ammonium carbonate, ammonium nitrate, sodium chloride, and particles of floating matter generally known as dust. By weight the proportion is 77 per cent. of nitrogen and 23 per cent. of oxygen. The proportion of carbon dioxide is variable, being on an average 0.03 per cent., but is found in cities often as high as 0.11 per cent. The amount of aqueous vapor is still more variable, and is affected by local causes, as temperature and altitude; the average is estimated at about 0.75 per cent. When air is freed from carbon dioxide and water vapor, the proportion of nitrogen to oxygen is found to be remarkably constant. All the great number of analyses made of air during the past fifty years, by many different investigators, show but slight variations from the figures above given, although the samples have been taken from all parts of the globe and from elevations of fourteen thousand feet. The constancy of these proportions led some of the earlier chemists to believe that air was a compound of oxygen and nitrogen, but in view of certain properties we must conclude that it is a mechanical mixture.

Properties.-The following are some of the characters which prove it to be a mixture:

(1) The physical and chemical properties of a mixture of seventy-nine volumes of nitrogen and twenty-one volumes of oxygen are exactly the same as air. When the two gases are mixed there is no evidence of chemical action, the temperature of the mixture remaining the same as that of the constituents.

(2) The proportion in which the two elements exist in air bears no relation to their atomic weights, and this proportion has been found to vary beyond the reasonable error of analysis, while a chemical compound does not vary in the proportion of its constituents.

(3) When air is drawn through a thin layer of caoutchouc, that which passes through is composed of about forty-two volumes of oxygen and fifty-eight volumes of nitrogen, on account of the property that oxygen has of passing through this medium more rapidly than nitrogen.

(4) On agitating air with water until the latter is saturated, the composition of the dissolved gas is found to be different from that of the original air on account of the solubility of oxygen in water being greater than that of nitrogen.

The question naturally suggests itself, whether the continual pouring into the atmosphere of so much carbon dioxide from all sources, the animal life on the globe as well as the product of so

much decay and combustion, has not changed the composition of the air by reducing the percentage of oxygen. In reply to this it may be said that calculations based on the inhabitants at one thousand millions, and the amount of oxygen used in other ways at ten times that consumed by human beings, demonstrate that in eighteen hundred years the proportion of oxygen in the air would be decreased by only 0.1 per cent.

Analysis.-Moisture is determined by passing known quantities of air over calcium chloride and noting the increase in weight of the latter. Carbon dioxide is estimated in the same manner, using potassium hydrate instead of calcium chloride, or the air is passed into solution of barium hydrate and the resulting barium carbonate filtered off and weighed. Oxygen is estimated by taking a definite volume of pure dry air in a eudiometer tube over mercury, adding a measured volume of pure hydrogen, and exploding the mixture by means of an electric spark. After cooling there will be a contraction equal to the volume of hydrogen and oxygen which have combined, the resulting volume of water being so small as to be disregarded.

Other less accurate methods have been used, but none of them are capable of the same exactness as the above. One of these is to pass the air over red-hot copper which will combine with the oxygen. By weighing the copper before and after the operation, and weighing or measuring the nitrogen, the proportion of the two gases may be estimated. Phosphorus has also been used for removing the oxygen. When this is carried out in a tube over mercury the resulting volume of nitrogen may easily be read off.

While the composition of air under ordinary circumstances is constant, it may in crowded halls and rooms become charged with injurious amounts of carbon dioxide and other compounds from the exhalations of the individuals present. The remedy for this is perfect ventilation, which consists in removing the impure air as well as in admitting that which is pure. Once in the outside atmosphere, the impurities rapidly disappear by dif fusion and by the oxidizing action of ozone and hydrogen peroxide.

PRACTICAL EXERCISES.

(1) Place a small piece of phosphorus in a little porcelain crucible, set them in a shallow vessel of water, so the phosphorus will not be wet, and ignite it by touching it with the hot end of a file or other convenient metal. Immediately bring over the crucible with its edge dipping into the water a large beaker or bell-jar. Some of the air is at first expelled by

the heat, but the oxygen is rapidly consumed and the phosphorus ceases to burn. When the apparatus is cooled the water rises in the beaker or bell-jar and occupies the amount of space previously held by the oxygen, the remaining space is filled with nitrogen.

(2) Prepare a larger quantity of nitrogen by heating, in a suitable flask, potassium nitrite dissolved in a solution of ammonium chloride. When the reaction begins, the heat must be removed, or carefully regulated to prevent too rapid evolution of the gas. Nitrogen may be collected over water like oxygen or hydrogen. The negative properties possessed by it may be noted during collection.

History. Ammonium chloride, or sal ammoniac, appears to have been known from the earliest times, and the aqueous solution of the gas was described by the alchemists under the name of "spirits of hartshorn." Priestley, in 1774, was the first to prepare gaseous ammonia, by heating together sal ammoniac and lime, and collecting the gas over mercury. He gave to this gas the name of "alkaline air," which later became "volatile alkali."

