LITERARY AND SCIENTIFIC.

LECTURE ON THE NATURE AND PROPERTIES OF ATMOSPHERIC AIR AND WATER.

On Thursday, the 22nd of January, Mr. Woolrich, Lecturer on Chemistry, &c. in the Birmingham School of Medicine, delivered a Lecture on the above subject to the Subscribers of the Kidderminster Public Library.

"Air and water," Mr. W. observed in his introduction, were formerly considered as elementary bodies; it was not until after the discovery of oxygen by Dr. Priestly, in 1774, that their real composition was ascertained. It is remarkable how little we are indebted to the ancients for discoveries in chemical science, and that amidst all the learning of the Egyptians, and of the Greeks and Romans, there is not the slightest intimation that they were acquainted with any of those elements which constitute atmospheric air and water; and, until Galileo discovered the pressure of the atmosphere, in the beginning of the 17th century, they do not appear to have been acquainted even with the physical properties of these bodies.

"The atmosphere is that transparent elastic fluid which surrounds the globe. It is highly elastic and compressible, and subject to all the physical properties of matter. Its pressure at the surface of the earth, on a level with the sea, is about 15 pounds on a square inch; but as its weight or pressure is dependent on attraction of gravitation, it diminishes in geometrical progression the higher we ascend. By this pressure, as Galileo first shewed, water is raised in pumps to the height of about 34 feet, and mercury is supported in the barometer-tube at the height of 30 inches. A column of mercury one inch in diameter and 30 inches high, being just equal in weight to a column of water of the same diameter 34 feet high, and to a column of air extending to the utmost limits of the atmosphere. Air is about 815 times lighter than water: 100 cubic inches weigh 31.0117 grains.

"The compressibility of air is only limited by the strength of the vessels containing it, and the space which it occupies is inversely as the pressure; thus, 100 measures of air under any given pressure will, when subjected to double that pressure, occupy the space of 50 measures; and when compressed by only half the original pressure, the same air will occupy the space of 200 measures. Upon the elasticity and compressibility of air all its mechanical properties depend. No means that have ever yet been employed have condensed it into a liquid state, therefore we know not what power would be required to effect this object. There are some bodies which exist in a gaseous state at the ordinary temperature and pressure of the atmosphere, which have been compressed into a liquid state, viz. carbonic acid gas, chlorine, sulphuretted hydrogen, &c. The term gas is applied to all aeriform bodies except atmospheric air.

"The extent of the atmosphere has often been a subject of discussion among philosophers, some contending that it is unlimited, while others conceive that it is confined to a distance not exceeding 50 miles. If its extent were unlimited it would be expected that the sun, and moon, and planets, as well as the earth, would each attract around themselves an atmosphere proportionate to their several masses. Dr. Woolaston, in his 'Essay on the finite extent of the Atmosphere,' remarks, that astronomers have not been able to discover any atmosphere about the sun nor about the planet Jupiter, hence he conceives that the atmosphere of the earth is limited, but he supports this opinion by other considerations. He conceives that atmospheric air is composed of particles or atoms incapable of division, (an opinion which now generally prevails as to the constitution of all kinds of matter,) and that when these particles are separated from each other to a certain distance, the further expansion of the air is prevented by the attraction of its particles, and the atmosphere thus becomes limited to that height where calorific repulsion and attraction balance each other; this is supposed to be at the distance of about 45 miles. Observations

which have been made on the refractive power of the atmosphere tend to support this opinion.

A considerable diminution of temperature prevails in the upper regions of the atmosphere, in consequence of the great tenuity of the air and the increase of its capacity for caloric. It has been calculated that the temperature diminishes about 1° Far. for every 300 feet; and at the distance of 7671 feet over this kingdom the temperature is perpetually at the freezing point of water. "The atmosphere contains small quantities of whatever is capable of evaporating from the earth, and which it can hold in solution; but its constant and essential constituents are two of the elementary bodies, oxygen and nitrogen, with variable proportions of carbonic acid gas and water. Its composition was first discovered by Lavoisier, and Scheele, soon after the discovery of oxygen; but the most correct analysis of the properties of its constituents was made by the Hon. Mr. Cavendish, whose statement accords almost precisely with the estimate of the most eminent chemists of the present day. Dr. Thomson, whose analysis is considered as correct, states the proportions of oxygen and nitrogen in 100 measures to be 20 of the former and 80 of the latter; or, if we take them by weight, of 8 of oxygen and 28 of nitrogen-it is remarkable that these weights correspond to one equivalent proportion of oxygen, and two equivalent proportions of nitrogen. The quantity of carbonic acid gas has been variously estimated. It has been generally stated to amount to one part in a thousand, but the proportion is constantly varying, and according to the recent experi. ments of Theodore Saussure, the maximum does not exceed 6.2 parts in 10,000 of air, and the minimum is 3.7 parts. It appears to be most abundant in elevated situations—more abundant in the night than during the day-and in the summer months more so than in the winter. The quantity of water in the atmosphere is also variable, even at the same temperature-at one time it is very dry, at another saturated with moisture. When air at 60° Far. is fully saturated with water, one cubic foot of it holds in solution about twelve grains

of water.

