analysis is of course of primary importance, a very slight admixture with other compounds being sufficient to vitiate the conclusions deducible from the experiments. Having by suitable means ascertained the purity of the substance to be analysed, the next care is to ensure its complete desiccation. Solids should be dried in the water oven. Volatile liquids that are unchanged by distillation should be allowed to stand for two or three days upon fragments of fused chloride of calcium; the liquid should then be decanted and distilled in a small retort. In other cases, as in the examination of fats or fixed oils, it may be more convenient to dry the material in a watch-glass placed in an ordinary waterbath, or in the hot-water oven. The further progress of the analysis will vary according to the form and composition of the substance to be examined. We shall describe the methods of analysing 1. A solid, which does not contain nitrogen. 1. Analysis of a Solid not containing Nitrogen. (1044) The combustible which is generally used in these experiments is charcoal; it is very manageable, but the dust occasioned by it is troublesome. Hofmann has contrived a furnace in which gas is made use of as a fuel, and which answers its purpose admirably. (Q. J. Chem. Soc., xi. 33.) The best form of furnace to be used with charcoal is represented at A, fig. 379: it is made of stout sheet-iron bent into the form of a trough, open at one end; the plate which closes the other end is perforated with an aperture three-quarters of an inch in diameter, to allow the passage of the combustion tube; the furnace is about 20 inches long, 5 inches wide at top, 2 inches at bottom, and 3 inches high; or about 50 centimetres long, 13 centim. wide at top, 7 centim. at bottom, and 8 centim. high. NOT CONTAINING NITROGEN. 15 Transverse slits are made along the floor at intervals of 2 inches for the admission of air, and in each of the spaces between these slits is riveted a vertical stiff piece of sheet-iron, 1 inch high, terminating in a concave edge above, for the support of the combustion tube. During the operation, the apparatus may rest on bricks, g,g. The tube in which the mixture is burned, the combustion or retort tube, (a, b, c,) should be about 15 inches long and half an inch in diameter (25 centimetres long and 12 millimetres in diameter), made of glass of difficult fusibility and free from lead: the hard Bohemian glass answers the purpose perfectly. The tube may on certain occasions be drawn out into a fine but strong tail bent upwards at an obtuse angle, and the mouth should be smoothed by making it red-hot in the flame of the blowpipe, so that a cork need not be torn in adjusting it. The water is collected in a bent tube, e, filled with chloride of calcium. At d is a small bulb, in which the greater part of the water is condensed; it may be emptied from time to time. The chloride of calcium with which the bent portion is filled must not be fused, but should be prepared merely by evaporating the solution of the chloride to dryness by a strong sand heat. A porous mass is thus obtained, which does not expand by absorbing moisture, as the fused variety does, to the destruction of the tube that contains it. The cork through which passes the bent tube by which it is connected with the potash bulbs, f, should be trimmed close to the large tube and covered neatly with melted sealing-wax; and lastly, air should be drawn through the apparatus by the mouth to ascertain that no obstruction exists. Pumice-stone moistened with concentrated sulphuric acid may be advantageously substituted for chloride of calcium in the drying tube. The connexion between this tube and the retort, a, b, c, is effected by means of a sound elastic cork, or a plug of vulcanized caoutchouc, which is made to close the mouth of the tube accurately; it is pierced with a round file, and fitted firmly upon the fine tube proceeding from the bulb d of the drying tube; the cork is dried on the sand-bath immediately before the apparatus is mounted. The solution of potash employed in the bulb tube, ƒ, should have a sp. gr. of from 125 to 127, and must be renewed for every experiment; the portions that have been used may be put aside, and afterwards, when sufficient has been collected, may again be rendered caustic in the usual way by means of quick 16 ANALYSIS OF A SOLID NOT CONTAINING NITROGEN. lime. To the free end of the potash bulbs, a tube h, filled with fragments of caustic potash, is adjusted by means of a cork; this is weighed with the bulbs, and is employed to dry the nitrogen and other gases which escape during the course of the analysis. The compound commonly used for supplying oxygen to the substance to be burned is cupric oxide; this oxide readily imparts oxygen to combustible matter which is in contact with it, but when heated alone, it bears a very high temperature without being decomposed. It is best procured by dissolving copper in pure nitric acid, evaporating to dryness, and decomposing the nitrate by heating it strongly in an earthen crucible. Ignition is to be kept up till red fumes cease to appear; if the heat be too great the oxide becomes agglutinated, and requires strong pounding in an iron mortar to pulverize it. The powdered oxide is afterwards sifted through a fine copper sieve, and secured in stoppered glass bottles. : Immediately before each analytical operation, a sufficient quantity of this oxide is to be ignited in a crucible, and while still hot, transferred to a dry tube, by plunging the mouth of the tube into the oxide in the crucible, and then shaking it in piecemeal the tube is to be immediately closed with a dry cork, and allowed to cool. Meantime the interior of the retort is to be completely dried by heating each portion of it in succession in the flame of a gas or spirit-lamp, beginning at the closed end, and drawing air through the heated tube by means of a narrower tube passed down just beyond the heated part, and exhausting by the mouth. When every part has thus been dried, the retort is to be corked and allowed to cool. Five or six grains, or about o'3 grm., of the substance to be analysed, when powdered and dried, are to be put into a perfectly dry test tube, and this tube with its contents very accurately weighed; its contents are then to be mixed with cupric oxide in a mortar, and the empty tube again weighed; the difference gives the weight of the substance employed. Much caution is requisite in charging the retort. The mortar, having been first made dry and warm, is to be placed on a sheet of glazed paper, and cleared out with a little of the dried oxide of copper, which is thrown aside. Oxide of copper to the depth of an inch (25 centimetres) is to be poured into the combustion tube; after which a small quantity of the oxide is to be put into the mortar, then the substance to be analysed, then more oxide: the mixture must be made quickly and carefully, adding so much oxide as shall be sufficient to fill a little more than half the retort ANALYSIS OF A SOLID NOT CONTAINING NITROGEN. 