3. The third class of changes is even more radical in its nature, having for its object the adaptation of the moist combustion method to oxidized or mineral nitrogen. The chief feature of this class of changes consists in the introduction of a substance rich in hydrocarbons, and capable of easily forming nitro compounds, for the purpose of holding the oxids of nitrogen which are formed during the combustion and helping finally to reduce them to the form of ammonia. The chief varieties of this class of changes were proposed by Asboth, Jodlbaur, and Scovell, and will be fully set forth in separate paragraphs. =188. Method of Wilfarth.=—The basis of this modification as already noted rests on the fact that certain metallic oxids have the power of carrying oxygen, and thus assisting in a katalytic way in the combustion of organic matter.[156] The copper and mercury oxids are best adapted for this purpose and experience has shown that mercuric oxid, or even metallic mercury gives the best results. The manipulation is carried out as follows: From one to three grams of the sample, according to its richness in nitrogen, are heated with a mixture of twenty cubic centimeters of acid containing two-fifths fuming and three-fifths ordinary sulfuric acid. To this is added about seven-tenths gram of mercuric oxid prepared in the wet way from a mercury salt free of nitrogen. The combustion takes place in the usual kjeldahl flask. If the boiling be continued until the liquid is entirely colorless, final oxidation with potassium permanganate is unnecessary. To save time the combustion may be stopped when a light amber color is reached, and then the oxidation finished with permanganate. Before distilling, a sufficient quantity of potassium sulfid is added to precipitate all the mercury as sulfid and thus prevent the formation of mercurammonium compounds which would produce a deficit of ammonia. A convenient strength of the sulfid solution is obtained by dissolving forty grams of potassium sulfid in one liter of water. Bumping at the end of the distillation is not usual, especially if potash-lye be used, but should it occur it may be stopped by the addition of zinc dust. Only when a large excess of potassium sulfid is used is there an evolution of hydrogen sulfid, the presence of which, however, does not influence the accuracy of the results. The presence of mercuric sulfid in the solution tends to prevent bumping during the distillation, but it is advisable, nevertheless, to use a little zinc dust. Other minor modifications consist of forming the acid mixture with equal volumes of concentrated and fuming sulfuric acid containing in one liter 100 grams of phosphoric acid anhydrid,[157] and using metallic mercury instead of mercuric oxid; or a mixture of half a gram of copper sulfate and one gram of metallic mercury,[158] or half a gram of copper oxid and a few drops of platinic chlorid solution containing 0.04 gram of platinum in a cubic centimeter.[159]