94 METHODS OF DETERMINING ALKALI 



alkali studies because the process is long and because the 

 quantity present ran be roughly estimated by elimination 

 when the other easier determinations have been made. 



Other Methods of Determining Soluble Salts. The 

 Electric Bridge. - - A modification of the Wheatstone 

 bridge has been found of considerable value in de- 

 termining the total salts in either soil or water in the 

 field without chemical analysis. The theory upon which 

 the instrument works is based upon the fact that the re- 

 sistance of the solution varies with the concentration of 

 its soluble salts. It has been found of great value for de- 

 termining the total salts in soils which do not contain ex- 

 cessive quantities of organic matter, and especially where 

 the salts are mostly sodium chloride and sodium sulphate. 

 It becomes unreliable where the organic matter is high 

 and it is necessary to determine the sodium carbonate 

 separately from the other salts because the resistance is 

 considerably different. 



In using the instrument, the soil in the cup is first 

 moistened until it becomes saturated or puddled and free 

 from air, and if the soil is very dry it should be allowed to 

 stand in this condition for about 20 minutes. The cup, 

 which is just levelful of the saturated soil, is placed be- 

 tween the clips through which the current passes, and 

 the pointer is moved back and forth until the neutral point 

 is reached where the buzzing in the receiver is at a mini- 

 mum. The instrument has coils with 10, 100, and 1000 

 ohms resistance, and it is necessary to adjust the coils 

 until the proper resistance is found. "The resistance of 

 the cup contents is found by multiplying the resistance 

 of the comparison coil used, shown on the rotary switch, 

 by the number on the scale opposite the pointer, when 

 a balance is established. Thus, if the comparison coil 



