=197. Gunning Method Adapted to Nitrates.=—The essential features of this modification are due to Winton and Voorhees.[168] The modifications of the kjeldahl method, for similar purposes, furnished the material details for the gunning modified process. Winton reports good results from digesting for two hours, from half a gram to a gram of the sample with thirty cubic centimeters of sulfuric containing two grams of salicylic acid, in a flask of half a liter capacity. Two grams of zinc dust are then slowly added, with constant shaking, and the flask heated, at first gently, until, after a few minutes boiling, dense fumes are no longer emitted. Three grams of potassium sulfate are next added and the boiling continued until the solution is colorless, or if iron be present, until a light straw color is produced. On cooling, when the mixture begins to solidify, water is added with caution, and afterwards sodium hydroxid, and the ammonia is obtained by distillation. In the process, as conducted by Voorhees, about one gram of the sample is digested with ten grams of potassium sulfate and thirty cubic centimeters of sulfuric containing one gram of salicylic acid, and three grains of zinc sulfid. The heat is kept down until frothing ceases, and then the mass kept in gentle ebullition until clear. The distillation is accomplished with the usual precautions. The voorhees process is superior to that recommended by Winton in adding the potassium sulfate at the beginning of the combustion. =198. Official Gunning Method Modified to Include the Nitrogen of Nitrates.=—In a digestion flask holding from 250 to 500 cubic centimeters, place from seven-tenths to three and five-tenths grams of the substance to be analyzed, according to the amount of nitrogen present. Add from thirty to thirty-five cubic centimeters of salicylic acid mixture, namely, thirty cubic centimeters of sulfuric to one gram of salicylic acid, shake until thoroughly mixed, and allow to stand from five to ten minutes, with frequent shaking; then add five grams of sodium thiosulfate and ten grams of potassium sulfate. Heat very gently until frothing ceases, then strongly until nearly colorless. Dilute, neutralize, and distil the same as in the gunning method. DETERMINATION OF NITROGEN IN DEFINITE FORMS OF COMBINATION.