TRANSACTIONS OF SECTION B. 513 



We are aware in the first place that it is the highest oxide of nitrogen known, 

 that, parting so easily and freely with its oxygen as it does, it stands highest in the 

 list of oxidising agents, and consequently amongst the most useful chemical re-agents 

 in manufacturing enterprises. 



There are two drawbacks in some cases fatal to its use : one is the high price 

 of the acid; and the other the injurious and malodorous gases which are evolved 

 during the deoxidation of the acid. The object, therefore, in these researches has 

 been twofold — firstly, to introduce the greatest possible economy in the use of nitric 

 acid ; and secondly, to get rid of the malodorous and injurious gases which are 

 evolved. In effecting the former the latter has been successfully attained. 



To make this operation clear it will be well to recall for a moment the severa 

 oxides of nitrogen. 



There are five compoimds of nitrogen with oxygen, containing respectively 

 1, 2, 3, 4 and 5 volumes of oxygen to 2 of nitrogen, viz., taking them in the as- 

 cending order of the oxygen — 



1. Nitrous oxide, N 2 0. 



2. Nitric oxide, N. 2 O r 



3. Nitrous anhydride, N 2 3 . 



4. Nitrogen peroxide, N. 2 4 . 



5. Nitric anhydride, N 2 5 . 



If water be added to the fifth or highest oxide, we have nitric acid H 2 N 2 G , or 

 2HN0 3 . 



When any oxidisable substance is presented to nitric acid, the nitric acid parts 

 with a portion of its oxygen to combine with the substance to be oxidised, and the 

 nitrogen is evolved, combined with the remaining oxygen as lower oxides of 

 nitrogen. 



Under the most favourable conditions not more than fifty per cent, of the oxygen 

 contained in the nitric acid can be used for the purpose of oxidation. In many 

 cases, however, the percentage is as low as twenty. Moreover, a fresh supply of 

 the acid will be required for each successive operation. 



Now it will be evident that if sufficient oxygen be made to combine with these 

 lower oxides of nitrogen, nitric anhydride will be formed, which, when combined 

 with water, reproduces nitric acid. To effect this reproduction economically, the 

 " means " employed must themselves be economical in order to be useful. 



We will deal with the gases evolved during the deoxidation of nitric acid. 

 These gases will be composed of a mixture of lower oxides of nitrogen, which will 

 be of a deep brownish red colour, caused by the presence of nitrogen peroxide. 

 These gases or fumes are conducted into the chamber or towers where they are to 

 be reoxidised. These towers consist of closed chambers about, for ordinary pur- 

 poses, thirty feet high and three feet in diameter. Their form and material may be 

 varied, but they may be constructed of glazed earthenware pipes or a slate. The 

 tower rests in a reservoir, into which the reconverted acid falls and from which it is 

 afterwards drawn. Into the sides of the tower may be fitted sight-holes glazed 

 with glass, so that the quantity of the gases may be roughly judged by the depth 

 of the colour inside the tower. The top of each tower is of a conical form, and in 

 the centre of the cone is fixed a jet, through which steam and hot water are forced. 

 This jet is so arranged as to be capable of being easily and accurately adjusted in 

 order to cause a cloud or spray of very finely divided hot water of about a tem- 

 perature of 100° 0. to fall slowly through the tower or chamber. Atmospheric air is 

 allowed at the same time to enter the tower. The construction of the jets should 

 be so arranged as that the quantities of hot water and the admission of air may be 

 regulated at will. As is obvious, the tower should be gas tight, except as hereafter 

 described. Now the gases or fumes coming from the vessel in which the nitric acid 

 is being used will gradually rise till the tower is charged. When so charged the 

 jet is brought into action ; steam and hot water are turned on into the jet in such 

 proportions as that the steam will strike and divide the water into a minutely 

 divided spray, the steam itself being condensed in the water, so that a misty spray 

 of hot water slowly falls down through the tower. 



In its descent the particles of hot water come into contact with the oxides of 



1878. l l 



