[6] Those desiring more explicit information on disinfectants and the principles of disinfection are referred to U. S. Department of Agriculture Farmers' Bulletin 926, "Some Common Disinfectants," and 954, "The Disinfection of Stables." and to publications of the U. S. Public Health Service. Of active disinfecting agents, heat from fire, live steam, and boiling water are the surest. The heat generated by the slaking of quicklime has proved effective with small quantities of excreta. Results of tests by the Massachusetts State Board of Health[7] show that the preferable method consists in adding sufficient hot water (120° to 140° F.) to cover the excrement in the receptacle, then adding small pieces of fresh strong quicklime in amount equal to about one-third of the bulk of water and excrement combined, covering the receptacle, and allowing it to stand 1½ hours or longer. [7] Annual Report, Mass. State Board of Health, 1914, pp. 727-729. [Illustration: Fig. 14.--Chemical closet. _A_, Water-tight sheet-metal container; _B_, metal or wooden cabinet; _C_, wooden or composition seat ring; _D_, hinged cover; _E_, 3 or 4 inch ventilating flue extending 18 inches above roof or to a chimney; _F_, air inlets] [Illustration: Fig. 15.--Chemical tank closet. _A_, Tank, 2 feet 3 inches by 4 feet 2 inches 5/64th-inch iron, seams welded; capacity, 125 gallons; _B_, 14-inch covered opening for recharging and emptying tank; _C_, 12-inch galvanized sheet-metal tube; _D_, 4-inch sheet-metal ventilating pipe extending above ridgepole or to a chimney; _E_, agitator or paddle] Among chemical disinfectants a strong solution of sodium hydroxide (caustic soda) or potassium hydroxide (caustic potash, lye) is very effective and is useful in dissolving grease and other organic substances. Both chemicals are costly, but caustic soda is less expensive than caustic potash and constitutes most of the ordinary commercial lyes. Chlorinated lime (chloride of lime, bleaching powder) either in solution or in powdered form is valuable. For the disinfection of stools of typhoid-fever patients the Virginia State Board of Health[8] recommends thoroughly dissolving ½ pound of best chlorinated lime in 1 gallon of water and allowing the solution to cover the feces for at least 1 hour. The solution should be kept in well-stoppered bottles and used promptly, certainly within 2 or 3 days. Copper sulphate (blue vitriol, bluestone) in a 5 per cent solution (1 pound in 2½ gallons of water) is a good but rather costly disinfectant. None of the formulas here given is to be construed as fixed and precise. Conditions may vary the proportions, as they always will vary the results. The reader should remember that few, if any, chemical disinfectants can be expected fully to disinfect or sterilize large masses of excrement unless the agent is used repeatedly and in liberal quantities or mechanical means are employed to secure thorough incorporation.