=169. Method of Estimating Surface Tension.=—The determination of surface tension is made by measuring the rise of the liquid in a capillary tube. A short piece of thermometer tubing is used, the diameter of the bore being determined by careful microscopic measurements with a micrometer eyepiece. The diameter of the tube should be about 0.5578 millimeter. The tube is very thoroughly cleaned after each observation, or set of observations, with a strong caustic potash solution, and, after washing, is allowed to stand for some time in a saturated solution of potassium bichromate in strong sulfuric acid. The height of the rise in the capillary tube is measured with a cathetometer. The following formula is used for the calculation of the results: T = (_h d_ ω)/(4 cos. _a_) Where T is the surface tension, _d_ is the diameter of the tube in centimeters; _h_ the height to which the liquid rises in the capillary tube in centimeters; ω is the specific gravity of the solution; and 4 cos. _a_ refers to the angle of the liquid with the sides of the glass tube. For a tube of the size given above, 5° 24′ is the value of this edge angle. In regard to saline solutions, Quincke[117] says, that the edge angle appears to increase a little with augmenting concentration of the saline solution, but otherwise to differ only inconsiderably from the edge angle of pure water. =170. Effect of the Solutions on Surface Tension.=—The mineral fertilizers, as a rule, increase the surface tension of water, while organic matters in solution decrease it. But it must not be forgotten in this connection that but little of the organic matter in the fertilizers employed for the experiment passes into solution. Moreover, with these substances, the accuracy of the work is impaired somewhat by the increased viscosity. In general, the results of the experiment are in harmony with the well-known effect of magnesium, sodium, and potassium chlorids, and sodium nitrate, to make the soil more moist in dry weather, and the opposite effect produced by the application of organic matter. =171. Method of Preparing Soil Extracts.=—The soil extracts used in determining the surface tension, as given in the above table, are prepared as follows: Ten grams of the soil are rubbed up with fifteen to twenty cubic centimeters of distilled water and allowed to stand for twenty-four hours with frequent stirring. Any fine particles not removable by a filter are neglected, although they may give a turbid appearance to the solution.