greater than all the other sources put together, and that if no causes of diminution existed, the amount of carbon dioxide would be doubled in about 380 years. This carbon dioxide is in part produced by slow processes of combustion continuously going on in the more superficial parts of the earth's crust, and in part from the diffusion of pent-up gas in natural reservoirs, large and small, produced in past ages.

2. The respiration of man and animals.

3. Processes of combustion.

4. Fermentation.

5. The burning of limestone, by which carbonate of lime is heated to redness. Under these conditions it parts with its carbonic acid and becomes caustic lime.

6. The escape of carbonic acid from natural waters in which it is held in solution.

Carbon dioxide is a colourless invisible gas; it is more than half as heavy again as air; 1 litre at 0° and 760 mm. pressure weighs 1.977414 grm. (or 100 cubic inches at 60° F. and 30 inches bar. weighs 47.303 grains); its specific gravity compared with air is 1·5203. On account of its density it can be poured from one vessel to another like water, and hence is liable to collect at the bottom of deep wells, of vats, or similar places. It does not support combustion, extinguishing flame immediately; a candle goes out when the oxygen sinks to 18.5 per cent. and the carbon dioxide rises to 25 per cent, but this quantity will not be immediately fatal, for most persons can breathe air containing 5 per cent. for a short period; anything over 5 per cent. causes insensibility in a longer or shorter time varying with different people; undiluted carbon dioxide is irrespirable, causing spasm of the glottis.

Carbon dioxide is quickly absorbed by solutions of caustic alkalies such as soda or potash, or the caustic alkaline earths such as lime or baryta; it is soluble in water, 100 volumes at 0° dissolving as much as 180 volumes of the gas; by increasing the pressure, water may be made to take up proportionately larger quantities; when the pressure is reduced the gas escapes, as witnessed every day in the effervescence of aerated waters, champagne, and similar liquids. Water charged with carbonic acid freely dissolves carbonates of lime and magnesia.

The amount of carbon dioxide varies somewhat according to

locality and season. Risler (Compt. Rend. xciv., 1390-1391) has made monthly estimations for a year of the carbon dioxide of the air at Nyon, Switzerland, situate 420 metres above sea level. His minimum number is 25, his maximum is 35 per 1,000 volumes of air; the mean of all the estimations being 30 per 1,000. The mean of numerous analyses by Saussure for the air of Geneva and Chambeisoy works out 45 per 1,000. London street air, Angus Smith found to average 36 per 1,000; Manchester, 40. In inhabited rooms, ill-ventilated stables, and near middens and other sources of carbonic acid much higher numbers are obtained. The carbonic acid derived from respiration has naturally a relation to the other excretory products of respiration in air, and as it can be very easily estimated its amount is a convenient measure of respiratory impurity: this will, however, be discussed farther on.

(28) The Fixation of Carbon by Plants.-Maintenance of the Composition of the Atmosphere.

It might be supposed that the atmospheric oxygen would continuously decrease and the carbon dioxide increase, but observation, so far, shows that the atmosphere preserves its composition, although in remote periods geologists believe it to have been of an entirely different composition. The great compensating agency at work keeping down the liability of carbon dioxide to increase is the action of the green chlorophyll of plants on carbon dioxide. This green chlorophyll, under the influence of sunlight, has the extraordinary power of splitting the gas up into its two constituents-carbon which it retains, and oxygen which it exhales. This fact was first pointed out by Priestly and Ingenhause; it is extraordinary, because carbon dioxide has hitherto only been decomposed in the laboratory by the action of the most powerful agents, such as potassium brought to a high temperature or metals raised to an intense heat or the electric spark; yet every little green cell in sunlight quietly fixes carbon and sets free oxygen; hence there is a continual circle, oxygen being changed into carbon dioxide, carbon dioxide falling again to pieces, the carbon going to build up manifold plant structures, the oxygen to again be changed into carbon dioxide. There is, however, a permanent loss of oxygen ever going on in the oxidation of inorganic matter; such, for example, as the conversion of the protoxides of iron into the

peroxides, and it is not easy to see how in this last case the oxygen can be returned under the ordinary processes of nature. It can, however, be shown that the reservoir of oxygen is so large that this diminution, even continuing for long periods of time, will not affect appreciably the percentage of oxygen. Professor Thorpe gives a very clear illustration of this. "If," says he, "we suppose the atmosphere to be put in a balloon, and suspended from the end of a balance, it would require 581,000 cubes of copper each having a side of 1 kilometre in length, to restore equilibrium."

