317 



GASOMETRIC ANALYSIS. 



GASOMETRIC ANALYSIS. 



313 



ascertained of its absorption by another gas, which he called nitrous 

 air, since called deutoxide or binoxide of nitrogen, or nitric oxide gas. 



This gas may be considered as nitric acid deprived of a large 

 portion of its oxygen, which is effected by dissolving a metal in it, as, 

 for example, copper, silver, or mercury ; and of these the last men- 

 tioned yields the gas in the greatest purity. The nitric oxide thus 

 obtained being disposed to regain the oxygen which the metal has 

 taken from it, absorbs it with great facility from all such gaseous 

 mixtures as contain it ; the evidences of its action are the formation 

 of a red vapour, condensation of volume, and the reproduction of one 

 of the acids of nitrogen ; and the quantity of oxygen absorbed is 

 determined by the degree of condensation which is produced by its 

 action. 



Dr. Priestley's method was extremely simple : he took a phial 

 capable of holding about an ounce of water, filled it with water, and 

 displaced the latter with atmospheric air, or with the gaseous mixture 

 to be examined ; the volume of this being noted, it was transferred, 

 over water, into an air-jar about an inch and a half in diameter. An 

 equal volume of nitric oxide was added to it, and they remained 

 together for about two minutes : if the diminution was very consider- 

 able, another volume of nitric oxide was added. When this part of 

 the process was over, the gas was transferred to a glass tube about two 

 feet long, one-third of an inch in diameter, and graduated into lOths 

 and lOOths. After noting the volume of the gas, the result was 

 expressed in measures and decimal parts ; thus, when equal volumes of 

 common air and nitric oxide were mixed, and they afterwards occupied 

 the space of one volume and two-tenths, Dr. Priestley, in speaking of 

 the air so tried, said the measures of the test were 1 '2 or the standard 

 of the air was 1 ~2. Owing to the formation of several compounds of 

 nitrogen and oxygen in this process it is now known to be entirely 

 unworthy of confidence. 



The eudiometer next to be mentioned is that invented by Scheele, 

 which was probably the first proposed after Dr. Priestley's. This was 

 a graduated glass tube containing a certain volume of air, which was 

 exposed to a mixture of sulphur and iron-filings made into a paste with 

 water. Although the oxygen was absorbed and the nitrogen left by 

 this operation, yet the process was not to be relied upon ; for, by the 

 formation of sulphuric acid, which occurred by the oxidisement of the 

 sulphur, the iron was acted upon, and water being decomposed, ita 

 hydrogen was evolved, and interfered with the results of the operation. 



This plan, however imperfect, had the merit of simplicity ; for the 

 quantity of oxygen absorbed was determined at once by deducting the 

 volume of the residual gas from that of the whole quantity submitted 

 to experiment. 



De Martc, instead of using sulphur and iron, employed a solution of 

 Mulphuret of potassium, prepared by dissolving sulphur in a solution of 

 potash. It is stated that tiis solution when newly prepared absorbs 

 a small portion of nitrogen ; but the fallacy arising from this source is 

 partially obviated by agitating the solution for a short time with a 

 little atmospheric air previously to using it, by which it is saturated 

 with nitrogen. A- tube divided into 100 parts and immersed in the 

 solution is sufficient for the use of it. 



Guyton also employed sulphuret of potassium in his eudiometer, 

 but he used it in a solid state, and applied heat to expedite its action. 

 In this case, as when the solution is used, both the sulphur and potas- 

 sium are oxidised. A description of this eudiometer, which has been 

 but little used, is given in ' Nicholson's Journal,' 4to, vol. i. 



The eudiometer of Seguin is a glass tube, about an inch in diameter, 

 eight inches long, and open at one end. It is to bo filled with and 

 inverted in mercury ; a small piece of phosphorus is then put under 

 the open end of the tube, and by its lightness it immediately rises to 

 the top of it, where it is to be melted by the approach of a red-hot 

 iron. A measured portion of the gas to be examined is then to be 

 passed into the tube ; the phosphorus inflames on each addition of the 

 gas, and the mercury rises, owing to the condensation of the oxygen. 

 ^ In 11 all the gas under examination has been thrown up into the 

 tube, the hot iron is again used to ensure tho completion of the pro- 

 cess. The quantity of the residual gas is determined by transferring it 

 into a graduated tube, and the difference between the quantity sub- 

 mitted to experiment and that left after it, indicates the volume of the 

 oxygen absorbed. 



In this operation, owing to the affinity existing between the phos- 

 phorus and the oxygen, they combine and form phosphoric acid : it ia 

 however stated that the nitrogen absorbs a small quantity of phos- 

 phorous vapour, and that, owing to the expansion which this occasions, 

 about jV) of the volume of the nitrogen gas is to be deducted. 



