CHAPTER II. INGREDIENTS OF THE ATMOSPHERE. The four Elements of the Ancients-Air a Mixture of various Gases-Oxygen-Its Qualities-Nitrogen-Ozone-Its characteristic Smell remarked by the Ancients and mentioned in the Odyssey-Carbonic Acid-Its Properties—Diffusion of Gases-Ammonia-Aqueous Vapours-Casual Ingredients of the Atmosphere Odorous Principles-Their extreme Divisibility-Self-purifying Power of the Atmosphere-The Molecules of the Atmosphere. THOUGH thousands of years have elapsed since man first appeared upon the stage of life, yet, strange to say, not quite a century has passed since he first gained an insight into the composition of the air he breathes. The Greek philosophers, who knew but little of the art of interrogating Nature by careful observations or accurate experiments, but whose speculative minds were extremely active in the invention of splendid theories, reckoned air, along with fire, water, and earth, among their four elements whose various combinations gave birth to all other existing substances. These four elements were placed in the universe according to their weight. Earth, as the heaviest, occupied the centre; water, the next in weight to earth, was principally spread over its surface; the lighter air occupied the middle zone of the universe; and fire, the lightest and most volatile of all, reigned in the highest regions, where it formed the sun and the planets. This system served to explain the composition of all the productions of nature. The meteors, for instance, were supposed to consist of fire and air; the inferior animals of earth and water; and man was indebted for the warmth of his blood to the addition of some of the fiery principle to the cold material mass of his body. For many centuries this ancient theory of the four ele ments was implicitly believed, until modern chemistry overthrew the whole fallacious fabric, and, dethroning the old elements, set up a far greater number of new ones in their place. As the ancient element earth has been decomposed into numerous metals, united with other primary substances; as water has been found out to be a compound of hydrogen and oxygen; as fire, from being a substance, has been converted into a mode of motion;-so also air has lost its ancient rank as an undecomposable element, and is now known to be a mere mixture of various gases. To Priestley (born 1733, died 1804), one of the great men who opened the era of modern chemistry, we are indebted for the discovery of oxygen, one of the two constituents of atmospheric air. In this gas it was found that combustion took place with extraordinary intensity; even iron wire heated red hot and plunged into it caught fire and burnt away! Other combustibles gave out showers of the most brilliant sparks, and produced the most intense heat when placed in the jar containing it; a lighted taper having been blown out, instantly rekindled when put into it, and blazed with much greater intensity than in air. Soon afterwards another gas was found also to form a part of the composition of air. This gas was the direct contrast of the other. Instead of increasing the brilliancy of flame, it extinguished it as effectually as if it had been plunged into water. Like the other, it was inodorous and invisible; but while the first exhibited the utmost avidity for combination, this was resolutely indifferent to every substance. Animals were intoxicated when immersed in the first, they were suffocated on immersion into this. Such are the opposing qualities of the important gases, oxygen and nitrogen, which, in the proportion of 23 to 6 in weight and of 2019 to 19:01 in volume, form the great mass of air we breathe, and though singly incapable of supporting life, are so mingled in the atmosphere-the inertness of nitrogen tempering the active energy of oxygen- as to produce the exact measure necessary for the existence of the whole organic world. Supposing the proportion of oxygen to be raised from one-fifth to one-fourth, a considerable acceleration of the vital process would be the result, and organic life forced 100'00 100 00 100 100 ANALYSIS OF OXYGEN. 17 to undergo such changes as to produce a total revolution in the existing state of things. On the other hand, life would languish under an augmentation of nitrogen, and either be extinguished altogether, or assume new forms, harmonising with the altered condition of the atmosphere. Until within the last few years, chemical investigation could find no difference whatever in the relative quantities of these two gases. Gay-Lussac, by means of the balloon, brought down air in a vessel from the height of between three and four miles. On being analysed, it was found to be in no respect different from the air of the lower strata of the atmosphere. De Saussure spent a considerable time on the high Alps in analysing the air of these altitudes, but he also found that air taken from the summit of the Faulhorn was of the precise composition of air at Paris. The air contained in particles of snow was also examined, being expected to furnish a fair sample of the qualities of air in those lofty regions where snow is formed; but in every case the results were similar. In whatever country air was analysed, it was impossible to detect the smallest difference in its