at the end of the heating it may be broken and a current of purified air or oxygen passed through, or a tube open at both ends is taken. In the latter case the combustion is carried out from beginning to end in a current of purified oxygen. The purification of the oxygen or of a current of air is effected in the apparatus shown in the cut as connected with the rear end of the combustion tube. Oxygen entering through the tube d passes through sulphuric acid in a, a chloride of calcium jar at b, and a U tube filled with stick potash at c c, and then through the tube f to the combustion tube. A second series of absorption vessels on the same stand serves for air. The water produced by the oxidation of the hydrogen of the organic substance is caught in a weighed tube containing fused and granulated chloride of calcium, and the carbon dioxide produced by the oxidation of the carbon of the organic substance is caught in a weighed bulb apparatus containing strong caustic potash solution. Instead of the older form of potash bulbs known as the Liebig bulbs shown in Fig. 91, the apparatus of Geissler shown in Fig. 92 is now more generally used because of the convenience in weighing. One-ninth of the increase of weight of the chloride of calcium tube represents the weight of the hydrogen, while three-elevenths of the increase of weight of the potash bulbs represents the weight of the carbon.

FIG. 92.

Geissler's potash bulbs.

If the organic compound is nitrogenous, a spiral of metallic copper or silver must be placed in the front end of the combustion tube and kept at a low red heat in order to prevent the oxides of nitrogen from going over in the absorption apparatus and vitiating the results. Organic compounds containing sulphur must be burned with chromate of lead, which will oxidize the sulphur and hold it as sulphate of lead. Halogens present in an organic compound are held as silver haloid salts by the use of the silver spiral before alluded to.

The nitrogen of an organic compound is either determined absolutely and its volume measured, or it is converted into ammonia by combustion with soda-lime as in the Will-Varrentrap method, or by heating with strong sulphuric acid and potassium permanganate as in the Kjeldahl method. In the determination of nitrogen by volume a tube closed at one end is used, and in

the farther end sodium bicarbonate or magnesite is placed. This is first heated so as to displace all the air of the tube by carbon dioxide, and then the substance is burned with copper oxide, a copper spiral being used of course in the front end of the tube to decompose oxides of nitrogen. The nitrogen gas is collected over strong potash solution, which absorbs the carbon dioxide and allows of the measurement of the volume of residual nitrogen. In the soda-lime process the substance is burned in a tube closed at one end, somewhat shorter than the ordinary combustion tube, and the ammonia produced is absorbed in pure strong hydrochloric acid. We may then either determine the ammonia here caught as sal ammoniac by the use of platinic chloride, or, if a measured amount of hydrochloric acid of known strength was taken, may titrate back with normal alkali solution and so determine the ammonia indirectly. In the Kjeldahl process the substance is heated with concentrated sulphuric acid for some time to a temperature near the boiling point of the latter, the addition of small portions of powdered potassium near the end of the action sufficing to complete the ammonia formation. The mixture is then diluted with water, supersaturated with caustic soda, and the ammonia distilled off and determined volumetrically.

The methods for the determination of the halogens, sulphur, and phosphorus have already been indicated in speaking of their qualitative detection.

Oxygen is always determined by difference, as no reliable general method for its determination exists.

3. Physical Properties of Organic Compounds.-The physical properties are just as important points for observation in the case of organic compounds as with inorganic substances, only, instead of crystalline form, hardness, lustre, color, etc., the important properties for consideration in this connection are fusing point, boiling point, vapor-density, and in some cases optical properties.

(a) Fusing Point.-Most organic solids, when sufficiently pure, fuse either with or without decomposition at a constant temperature. To determine the fusing point, a small quantity of the substance carefully dried and pulverized is placed in a capillary tube sealed at one end, and this is attached by a rubber band to a thermometer in such a way that the capillary tube with the substance is immediately adjacent to the bulb of the thermometer. A round-bottomed flask with a long neck is then taken, and in this is placed concentrated sulphuric acid or paraffin so that the

bulb of the flask is three-quarters full. The thermometer with capillary tube attached is held in position by passing it through a perforated cork fitted in the neck of the flask, and dips into the liquid far enough to allow the bulb and substance in the capillary to be covered. A small side tube fitted in the perforated cork allows the air to escape when the flask is heated. The heat is applied gradually, and the moment the substance in the capillary is in clear fusion the temperature is read off.

