38 Instructions to Field P<ii'tie><. 



6 feet will be determined according to the directions in the front 

 of the Alkali Field Book. In many cases, after the observer 

 becomes familiar with the soils of a locality, the field work may 

 be very much shortened by making the alkali determinations in 

 alternate foot sections or by mixing 2 or more feet for one salt 

 determination. 



The percentage of alkali, as indicated by the various colors 

 upon the alkali map, is to be a mathematical average of the alkali 

 in the foot sections. In case there should be a marked accuivm- 

 lation of the alkali at any one part of the vertical flection, such an 

 the surface of the ground or in an alkali hardpan, the judgment 

 of the observer is to decide whether the strict mathematical aver- 

 age should be followed or whether the soil is to be mapped as of 

 the next higher grade of alkali content. If these areas, where 

 there is a zone of accumulation of the alkali such as an accumula- 

 tion at the surface, are of considerable extent, they are to be 

 indicated upon the field map by special rulings in lead pencil, 

 and when the field maps ai-e published, the rulings will be de- 

 scribed in the legend of the map. Thus, if a soil contain on an 

 average 0.2 per cent of alkali, but has an accumulation of 0.5 

 per cent of alkali on the surface, this soil should be colored to 

 show 0.2 per cent alkali, and the fact that there is a surface accu- 

 mulation should be indicated by black rulings across the area 

 affected. If the accumulation is below the surface foot another 

 kind of ruling can be used to indicate that fact. Appropriate let- 

 tering upon the maps will render these rulings intelligible. 



Determination of salts in water. — Fill the electrolytic cell with 

 water and take the resistance. Take the temperature with an 

 ordinary thermometer, and reduce to 60° F. by use of the tables 

 o\\ pages 33, 34, and 35. The salt content corresponding to this 

 resistance may be found from the following table, compiled by 

 !Mr. Seidell from a large number of laboratory determinations. 

 The curve varies with the character of salts present. Where no 

 carbonates are present in the water, the figures in the colunm 

 marked "Chlorides" should be used. Where the percentage of 

 carbonates is high (more than 50 per cent from a preliminary 

 estimate of the total salt content), the figures in the column 

 marked "Car})onates" should be used. For intermediate per- 

 centages of carl)onates a corresponding intermediate value be- 

 tween those given in the two columns should be used. 



