The original mixture used by Gunning had the following composition; _viz._, one part of potassium sulfate and two parts of strong sulfuric acid. The substances are united by heat and, on cooling, are in a semi-solid state, melting, however, easily on the application of heat and assuming a condition to be easily poured from vessel to vessel. The quantity of the sample taken should vary in proportion to its nitrogenous content from half a gram to a gram. The combustion takes place in flasks entirely similar to those used in the ordinary kjeldahl process. In the case of liquids, they should be previously evaporated to dryness before the addition of the oxidizing mixture. At the beginning of the combustion there is a violent foaming attended with evolution of some acid and much water, and afterwards of stronger acid. This loss of acid should not be allowed to go far enough to produce too great concentration of the material in the flask. One of the best ways to avoid it is to place a funnel in the flask covered with a watch-glass which will permit of the condensation and return of the escaping acid. As soon as the foaming ceases, the flame should be so regulated as to permit of the volatilized acid being condensed upon the sides of the flask. In the end a colorless mass is obtained in which no metallic oxids are present, and this mass can at once be diluted with water, treated with alkali, and distilled. According to the nature of the substance from half an hour to an hour and a half are required for the complete combustion. _Modifications of the Gunning Method._—As in the case of the kjeldahl method, numerous minor modifications of the gunning method have been made, the most important of which relate to its application to substances containing nitrates. In general the same processes are employed in this case as with the kjeldahl method. One of the best modifications consists in the use of the mixture of salicylic and sulfuric acids followed by the addition of sodium thiosulfate or of potassium sulfate or sulfid. These modifications will be given in detail under the official methods. =195. Reactions of the Gunning Process.=—The various reactions which take place during the combustion according to the gunning method have been tabulated by Van Slyke.[166] The first reaction to take place is the union of sulfuric acid and potassium sulfate to form potassium acid sulfate in accordance with the following equation: (1) K₂SO₄ + H₂SO₄ = 2KHSO₄. When heated, the potassium acid sulfate decomposes, forming potassium disulfate and water, thus: (2) 2KHSO₄ = K₂S₂O₇ + H₂O. The potassium disulfate at a higher temperature decomposes, forming normal potassium sulfate and sulfur trioxid, thus: (3) K₂S₂O₇ = K₂SO₄ + SO₃. At a sufficiently high temperature the two preceding reactions may take place in one, thus: 2KHSO₄ = K₂SO₄ + H₂O + SO₃.