If we also assume that each individual consumes 1 kilo of oxygen per diem, and that the population of the earth is 1,000,000,000, and further, that the oxygen consumed in the respiration of other animals, and in the oxidation of organic matter amounts to four times that required by man, and also that the oxygen disengaged by plants compensates only for the causes of diminution of oxygen not specified, then even in this exaggerated case, the amount of oxygen abstracted from the air in a century would only amount to fifteen or sixteen of the copper cubes; or, in other words, the abstraction in a century would be onlyth of the total quantity of oxygen contained in the air -an amount inappreciable by the most exact eudiometric methods known to us.

(29) Other Gases more or less constantly present in Air.

3'

Ozone is an allotropic form of oxygen, its chemical formula being denoted by Og, which signifies that the molecule contains three atoms, whereas ordinary oxygen, O,, contains but two. 22.4 litres at standard temperature and pressure weigh 48 grammes; it is 1 times the density of oxygen. Ozone is readily changed into ordinary oxygen by heating it to 300°. It has been liquefied by cold and pressure and is then seen to be a blue liquid. Its oxidizing power is very great. It rapidly corrodes caoutchouc, bleaches vegetable colours, and oxidizes metals. It has such an irritating effect on the mucous membranes of the eyes and nose, that it has even been credited as the general cause of catarrh. It may be produced in considerable quantities by the silent electrical discharge acting on oxygen. Its occurrence in the atmosphere is referred to electrical agency. From the experiments of Houzeau, country air contains one volume in 700,000. In the air over

marshes, in the air of crowded dwelling rooms, and in the air generally of cities it is absent.

The most usual test of its presence is filter paper which has been steeped in a solution of potassic iodide and starch; moistened paper of this kind becomes blue in the presence of ozone, because ozone decomposes potassic iodide and sets free iodine; unfortunately the test is not conclusive because other oxidizing agencies have the same property.

Acids. Nitric acid in very small quantities exists in most air. Sulphurous acid is ever present in the air of towns. atmosphere is, from the continuous combustion of coal containing sulphur compounds, sufficiently acid to turn in a few hours moistened blue litmus paper red.

A. Ladureau (Ann. Chim. Phys. 5, xxix., 427–432) examined the air of Lille for sulphurous acid gas: he found on an average a cubic metre to contain 18 c.c.; on calm days this was found to increase to 2 c.c., on stormy days it decreased to 14 c.c.

Ammonia.-Traces of ammonia are always present even in the purest air. Mr. Horace T. Brown found a million parts of country air, when the samples were collected two metres from the ground, to contain from 5:102 to 6:085 parts of ammonium carbonate; the air of a town contained from 4059 to 8732 parts of ammonium carbonate. He considers the normal amount to be about 6 parts per 1,000,000. Heavy rain decreases the amount of ammonia, but only for a time.

Sulphuretted Hydrogen or its ammonia compound is constantly in the air of large cities like London, as proved by the tarnishing of silver coinage and ornaments. The amount has not been accurately estimated.

Marsh Gas.-There is always in the air a small quantity of gas which is a compound of carbon, probably marsh gas. A. Muntz and E. Aubin (Comptes Rendus, cxix., 871-874) have found the amount of carbon dioxide which may be obtained by passing air, previously filtered from dust and deprived of its carbon dioxide, over red hot copper oxide, to amount to between 3 and 10 volumes per 1,000,000 volumes of air. It may be stated generally that the amount of the carbon gas or vapour is represented by a volume of carbon dioxide equal to the 100th part of the volume of carbon dioxide existing as such in the atmosphere.

(30) Other Substances floating in Air.-Micro-organisms. Common Salt is an invariable constituent of all natural air. It is probably in the minute dust floating in the lower layers of the atmosphere; its presence is proved by a Bunsen flame always giving the line called the sodium line when examined by the spectroscope. All air also contains micro-organisms, the spores of fungi bacteria, micrococci and bacilli. Hesse was one of the first who made quantitative estimations of the micro-organisms in air. A tube was coated on the inside with sterile nutrient gelatine and the air to be examined slowly aspirated through the tube; in this way the air was made to give up its tiny load of germs, for in passing over the moist sticky gelatine, the minute seeds would adhere. On putting on one side the tube, and keeping it at common room temperatures, the little colonies would grow at all the points which had received a seed or germ, and the points would in a few days be visible to the naked eye. They were afterwards examined by the microscope and their general nature determined.

Dennis, Miquel, Carnelly and Haldane, Greenleaf Tucker, and others have all made interesting researches as to the contents of the air of different places in micro-organisms. The chief results are summarized in the table on the following page. In the latest research, that of Greenleaf Tucker, the micro-organisms were filtered through and arrested by a closely-packed layer of sterilized sugar to which afterwards the nutrient liquid was added.

(31) The General Difference between Town and Country Air. The following is a representation of what is constantly found in town and country air, omitting in the enumeration strictly local airs, which may contain any gas almost that is known to science, and also omitting mineral and non-living but organic dust.