21 



measuring from 60 /x to 250 /x from tip to tip. Contractile 

 vacuoles numerous and minute. Encystation into single- walled cysts ; 

 excystation of original organism. 



All three forms feed on bacteria and small amoebae and 

 flagellates. 



XX. "The Estimation of Phosphates in Soil Extracts." 



J. A. Prescott. Journal of Agricultural Science, 

 1914. 6, 111—120. 



The volumetric method originally proposed by Pemberton for 

 the determination of phosphates has been investigated, and the con- 

 ditions for obtaining accurate results with soil extracts have been 

 worked out. 



The following is the method finally adopted : — 

 REAGENTS. (1) Ammonium nitrate: 500 gms. to 1 litre of 

 water. 



(2) Ammonium molybdate : 150 gms. molybdate 

 dissolved in 1,000 c.c. of water and poured into 1,000 

 c.c. of nitric acid (S.G. 1.2). 



(3) 2 per cent, sodium nitrate. 

 Preparation of Soil Extracts. A volume of the extract 



containing 5 to 10 m.g. of P 2 6 is evaporated to dryness, ignited 

 for 15 minutes at dull red heat, and where much silica is present, 

 roasted for 2 hours at 140°C to render the silica insoluble. The 

 residue is extracted with 50 c.c. of 10 per cent, sulphuric acid by 

 digestion for half an hour on a sand bath ; the extract is diluted and 

 filtered. 



Precipitation of the Phospho-molybdate. To the 

 above extract, 25 c.c. of the ammonium nitrate solution is added and 

 the mixture brought to 55°C. 25 c.c. of the molybdate solution, 

 previously brought to the same temperature, is then added and the 

 mixture stirred, allowed to cool, and filtered after 2 hours. The 

 precipitate is washed free from acid with I per cent, sodium nitrate 

 solution, dissolved in standard alkali and titrated back using phenol 

 phthalein as indicator. The factor recommended is 1 c.c. of tu 

 alkali = "0003004 gms. P.0 5 . 



XXI. "The Reaction between Dilute Acid Solvents and Soil 



Phosphates." J. A. Prescott. Proc. Chem. 

 Society, 1914. 30, 123. 



A preliminary note showing that the reaction is more readily 

 interpreted as a colloidal absorption phenomenon than as a simple 

 chemical reaction (see p. 9). 



XXII. ' The Nature and Amount of the Fluctuations in 



Nitrate Contents of Arable Soils." E. J. RUSSELL. 

 Journal of Agricultural Science, 1914. 6, 18 — 57. 



The amount of nitrate in the soil of arable land fluctuates 

 regularly, but in these experiments it rarely exceeded the following 

 values . Per million Per cent Lbs per Acre _ lb ins 



Sand ... 6 '0006 28 



Loam ... 23 '0023 115 



(excepting on heavily dunged land, when it rose to 37 parts per million). 



Clay ... 14 '0014 60 