"Although the two elementary bodies oxygen and nitrogen appear to be most essential to the constitution of atmospheric air, as far as regards animal respiration and ordinary combustion, yet as carbonic acid gas is always present, it is no doubt equally necessary, if not to the animal at least to the vegetable creation. The presence of water, too, is of great importance; a deficiency of it in atmospheric air often proves fatal both to animal and vegetable life, as is witnessed in the extensive deserts of Asia and Africa, where what is called the Samiel Wind destroys both man and beast; even in this country the unhealthiness of a dry easterly wind is proverbial."

Mr. Woolrich proceeded to explain the particular properties of the several components of atmospheric air, and illustrated them by numerous interesting experiments.

Oxygen gas was first discovered by Dr. Priestly, in the year 1774. Scheele afterwards discovered it without any knowledge of the prior discovery of Priestly. It was formerly called vital air, and dephlogisticated air. Its present name was given to it by Lavoisier, from ožus acid, and yeyyaw I generate, in accordance with a favourite theory which he had proposed as to the cause of acidity. Priestly first obtained it by heating the per-oxide of mercury in the focus of a burning lens. It may be obtained by heating to redness several of the metallic oxides; also from nitrate of potash, and from chlorate of potash; the latter yields it in a very pure state. For general purposes it may be easily obtained from the per-oxide of manganese, by heating it to redness in an iron retort, and collecting the gas in receivers placed in the pneumatic trough; a small quantity of quick lime put into the water through which the gas passes, will free it from any carbonic acid which it may contain, and render it sufficiently pure for ordinary purposes. When pure it is colourless, and has neither taste nor smell; its specific gravity is 1.111, atmospheric air being 1000. One hundred cubic inches, at the temperature of 60° Far., and at 30 inches barometric pressure weigh 34.454 grains. Water absorbs this gas very sparingly; 100 measures of water when deprived of air, by boiling, absorb between three and four measures of this gas.

"Of all the elementary bodies oxygen is the most universally distributed. It occurs abundantly in the composition of most substances in the animal,

vegetable, and mineral kingdoms.

It is essential in the important process

of respiration, and in all ordinary cases of combustion. When the air is deprived of only one half of its oxygen it becomes incapable of supporting animal life, and also incapable of supporting combustion. Air that has passed only once through the lungs, loses nearly one third of its oxygen; and in the air that is given out from the lungs there appears about an equal bulk of carbonic acid and when an animal is confined in the same air, death usually ensues when little more than half the oxygen is consumed. An atmosphere containing, however, much less of oxygen, will still continue to support respiration in small animals and insects. Although oxygen is so necessary to respiration, when properly diluted with nitrogen, (the other constituent of air) yet alone it cannot long support animal life-its stimulating properties speedily bring on debility, and death usually ensues in a few hours.

"Two most important effects result from the process of respiration. The first is the change which venous blood undergoes in its passage through the lungs, by which it is converted from a dark, almost black, colour into a florid crimson, and becomes what is called arterial blood, suitable for the supply of all the various wants of the animal economy. The second is the production of animal heat. The healthy temperature of the human body is about 95 Far. in all climates, the same in Siberia as in the East Indies, and this uniformity of temperature is mainly dependent upon respiration.

