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E. H. TOOLE AND W. E. TOTTINGHAM 
separate salts of M/2 concentration; the watery suspension was in- 
corporated in making the solution up to volume. 
Two series of cultures were run for each material. In one series, 
there was incorporated in each 400 cc. portion of nutrient solution the 
equivalent of 2.5 grams of dry substance of the solid to be added. In 
the other, the equivalent of 0.5 grams of dry substance was used. 
The relative fineness of division of the solids is indicated by the fol- 
lowing volumes of each suspension required to supply 2.5 grams of dry 
substance: H2Si03, 93.5 cc; Fe(0H)3, 85.6 cc; carbon black, 18.3 cc. 
Thrifty seedlings about five days old, selected to be as uniform in 
size and appearance as possible, were supported in jars by the method 
used by Tottingham (5). About two thirds of the pea cotyledons 
were removed to hasten the effect of the nutrient solution. The plants 
were grown in a greenhouse at Madison, Wis., from July 20 to August 
10, 1914, the cultures being shaded from the intense sunlight of the 
period by a cheesecloth curtain. Daily aeration of the solutions was 
accomplished by means of an atomizer bulb, and the solutions were 
renewed every three days. At the end of the growth period of twenty- 
one days, the plants were removed and the residual nutrient solutions, 
after being restored to their original volumes, were tested for hydrogen- 
ion concentration. Later, fresh nutrient solutions were prepared to 
determine their initial acidity and to test for any possible adsorption 
after contact with the solids. 
In analyzing the culture solutions, the suspensions were allowed to 
settle, the clear liquids were filtered off, and aliquot parts were taken 
for the various determinations of hydrogen-ion concentration, phos- 
phorus, and calcium. The concentration of hydrogen ions was deter- 
mined by the indicator method, using as guides the papers of Sorensen 
(7), Henderson (8), Hawk (9), and Clark (10). The buffer solutions 
used as standards were made by dissolving the proper amounts of mono- 
and di-basic sodium phosphate for alkalinity, and acetic acid and 
sodium acetate for acidity (8). Neutral red was found satisfactory as 
an indicator for the more alkaline solutions studied and mfethyl red 
for the more acid solutions. Phosphorus was determined by pre- 
cipitation with molybdate reagent followed by magnesia mixture. 
The pyro-phosphate thus obtained was dissolved in hydrochloric acid, 
the solution was filtered, and the phosphoric acid was finally deter- 
mined by precipitation with an excess of magnesia mixture and by 
ignition in the usual manner (11). Calcium was determined by 