Bcrthollet also employed phosphorus in his eudiometer, but instead 

 of hating it, as in the above-described method, he allowed combination 

 to take place between it and the oxygen, by what is termed slow com- 

 bostion. He exposed a stick of phosphorus fastened to a glass rod in 

 a narrow graduated glass vessel, filled with air, and standing over 

 water : the phosphorus immediately begins to act on the oxygen of the 

 air, an shown by the formation of the white vapour of phosphorous 

 acid ; but this occurs without visible combustion. In six or eight 

 hours the whole of the oxygen disappears, and its quantity is, of 

 immediately apparent, making the deduction above stated from 

 the nitrogen gas. 



Dr. Hope's eudiometer is represented in the annexed figure. It is 



used with a solution either of sulphate of iron impregnated with nitric 

 oxide or with solution of sulphuret of potassium. This eudiometer 

 consists of a small bottle, capable of holding about three 

 ounces, for containing the eudiometric fluid, arid it is per- 

 forated and furnished with a stopper at 6. Into the neck 

 of the bottle a hollow graduated tube, a, closed at the 

 upper end, is accurately fitted by grinding. It holds pre- 

 cisely a cubic inch, and is divided into 100 equal parts. 

 To use the apparatus, the bottle is first to be filled with 

 the solution to be employed, and. covering the mouth with 

 a flat piece of glass, it is to be immersed in water ; the 

 glass being then removed, the open end of the tube con- 

 taining the gas to be examined is to be inserted. 



The instrument being removed from the water, is to 

 be inverted. The gas ascending into the bottle, it is to be 

 brought into thorough contact with the liquid by brisk 

 agitation, by which absorption of gas occurs. To supply 

 its place, the stopper at 6 is removed under water, a 

 quantity of which rushes into the bottle ; the stopper is then re- 

 placed, and these operations are alternately renewed till no further 

 diminution takes place. The tube a is then withdrawn, the neck of 

 the bottle being in water, and it is held inverted for a short time, and 

 the diminution is then measured by the graduated scale. 



Dr. Henry has pointed out some difficulties attendant upon the use 

 of this eudiometer, to obviate which he has substituted 

 a caoutchouc bottle for the glass one, as shown in the 

 annexed figure at 6. The tube a is accurately ground 

 into a short piece of very strong tube of wider bore, as 

 shown at c, the outer surface of which is made rough by 

 grinding, and shaped as represented, that it may more 

 effectually retain the neck of the elastic bottle when 

 fixed by waxed thread. This instrument is used in 

 every respect in the same manner as Dr. Hope's. The 

 only difficulty is in returning the whole of the residuary 

 gas into the tube ; but the art of doing this is readily 

 acquired by practice. 



Pepys contrived a eudiometer in which a caoutchouc 

 bottle was employed, as in Dr. Henry's. This instru- 

 ment, from the inventor's statement, appears to be 

 susceptible of great accuracy ; for he states that he is 

 able to measure an absorption of only jJ-^ of the gas 

 employed. For an account of this eudiometer, and of 

 some experiments performed with it, see ' Phil. Trans.,' 1807. The 

 parts are too numerous for us to insert figures of, and without them 

 a description would be scarcely intelligible ; besides, the apparatus ia 

 now superseded by others. 



Having now described the use of nitric oxide, sulphuret of potassium, 

 and phosphorus, as eudiometrical substances, and mentioned the more 

 important instruments in which they are employed, we shall notice 

 the eudiometer of Volta, and the eudiometric body which he made 

 use of. 



t's method of determining the composition of atmospheric air, 

 or of the quantity of oxygen contained in gaseous mixtures, was by 

 means of combustion with a known volume of hydrogen 

 gas ; for it having been ascertained that when a mixture of 

 oxygen and hydrogen gases is fired, one-third of 

 the diminution is owing to the condensation 

 of oxygen, we have only to observe the measure 

 of the contraction of volume to ascertain that of 

 the oxygen which was present. Of Volta's 

 eudiometer various modifications have been pro- 

 posed, all agreeing however in the principle 

 above mentioned. According to Berzelius, that 

 invented by Mitscherlich is to be preferred, on 

 account of the simplicity of its construction and 

 the accuracy of its results. We shall, therefore, 

 describe it instead of the original one, and it 

 will at once occur to the chemical reader that it 

 is a slight modification of Priestley's and Caven- 

 dish's detonating tubes. 



This instrument, as represented by the annexed 

 figure, consists of a very thick glass tube, from 

 18 to 24 inches long, and about 4 lines internal 

 diameter ; it ia graduated, open at one end, and 

 closed at the other. Near the top (A) the tube 

 is perforated with platinum wires, placed at such 

 a distance as readily to allow of the passage of 

 the electric spark between them, and exter- 

 nally hooked ; near the orifice (u) the tube is 

 laterally perforated and furnished with a glass 

 cock, which is shut after filling the tube with the gas to be 

 examined : this is, of course, to prevent the loss of gas by 

 the expansion accompanying the detonation by the spark. 

 When this is over, the cock is turned under either water or 

 mercury, and the fluid rising in the tube, tho condensation 

 is exactly noted by bringing the fluid within and without the tube to 

 the same level. 



Another variety of Volta's eudiometer is represented by the,opposite 





B 



P 



