32 EXPERIMENTAL PLANT PHYSIOLOGY 



It has been found to consist of water in which are 

 dissolved those mineral salts that are capable of solution 

 in the water as well as those that can be dissolved by 

 the acid of the root. 



The method of absorption, the passage of the food 

 solutions up the stem, and the transpiration of the 

 surplus water have each been studied in turn. 



We are now ready to turn to the second source of 

 food for the plant, namely, the air. 



CHAPTER II 

 I. THE COMPOSITION OF THE AIR 



Before an attempt is made to solve any of the prob- 

 lems relating to absorption from the air it is necessary 

 to know something of its composition. The air is 

 gaseous and is without colour, taste, or smell. Whether 

 it consists of one or of several gases must be determined. 

 If there are several the special properties of each one 

 must be investigated. 



To find out whether two gases are alike or different 

 may not seem at first a very easy matter. Gases 

 cannot, as a rule, be seen, nor can they be handled, 

 and therefore they are not capable of comparison by 

 the methods applied in dealing with solids or with 

 liquid bodies. They have nevertheless very character- 

 istic properties, and each gas should be submitted to 

 the following tests : 



1. Its general appearance should be noted, its colour, 

 and its smell, if any. 



2. Its solubility in water must be determined. It is 

 not always easy to determine the degree to which a 

 gas is soluble. Various methods by which a rough 

 estimate of the solubihty can be obtained will be con- 

 sidered when dealing with individual gases. 



