1919] Pendleton: A Study of Soil Types 185 



of the iron mortar was necessary to supplement the rubber pestle." The samp 

 were thoroughly mixed after screening, when they were weighed and placed in 

 sterile containers — glass jars and large bottles. Precautions were taken as Ear 

 as possible to avoid contamination of the samples during this preparatory proci 

 The screens, mortars, scoop, and pans were flamed out between samples. Obvi- 

 ously contamination could not be avoided absolutely without too greal a prolon 

 tion of the work. 



The material not passing the 2 mm. screen was subsequently washed on the 

 screen, with a stream of water to remove the liner material. The residue not 

 passing the screen by this treatment was dried and weighed. It seemed mine. 

 sary to adopt elaborate precautions, like those described by Mohr, 41 to obtain tin- 

 exact quantities. 



Mechanical Analysis 



The Hilgard elutriator was used for the purpose of making the mechanical 



analysis of the samples (surface horizon only). For the purpose of this work 

 the method described by Hilgard" lias been modified in several respects. The pre- 

 liminary preparation by sifting through the 2 mm. sieve in the dry state, and 

 through the 0.5 mm. sieve by the aid of water was used. One hundred grams was 

 sifted with the 0.5 mm. sieve, and the fine material plus the water was evaporated 

 to dryness on the water bath. The dry material was broken up and from this 

 the samples were weighed out for the analysis. 



The samples were not disintegrated by boiling, since it was believed that such 

 treatment would affect the "colloid" content of the sample. Instead, the samples 

 were shaken with water in sterilizer bottles for three hours, similar to the treat- 

 ment preparatory to the mechanical analysis by the Bureau of Soils method. 

 However, not boiling the samples caused more work later. 



The colloidal clay was removed by placing the previously shaken sample in a 

 large precipitating jar and stirring up with several liters of distilled water. ( Dis- 

 tilled water was used throughout the analysis.) The quantity of water was nut 

 important, but rather the depth of the suspension, which was 200 mm. After 

 allowing to stand for 24 hours the supernatant turbid water was siphoned off, 

 when the residue in the bottom of the jar was again stirred up with water and 

 the clay again allowed to settle out of a 200 mm. column. This was repeated 

 until the supernatant liquid contained practically no material in suspension after 

 standing for 24 hours. The clay suspensions were placed in large enamelware 

 preserving kettles, and the solutions reduced in volume by boiling. The final 

 evaporations were carried on over the water hath, so as to avoid too high a tem- 

 perature. 



A large portion of the finest sediment (0.25 mm. hydraulic value) was removed 

 as follows: After the greatest portion of the clay had been removed by the 21 

 hour sedimentation and deeantation, the sample was placed in a 1 liter beaker and 

 stirred up with sufficient water to make a 100 mm. column. After standing 

 to S minutes the suspended material was decanted off. This was repeated until 

 the supernatant solution was practically clear. The entire time for these decanta- 

 tions usually occupied 2.5 or 3 hours. The decanted material was allowed to stand 

 for 2-4 hours, as before, and the 200 nun. column decanted as with the original 

 (day suspension. This was continued until the clay was practically all removed. 



"Hilgard, Calif. Agri. Exp. Sta., Circ. <',. June Tie:; 



47 Bull. Dept. Agr. Indes Neerhmd.. no. 11. 1910. 



48 Calif. Agr. Exp. Sin.. Circ. 6 (1903), pp. 6-15; see also Wiley. Agricultural Analysis, 

 vol. 1 (1906), pp. 246-62. 



