It will be seen that there was no absorption in the case of the nitrate, while with each of the other salts there was quite a marked absorption. It will also be noticed that the percentages of absorption are not very different, and especially is this true of the potassium and ammonium salts, the P₂O₅ being somewhat higher. Whether this fact is merely an accidental occurrence or is due to the law of combination by equivalents could hardly be predicted from the single soil experimented upon; but taking into consideration the possibility of difference in solubility of the resulting compounds in the saline solutions used, and of other varying conditions, the percentages are evidently not far enough apart to exclude the possibility of the bases uniting in equivalent proportions. =138. Preparation of Salts for Absorption.=—The salts employed in the foregoing determinations are conveniently prepared, in fractional normal strength. In grams per liter the following quantities in grams are recommended, _viz._, 5.35 g NH₄Cl; 10.11 g KNO₃; 16.40 g Ca(NO₃)₂; 24.60 g MgSO₄ + 7H₂O; 23.4 g CaH₄(PO₄)₂, etc. The ammonium chlorid, potassium nitrate and magnesium sulfate can be weighed as chemically pure salts and the standard solution be directly made up. Calcium nitrate is so hygroscopic that a stronger solution must be made up, the calcium determined and the proper volume taken and diluted to one liter. Monocalcium phosphate is prepared as follows: A solution of sodium phosphate is treated with glacial acetic acid and precipitated with a solution of calcium chlorid. It is then washed with water until all chlorin is removed. The fresh precipitate is saturated with pure, cold phosphoric acid of known strength. After filtering the solution is placed in a warm room and left for two or three weeks until crystallization takes place. The crystals are pressed between blotting papers and finally dried over sulfuric acid and washed with water-free ether, and again dried. Since this salt is decomposed in strong solutions it should be used only in one hundredth normal strength, viz., 2.34 grams per liter. POROSITY AND ITS RELATIONS TO MOISTURE.