APPENDIX I 353 



Available Potash and Phosphoric Acid. — Dyer's directions are as 

 follows : 200 grms. dry soil are placed in a Winchester quart bottle 

 with 2 litres of distilled water, in which are dissolved 20 grms. of 

 pure citric acid. The soil is allowed to remain in contact with the 

 solution at ordinary temperatures for seven days, and is shaken a 

 number of times each day. The solution is then filtered, and 500 

 c.c. taken for each determination ; this is evaporated to dryness, and 

 gently incinerated at a low temperature. The residue is dissolved 

 in hydrochloric acid, evaporated to dryness, redissolved, and filtered ; 

 in the filtrate the potash is determined. For the phosphoric acid 

 determination the last solution is made, as before, with nitric acid. 



Mechanical Analysis. — The object is to obtain information about 

 the size of the ultimate particles of which the soil is composed ; the 

 compound particles are therefore broken down by treatment with 

 hydrochloric acid, and afterwards with ammonia. Direct measure- 

 ment of the ultimate particles is found to be impracticable ; indirect 

 methods have to be adopted, depending on the time taken to fall 

 through a column of water of given height. When a body falls 

 through a vacuum the time taken is independent of its size or weight, 

 but if air or any other fluid is present the case becomes more com- 

 plicated and the proper mathematical relationship has been found 



22'(Z ((T "" O) 



by Stokes to be z; = ~ — ^^ —, where v = velocity of the falling 



9V 

 particle, o- its density, a its radius (assuming it to be a sphere), and 

 p the density and rj the coefficient of viscosity of the medium {Trans. 

 Camb. Phil. Soc, 185 1, vol. ix., p. 8). 



The numerical values at 16" C. are : g = 981, a = 2*5, p = i, 

 7j = •01 r, and the equation therefore reduces to v = a"^ x 29730, 



or a =r J^iL— cm. 

 172 



The calculated and observed values are found to agree fairly 

 well, dififerences being due to the fact that the particles are not true 

 spheres, and to the existence of convection currents produced by 

 changes of temperature. The effect of variations in temperature is 

 discussed by Robinson mjourn. Agric. Sci., 19 14, 7, 142. 



The method adopted by the Agricultural Education Association 

 {seejourn. Agric. Sci., 1906, i., 470) is as follows: — 



this case the residue from the citric acid extraction has first to be heated two 

 hours at 120° to 160° to render the silica insoluble. The older method is de- 

 scribed by Dyer (gi). 



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