operation requires an apparatus permitting the placing of six or eight tubes.* In default of an apparatus of this kind, the operator must have an assistant at hand to hold the filled tubes until they are required for measurement; care should be taken to mark the order in which the tubes are respectively filled.

When all the tubes are filled, the gas is measured successively in the following manner. The tube is, in the first place, thoroughly immersed in the mercury with which the cylinder illustrated by

PLATE LXVI. is filled; this complete immersion establishes at

once a perfect uniformity of temperature between the gas and the mercury. The tube is then raised until the mercury in it is on an exact level with that in the cylinder, when the volume is read off and noted, (vide § 12). A small quantity of potass ley is then made to pass from the syphon B, which is nearly filled with this liquid, into the tube; (this is effected by blowing cautiously into the syphon). The syphon is then withdrawn, and the absorption of the carbonic acid by the potass ley is promoted by

moving the firmly held tube up and down in the mercury, keeping the opening firmly fixed against the wall of the cylinder; the tube is now finally again thoroughly immersed in the mercury, raised again until the mercury in it is on an exact level with that in the cylinder, and the volume is then accurately read off. (The pressure which the small column of potass ley exercises, may be altogether disregarded.) The volume of the carbonic acid is now found by subtracting the figure found in the second measurement (the nitrogen

to compel the small bubbles of air which may adhere to the walls to unite gradually with the large bubble at the top. The tube is then again turned upwards, and slowly filled with mercury.

* Compare "Das chemische Laboratorium zu Giessen," by J. P. HOFMANN, Heidelberg, 1842.

gas) from that noted in the first, (nitrogen gas+carbonic acid gas).

When the contents of the one tube have thus been determined, the mercury is cleaned by washing with water slightly acidulated with hydrochloric acid, and subsequently with pure water, and drying between blotting paper; the contents of the second tube are then determined in the same manner.

The results which the respective measurement of the contents of the various tubes yields are usually found tolerably corresponding; in many instances, however, where the nitrogenous substance, previously to complete combustion, is resolved into products of decomposition of different degrees of volatility, considerable differences are observed between the respective results afforded by the measurement of the several tubes. Under all circumstances, the average is taken as the correct result, and this may be considered the more accurate and safe, the less the respective results of the individual tubes diverge from it. Should the first tubes, however, show a considerably larger proportion of nitrogen than those which are filled subsequently, this is a sign that the air was not yet completely expelled when the first tubes were filled; in this case the contents of the first tubes are not taken into account.

The relative proportion of the carbonic acid to the nitrogen gas expresses immediately, and without any further calculation, the proportion of the equivalents of carbon to those of nitrogen, since one equivalent of carbon is consumed by two equivalents of oxygen without altering the volume of the latter element, and yields, therefore, two volumes of carbonic acid;-and one equivalent of nitrogen yields equally two volumes of nitrogen gas. Suppose we find the proportion of the carbonic acid to the nitrogen, like 4: 1, the analysed compound contains to four equivalents of carbon=4 × 75=300, one equivalent of nitrogen

=

175. If we had found, therefore, twenty-six parts of carbon in one hundred parts of the analysed substance, the latter would contain 15.17 of nitrogen, since 300 175: 26: x; x=15.17.

There is one irremediable source of error inherent in this method, viz., it is impossible to remove the air completely from the tube, and thus the resulting figure for the nitrogen is somewhat too high. This defect, however, does not materially interfere with the determination of the correct proportion, if the relative amount of nitrogen is considerable; thus, for instance, were the proportion found like 1 4.1, it would be evident at once that the correct proportion should be taken as 1 : 4. But if the relative proportion of the nitrogen is inconsiderable, this defect vitiates the results, and experience has proved that this method is no longer applicable to substances which contain less. than one equivalent of nitrogen to eight equivalents of carbon.

bb. QUANTITATIVE METHOD. (DUMAS.)

§ 148.

