The alternate application of washes of castile soap and alum, each being dissolved in water, is known as the Sylvester process of waterproofing. Castile soap is dissolved in water, 3/4 of a pound of soap in a gallon of water, and applied boiling hot to the concrete surface with a flat brush, care being taken not to form a froth. The alum dissolved in water - 1 pound pure alum in 8 gallons of water - is applied 24 hours later, the soap having had time to become dry and hard. The second wash is applied in the same manner as the first, at a temperature of 60° to 70° F. The alternate coats of soap and alum are repeated every 24 hours. Usually four coats will make an impervious coating. The soap and alum combine and form an insoluble compound, filling the pores of the concrete and preventing the seepage of water. The walls should be clean and dry, and the temperature of the air not lower than 50° F., when the composition is applied. The composition should be applied while the concrete is still green. This method of waterproofing has been used extensively for years, and has generally given satisfactory results for moderate pressures.