This must now be brought upon a weighed filter, (or better, upon two filters, one of which has been counterpoised against the other,) and washed repeatedly with the mixture of two parts of alcohol and one of ether until nothing further is taken up; the precipitate and filters must be dried by a water heat and the weight accurately observed. According to Varrentrapp and Will, 220.52 grs. of the ammonia-chloride are equivalent to 14 grs. of nitrogen ; the esti mate of these writers is too high, and 225 grs. a more nearly equal to one equivalent or 14 grs. of nitrogen. Practically, however, their calcu lation is very near the truth, as during the operation a minute quantity of hydrochlorat of atnmonia escapes uncondensed, and the tw errors compensate each other.
This method of determining nitrogen an4 swers for all cases excepting those in which occurs in the form of nitric acid, when it mill be determined by volume and its weight thence deduced. For this purpose the process of Dumas is the most trustworthy. A retort tube of about twenty inches long is employed ; not drawn out into a tail, but sealed with a rounded extremity • about two inches of the tube arc filled with' carbonate of copper or of lead, and then the mixture with oxide of copper addeli and covered as usual with a layer of purl oxide ; beyond this the last two or three ineliet of the tube are filled with clean copper tura ings, as already directed, to decompose any a the oxides of nitrogen which may be formel The retort tube is then connected with a three legged apparatus of brass or copper (fig.43 1, 1) 1)), one limb of which is furnished with a still cock (c). The connection with the retort best inade by pssing a piece of glass tube through a cork fitting accurately into the mouth of the combustion tube and connecting- the brass apparatus to this small glass tube by a caoutchouc connector; the limb (b) is fastened by a similar joint to a glass tube (e) bent at right angles near one end, with a straight por tion upwards of thirty inches long, the other extremity of which is turned up at an acute an gle for the convenience of safely delivering the gas ; the tube is placed in a vertical direction svith its lower upturned extremity dipping into a small mercurial trough; the stop-cock tube (e) is connected with an exhausting syringe and a vacuum producesl; the apparatus is left for half an hour to ascertain that all the joints are tight: if the mercury after this lapse of time still stands at the satne level, the experiment is proceeded with ; a moderate heat is applied by a spirit larnp to the end of the retort contain ing the carbonate; by this means carbonic acid is set free and displaces the last portions of air; the exhaustion and disengagement of gas are repeated alternately three or four times, taking care to leave sufficient carbonate undecom posed to renew this expulsive process at the termination of the experiment. The stop-cock
(c) is now closed, the air-pump is removed, and a graduated jar containing sonie solution of potash is inverted in the mercury over the recurved extremity of the long glass tube. The copper turnings are then brought to red ness in the usual way by charcoal, and the experiment conducted with the customary pre cautions, the decomposition being caused to take place less rapidly than usual ; when the part of the retort containing the matter for analysis is red-hot through its entire extent, heat is gradually applied to the carbonate at the end, and the last portions of gas from the combustion in the apparatus are driven into the receiver by the disengaged carbonic acid.
As the products of combustion are only water, carbonic acid, and nitrogen, the two former are retained by the solution of potash, whilst the latter alone presents itself for mea surement. I need hardly say that the height of the barometer and thermometer must be carefully noticed, when the apparatus by stand ing for an hour or two has reached the tem perature of the atmosphere ; as the gas will be saturated with moisture, its volume must be corrected by the known methods for the three points of temperature, pressure, and moisture ; then, since 100 cubic inches of nitrogen at standard temperature and pressure weigh 30.15 grs., the sveight of the nitrogen that a given quantity of the matter analysed contains is easily determined. In this process, as in every case where the proportion of nitrogen alone forms the object of our experiment, after the weight of the material for analysis has been once accurately determined, it is evident there is nothing to fear from the absorption of moisture.
Occasionally the quantity of nitrogen, where large, is advantageously determined by making the combustion just as though we were going to ascertain the proportion of carbon and drogen; but, instead of condensing the car bonie acid and weighing it, the whole of the gases produced are collected over mercury. A bent gas-delivering tube is substituted for the usual drying,apparatus. ( Fig. 431.) In this case it is best to-trigin at the closed ex tremity of the tube, and havine expelled the atmospheric air by a portion of gas generated from the substance, to collect the rest of the gaseous products in a graduated jar ; by agi tating the gas with solution of potash the pro portion of nitrogen to carbon is at once deter mined, as equal volumes of carbonic acid and nitrogen represent single equivalents of carbon and hitrogen. Ir is not necessary in this case to determine accurately the quantity of mate rial acted upon.