composition, and even the air which was spreading the most fatal pestilence, the air of a great and crowded city, the air of a hospital ward, were all found identical as far as their proportions of oxygen and nitrogen went. But where the utmost accuracy of a Dumas or a Boussingault failed to render appreciable the smallest essential difference between the hot and dusty air of towns and the balmy breathings of a country wind, laden with the odour of a thousand flowers, so great is the perfection to which chemical analysis has now been brought, that errors have since their day gone down from six per cent. to the second place of decimals. Thus Dr. Angus Smith' not only tells us that the air of mountains and plains differs from that of towns and dwellingrooms, but he shows us exactly in what the difference consists. The sea-air on the coast of Scotland contains 20.999 per cent. of oxygen; that of a suburb of Manchester on a wet day, 20-9800; the same on a dry day, 20.947; in Manchester during fog and frost, 20-910; whilst in a sitting-room the 4 1 Air and Rain, the Beginnings of a Chemical Climatology.' London, 1872. quantity is reduced to 20-89; in the pit of a theatre to 20-74; in the Court of Queen's Bench to 20-65; and in the sumpt of a mine to 20·14. Though the differences are minute, they are definite. The air of impure places is universally characterised by a diminished quantity of oxygen. The air of a closet or midden contained only an average of 20·70 per cent., whilst the air of the street contained 20.943. In the open spaces of London, the proportion of oxygen is 20-950 per cent.; in the eastern and crowded districts it is 20-857; in the Metropolitan Railway tunnel it is only 20-70; in the middle of Hyde Park the air is particularly pure, and was found to contain 21.00 per cent. of oxygen. The air contains the largest quantity of oxygen after rain, and the smallest in dull, foggy weather. The senses seem to estimate a difference of only 0-07 per cent., but it is probable that they estimate the impurities produced rather than the loss of oxygen. During a visit to London, Dr. Smith analysed the air of a law-court. One specimen had 20.65 per cent. of oxygen; and a second, taken from the lantern, only 20-490. No such loss of oxygen was found in any factory in Manchester, or in any other inhabited place above ground during the day. To find air similarly degraded, it is necessary to descend the shaft of mines, where the deterioration is the same. Had Dickens known of this circumstance, he might perhaps have made it the subject of some cutting remarks upon his good friends the lawyers. As in science one discovery leads to another, the researches of the present age have shown us oxygen in a new state, investing it with properties such as were never thought of by Priestley or Lavoisier. The name ozone (from the Greek oliv, to emit an odour), was given in 1839 to a smell, by Professor Schönbein, of Basel, the inventor of gun-cotton. Most persons will be acquainted with the peculiar odour which pervades a room in which an electrical machine has been kept in motion. Many will have experienced the sensation of a strange odour filling a house which has been enveloped in discharges from a thunder-cloud. This smell has been compared to that of sulphur from the days of Homer to our own days OZONE. Then Jove in anger bids his thunders roll, Odyssey, Book XII., 485. 19 And it was explained by assuming that it was due solely to a peculiar action of electricity on the olfactory organ. Schönbein was the first to detect a material substance in that which had previously been regarded as a sensation, and to prove it to be oxygen in a remarkable condition of energy. As carbon exists in the state of charcoal and of the diamond; as phosphorus is known in two conditions, the yellow variety, which is poisonous in an extreme degree, and the red kind, which is comparatively inert, and as this double condition has been discovered to exist in many other simple bodies, so oxygen gas is known to possess at least two very distinct states, one of these being the oxygen of the air we breathe under ordinary circumstances, odourless and tasteless, supporting life and acting gently in maintaining all the conditions of animal and vegetable existence, and a comparatively feeble oxidising agent; the other possessing a powerful odour, and a flavour of lobsters, acting as a strong irritant, injurious when in a free state to animals and plants, a very active oxydising agent, and an energetic bleacher. Ozone forms in the air in extremely minute quantities, nor has chemistry as yet been able to collect it in a pure state. It is also still uncertain whether it is solely formed under the influence of electricity, or whether an intense solar light may have a share in its production. Its quantity increases with the moisture and motion of the atmosphere, and seems to diminish in high temperatures, and when the barometer stands very high. Atmospherical precipitations, rain, snow, and hail, gales and thunderstorms, are favourable to its development. It is the great purifier of the air, the arch-enemy of miasmatic and contagious diseases. Although throughout the atmospheric ocean the composition of the air is apparently as constant as if it were fixed |