(b) Boiling Point and Fractical Distillation. Most organic liquids boil without decomposition at a fixed temperature, which is called their boiling point. Even in the case of such liquids as cannot be distilled without decomposition under ordinary atmospheric pressure, it is often possible by distilling them under reduced pressure or in vacuo to get them to vaporize at a constant temperature. A boiling point, constant for the same atmospheric pressure, is taken as one of the most reliable indications of purity and identity of organic liquids.

To determine the boiling point the liquid is placed in what is termed a distillation bulb. This consists of a bulb with tall narrow neck into the side of which at some height above the liquid is fused a delivery tube bending downward at an oblique angle. The neck of the distillation bulb is closed by a tight-fitting cork perforated for a slender thermometer. The latter must extend so far into the neck that the mercury bulb comes just below the lateral exit for the vapors, but never dip into the liquid. If in making a boiling-point determination any part of the mercury column extends above the neck of the distillation bulb where it is surrounded by vapor, a correction must be made depending upon the length of the column not surrounded by vapor. Of course, the boiling point is always dependent upon the atmospheric pressure, as indicated by the barometer (see page 41), and therefore the pressure must be noted in connection with each boiling-point determination.

A mixture of organic liquids of different boiling points can often be separated into its components by what is termed fractional distillation, especially when the boiling points of the individual components are moderately removed from each other. In this case fractions of the distillate are caught separately at fixed intervals, say every five or ten degrees, and these are then distilled by themselves and the portions coming over at approximately the same temperature added together. By repeating the operation several times the distillates show a tendency to accumulate at a

few fixed temperatures corresponding to the boiling points of the components of the original mixture.

The process of fractional distillation is much facilitated by the use of distillation bulbs with special arrangements for condensation in the vertical neck attached to the bulb or flask, such as the apparatus of Wurtz, Linnemann, and Hempel. These accomplish a fractional condensation of the vapors corresponding to that effected in the column apparatus of the rectifier or tar distiller.

(c) Vapor-density.-We have already explained the relationship between vapor-density and molecular weight and the principles upon which it is based (see page 121). This determination has considerable importance in the study of organic compounds, as, for reasons to be explained later, ultimate organic analysis does not generally enable us definitely to determine molecular weight and molecular formulas. Hence a physical determination which can go so far towards settling the question of chemical composition becomes very important.

For determination of vapor-density we can ascertain the weight of a definite volume of the vapor, which is then compared with the weight of the same volume of air under similar conditions of temperature and pressure, or the volume occupied by the vapor obtained from a fixed weight of the substance, or thirdly, the volume of another substance like mercury or air displaced by the vapor from a definite weight of the compound under investigation. To the first class of determinations belongs the Dumas vapordensity method, in which globes of heavy glass with narrowed neck are used. These are weighed full of the vapor after the neck has been sealed by fusion of the capillary portion, and then opened under mercury so that the cubic contents of the globe can be accurately determined by measuring the mercury which fills it. The method is now rarely used.

To the second class of determinations belongs the Hofmann vapor-density method. In this case we have a tall barometer tube filled with mercury and dipping into a mercury trough. This is surrounded by a wider mantle-tube through which the vapor of water, aniline, or other higher-boiling liquid may circulate. Before the vapor is made to pass through the mantle, the weighed portion of the substance whose vapor-density is required is passed up into the barometer tube. Here, as the tube becomes heated by the vapor surrounding it, the substance vaporizes in When the level of the mercury becomes constant in the tube, it is read off and the volume of vapor is calculated.

vacuo.