This method is applicable for the analysis of all nitrogenous organic compounds, and likewise for that of nitrates. The performance of the mercury processes requires, besides the objects enumerated at § 147, sub. 4 to 9, and those named at § 141, sub. 2, 9, and 12, a barometer and thermometer, and a graduated glass cylinder of a capacity of about 200 cubic centimetres; and finally a glass plate with a ground surface, to cover the latter.

The combustion tube should be about twenty-eight inches long, and closed at one end by fusion in the manner of a test cylinder; a layer of dry bicarbonate of soda, from five to six inches in length, is introduced into the tube; this is followed by a layer of oxide of copper, one inch and a half in length; an accurately weighed portion of the substance intended for analysis—(0.300— 0.600 gram., or more, in the case of compounds containing but an inconsiderable proportion of nitrogen) is most intimately mixed with a quantity of oxide of copper sufficient to fill the tube to about one half; and this mixture is then likewise introduced into the tube and covered with the oxide which has served to rinse the mortar, and also with a layer of pure oxide;

the tube is then finally filled up with a layer of copper turnings, about five inches in length. The tube thus prepared is connected with the gas conducting tube a, (vide infrà, Plate LXVII.), and placed in the furnace. The hindmost end of the tube (to the extent of about two inches) is then gradually heated to redness, the heat being prevented by a screen from reaching any other part of the tube. The bicarbonate of soda is decomposed by the intense heat applied to this part of the tube, and the carbonic acid thus evolved effects in its passage the expulsion of the air from the tube. When the evolution of gas has proceeded for some time, the extremity of the gas conducting tube is immersed under mercury, and a test cylinder, filled with potass ley, placed over it; the red-hot charcoals are then slightly advanced from the posterior part of the tube. If the gas bubbles entering the cylinder are completely absorbed by the potass ley, this is a sign that the air is thoroughly expelled from the tube, and the actual combustion may now be at once commenced; but should this not be the case, the evolution of carbonic acid is to be continued until the desired point is attained. The gas evolved is made to enter the graduated cylinder, which is filled to two-thirds with mercury, and one-third with strong potass ley, and placed in the mercurial trough, over the aperture of the gas conducting tube,* as the annexed illustration shows.

The following is the best way of filling the cylinder and placing it over the opening of the gas conducting tube. The mercury is introduced first, and the air-bubbles which adhere to the walls of the vessel are removed

The actual combustion is then proceeded with in the usual manner, by heating first the anterior part of the tube to redness, and advancing gradually towards the posterior part. At the conclusion of the process, the other half of the bicarbonate of soda is decomposed, and thus the whole of the nitrogen gas which still remains in the tube is forced into the cylinder. The operator must now wait until the volume of the gas in the cylinder decreases no longer, even upon slightly agitating the latter, (that is, until the whole of the carbonic acid in the cylinder has been absorbed.) The cylinder is then placed in a large and deep glass vessel, filled with water, the safe transport from the mercurial trough to this vessel being effected by keeping the aperture closed with mercury contained in a small basin. The mercury and potass ley sink to the bottom, and are replaced by water. The cylinder is totally immersed in the water, and subsequently raised again until the water in the cylinder is on an exact level with that in the vessel; the volume of the gas is then accurately read off, and the temperature of the water, and the point at which the barometer stands, are carefully marked; the weight of the nitrogen gas is finally calculated from its volume, after previous reduction to 32° of the thermometer, and 29.8 of the barometer, and with due regard to the tension of the aqueous vapor,— (compare infrà, "Calculation of analyses.") The results are generally somewhat too high, viz., by about 0.2 to 0.5 per cent. ; this is owing to the circumstance, that even a protracted transmission of carbonic acid through the tube fails to expel every trace of the air adhering to the oxide of copper.

in the usual manner. The potass ley is then poured into the cylinder, leaving the top free, to the extent of about two lines; this is then cautiously filled up to the brim with pure water, and the ground glass plate placed over it. The cylinder is now inverted, and the opening placed under the mercury in the trough; the glass plate is then finally withdrawn. In this manner the operation may be performed easily and without soiling the hands with the ley.