Occurrence and Formation.-Ammonia, in combination with carbonic, nitric, and nitrous acids, exists in the air in minute quantities, being produced by the decomposition of organic matter. It is found in the Tuscan lagoons, probably combined with boron nitride, BN. It is also found as sulphate and chloride near active volcanoes, having been produced by the hot lava flowing over fertile soil containing nitrogen. Ammonia and its salts are formed in the dry distillation of many organic substances.

Formerly horns, hoofs, urine, and other animal products were distilled, and the ammonium carbonate thus produced neutralized with hydrochloric acid, the product after sublimation being known as sal ammoniac. At the present time our supply is obtained by a similar process, in which, however, coal is the material used. Bituminous coal yields on distillation about 2 per cent. of ammonia, which, therefore, occurs in the ammoniacal liquor formed during the manufacture of illuminating gas. In addition to free ammonia there are present in this ammoniacal gas-liquor the carbonate, sulphide, sulphate, and thiosulphate. The gasliquor is usually distilled with lime, by which the ammonia is liberated and collected in a suitable receiver with some water. This distillate is neutralized with hydrochloric or sulphuric acid,

and the resulting ammonium chloride or sulphate purified and used for preparing the other ammonium salts.

Ammonia is also formed by the action of nascent hydrogen on nitrous or nitric acid. Finally, ammonia is produced in minute quantity by passing the electric spark through a mixture of nitrogen and hydrogen. It has been said that the elements do not unite directly, and this appears to be an insignificant exception to that statement. It is improbable that such combination should take place to any great extent, since ammonia gas is decomposed by the electric spark into its constituents.

Preparation.-Ammonia gas is prepared on a large scale by heating together in a glass or iron retort calcium hydrate and ammonium sulphate or chloride :

(NH4)2SO4 + Ca(OH), = CaSO4 + 2NH3 + 2H2O

For laboratory purposes it is usually preferable to heat the solution of ammonia which is supposed to contain ammonium hydrate :

NH,CH = NH3 + H,O.

When required pure the gas must be passed over calcium oxide (quicklime) to remove moisture, and collected over mercury. As ordinarily needed it may be collected by "upward displacement;" that is, by holding an inverted vessel over the tube from which the gas is escaping, which gas, being lighter than air, rises and fills the vessel.

Properties.-Ammonia is a colorless gas, of a pungent, suffocating odor and a caustic taste. It has a density of 8.5, being a little more than half as heavy as air. The gas is liquefied by passing into a tube cooled to -40° by means of a mixture of ice and calcium chloride. It may also be liquefied by subjecting to a pressure of from six to seven atmospheres at ordinary temperatures. This liquid is colorless and highly refractive, and may be solidified by cooling to -75°. At-38.5° it boils, and in vaporizing absorbs large quantities of heat. On account of this property it is used in ice machines and in the several varieties of cooling apparatus used in breweries.

Ammonia is very soluble in water, one volume of the latter absorbing 1148 volumes of the gas at o°, and about 600 volumes at ordinary temperatures. This solution, which is official in two strengths, will be described later.

Ammonia is not a combustible gas, although if it be mixed

with a small quantity of oxygen combustion readily takes place. on the application of flame, with the formation of water, nitrogen, and nitric acid. Ammonia is alkaline to litmus paper, and combines with acids, forming the well-known ammonium salts. The formula NH, represents a hypothetical compound which has many properties in common with those of sodium and potassium, and will be treated of in connection with them.

Uses.-Anhydrous liquid ammonia is prepared on a large scale for use in ice machines. It comes in commerce in large iron drums strong enough to withstand the pressure necessary to keep it in the liquid condition. The aqueous solution of ammonia is used largely in medicine and in the manufacture of pharmaceutical preparations.

Detection.-Ammonia and its solution are easily detected by the odor; in smaller quantities by bringing over the suspected solution a piece of moistened red litmus paper, which will be turned blue. Still more delicate is the reaction with fumes of hydrochloric acid. A rod moistened with the diluted acid is brought into some of the gas or over some of the warm solution, when immediately white fumes of ammonium chloride will form. When combined with acids, ammonia is detected by first liberating it from its combination with an alkali like potassium or sodium hydrate, and then applying one of the above tests for the gas, or the solution is acidified with hydrochloric acid and solution of platinic chloride added, when a yellow precipitate of ammonio-platinic chloride-(NH4),PtCl-will slowly separate out in minute crystals. Einbrodt's reagent, consisting of a solution. of mercuric chloride to which a minute quantity of an alkaline carbonate has been added, will detect the merest trace of ammonia or its compounds, if the latter are previously made alkaline, by forming a white precipitate or cloudiness. Nessler's reagent produces a brown precipitate with ammonium compounds, or if in very dilute solution a brown or yellow color. This reagent is made by adding to a solution of mercuric chloride a solution of potassium iodide until the precipitate at first formed is nearly all redissolved. Solution of potassium hydrate is then added to strongly alkaline reaction and the liquid allowed to settle until it becomes clear, when it is decanted from any sediment.

Aqua Ammoniæ, U. S. P.-The solution of ammonia is official in two strengths, one, aqua ammonia, containing 10 per cent., by weight, of the gas, and the other, aqua ammoniæ for