17 tube; the mortar is then to be taken in the palm of the left hand, and the mixture introduced, carefully picking it up piecemeal by the retort tube itself: fresh portions of oxide are to be rubbed in the mortar to clear out the last traces of the mixture, and the retort is finally to be filled up with pure cupric oxide to within two inches (5 centimetres) of the extremity. The proportions of the mixture are represented in fig. 379: the portion from the tail of the tube to the letter a consists of pure oxide of copper, from a to b of the mixture, from b to c of the rinsings of the mortar, and from c to within an inch of the cork is pure oxide. The tube having been closed by a cork is to be struck smartly in a horizontal position on the table, so as to clear the tail-like prolongation, and make an air-way above the oxide from end to end. The drying tube e having been accurately weighed, is next to be fitted to the dry perforated cork, and connected by it airtight to the retort tube, a, b, c; this is now to be placed in the furnace; the potash apparatus f, also previously weighed, is to be attached to the drying tube by means of a connecting piece of caoutchouc, taking care that the largest bulb is on the arm connected with the drying tube; the potash apparatus should be slightly inclined by placing a cork under the end of the horizontal portion nearest the open extremity. Matters being thus arranged the next process is to ascertain if the whole be tight, and for this purpose the air in the large bulb is to be gently heated so as to expel a few bubbles; if, on cooling, the liquid rise in the limb and maintain its elevation steadily for a few minutes, the combustion may safely be begun. Charcoal broken into pieces about the size of a walnut is to be ignited, and applied to the portion of the tube nearest the cork, where the pure oxide of copper lies: the action of heat is limited by a double sheet-iron screen which fits into the furnace, and has a central slit in order to allow it to bestride the tube; this screen can by degrees be moved further and further down the furnace until the whole tube is heated. An additional screen of single iron plate is hung over the closed end of the furnace to protect the cork, care being taken that the heat never rises so high as to scorch it, or falls so low as to allow of the condensation of moisture in the portion of the retort which projects from the furnace. When the fore part of the retort is red-hot and the escape of air due to expansion has ceased, about an inch more of it may be heated; and in this way the fire is gradually carried down. At first but a small portion of the gas is absorbed, but when the sub 18 ANALYSIS OF A LIQUID NOT CONTAINING NITROGEN. stance is fairly undergoing decomposition, and the atmospheric air in the apparatus has been expelled, the gas is almost entirely taken up by the potash-ley. When the whole tube is ignited, the heat must be continued till bubbles are no longer disengaged; the potash-ley will now gradually recede into the large bulb; when this is observed to commence, the charcoal must be removed from the tail of the tube; and as soon as the solution of potash has risen sufficiently to fill half the large bulb, the tip of the tail must be nipped off; gentle suction is then to be effected by means of a tube of vulcanized caoutchouc, which is to be fitted to the free extremity of the potash apparatus, and air drawn through the combustion tube by the mouth, in order to displace the carbonic anhydride and aqueous vapour which the apparatus contains. Dumas prefers to connect the extremity of the retort with a drying tube, and this again with a receiver containing oxygen, which gas is carefully driven over the contents of the tube in the manner shown in fig. 288, part ii., p. 79. This renders the operation somewhat more complicated, but it is unquestionably more exact, especially for compounds where the proportion of carbon is great. The tube for supplying oxygen is easily adjusted to the retort by drawing out the tail horizontally instead of obliquely, and fitting it on by a caoutchouc connector, care being taken to screen the junction from the influence of heat. The apparatus is now to be dismounted, and allowed to cool: in about an hour the drying tube e may be weighed; one-ninth of the gain which it has experienced indicates the quantity of hydrogen contained in the compound analysed; the potash apparatus, f, is also to be weighed, and three-elevenths of what it has gained shows the quantity of carbon. The deficiency is reckoned as oxygen. The oxide of copper used in these experiments may again be rendered serviceable by moistening it with nitric acid, and igniting it as before, or by reoxidizing it whilst red-hot in a current of air directed through the tube. 2. Analysis of a Liquid not containing Nitrogen. (1045) If the liquid be volatile, a piece of tube rather less than a quarter of an inch or 6 millim. in diameter is to be heated in the blowpipe flame, and drawn out into a capillary portion, a (fig. 380), about four inches (10 centimetres) long: about a quarter of an inch below this the tube is to be sealed, and the little piece of tube, b, thus left connected with the capillary part, heated |