. A substance soluble in water cannot be separated from the 

 water by filtering, but passes through the filter paper. An insoluble 

 substance remains behind on the paper. 



The water that is always to be found in large or 

 small quantities in the ground contains dissolved matter 

 of many kinds which the rain has washed out of the 

 earth. When the tiny root hairs of which we spoke 

 last term suck up water, they suck up with it these 

 dissolved substances, and only those that are so dis- 

 solved ; for no others will pass through the walls of the 

 root hairs and into the root. The solution inside the 

 root is clear and almost colourless and is called sap. 



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