"Substances that burn very feebly in the atmosphere burn with great brilliancy in oxygen gas. Iron wire burns very rapidly and beautifully in oxygen, and so do phosphorus and charcoal, giving out the most intense heat and light. A taper that has been blown out but still retaining a small spark at the end of the wick, is instantly rekindled into a flame on being introduced into a jar filled with oxygen. Oxygen has, accordingly, been termed a supporter of combustion, and Lavoisier founded a theory on the supposition that oxygen was the sole cause of combustion; but this theory, like the one that was supplanted by it, (the Stahlian) has been found not to stand the test of rigid investigation. Stahl's theory of combustion, which was universally adopted by the chemists of the 17th century, was founded on the supposed existence of phlogiston, or the element of fire. When a body had undergone combustion it was supposed to have lost its phlogiston, and therefore it became incombustible. Metals being combustible were considered as bodies combined with phlogiston, and after the combustion of a metal, the substance remaining was called the cala, or the earth of the metal-lime (calx) being similar to the calces of metal. Some combustible bodies, such as sulphur and phosphorus were observed to be converted into acids instead of calces; and other bodies, such as charcoal, alcohol, oil, &c., were almost entirely dissipated during combustion, it was supposed that these latter substances abounded in phlogiston, and accordingly when any of the metallic calces (now called oxides) were mixed with charcoal and heated, the phlogiston of the charcoal was supposed to combine with the calx and convert it into a metal. The sulphuric and phosphoric acids were in like manner supposed to be converted into sulphur and phosphorus by union with the phlogiston of charcoal. But this theory of combustion, which had been so ably supported by Stahl, was completely overthrown by Lavoisier, soon after the discovery of oxygen. It was shewn, by the experiments of the latter, that during the combustion of bodies, oxygen is absorbed from the atmosphere, and, uniting with the combustible body, converts it either into an oxide or an acid; and that when charcoal or any other substance that has a powerful affinity for oxygen is heated in contact with the oxide, or the acid, it deprives them of oxygen, and reduces them to their original bases. The fact which had been already observed by Mr. Boyle, of a metal becoming heavier after combustion, was employed by Lavoisier to shew the absurdity of the phlogistic theory;-100 grains of lead on being heated in contact with air, are gradually converted into red-lead, weighing about 111 grains-thus shewing, that instead of losing anything, the metal gains considerably during combustion. It was soon proved by experiment that the increase of weight is owing to the absorption of oxygen. But the supporters of the phlogistic theory, still unwilling to renounce their favourite opinions, endeavoured to account for the above fact, by giving to their imaginary substance, phlogiston, the principle of levity; the separation of it from the lead March, 1835.-VOL. II. NO. VIII.

T

was, according to their view, the cause of the increase of its weight during combustion. But this assumption of a property which no experiment could support, soon fell to the ground, and the disciples of Stalh gradually declined in number, and were at length compelled to adopt another theory. That which succeeded and which was so ably promulgated by Lavoisier, was founded on the supposition that the heat and light which is given out during combustion, is derived principally from the oxygen during its combination with the combustible body. It was soon shewn, however, that to oxygen alone, the term supporter of combustion could not be exclusively confined. Other substances were found capable of supporting combustion. Copper, in thin leaves, on being introduced into chlorine gas, burns spontaneously, giving out heat and light; and potassium unites powerfully with arsenic and tellurium. All the later researches of chemists tend to shew that combustion is not dependent upon any particular species of matter, but that it is an effect which always takes place whenever any two substances unite together with sufficient energy to produce heat and light. Substances possessing opposite electrical relations generally unite with the greatest energy, and produce the phenomena of combustion-hence it has been considered by some chemists entirely as an electric exhibition. Oxygen combines extensively with the metals-with one class it forms oxides, with another acids, and with a third alkalis. It also unites with all the non-metallic bodies, and converts them either into acids or oxides. Lavoisier conceived that oxygen was the sole cause of acidity, but in this conclusion he was wrong; there are acids, as the prussic and muriatic acids, which do not contain oxygen.

"It is evident that during the respiration of animals, and during all ordinary cases of combustion, immense quantities of oxygen must be constantly consumed, and its place supplied with carbonic acid gas, yet wherever atmospheric air has been examined, in the highest and lowest situations, in cities and in the country, no difference in the proportion of oxygen can be detected. Whatever may be the causes of the occasional unhealthy state of the atmosphere, it cannot be attributed to any change in the proportions of oxgen and nitrogen, but rather to the excess or deficiency of watery vapour, or to the presence of very small quantities of matter foreign to its constitution. The only process in Nature, with which we are acquainted, that tends to preserve this constant uniformity in the proportion of the elements of atmospheric air, is in the growth of vegetables; and a more perfect and admirable instance of the wisdom and contrivance of the Almighty cannot be adduced. During respiration and combustion carbonic acid gas is thrown abundantly into the atmosphere, and if it were allowed to accumulate would eventually become injurious to animal life; but vegetables, during their growth, absorb this gas, partly from the rain-water which receives it from the atmosphere, and conveys it to their roots, and partly also by their leaves. In passing through their vessels it is decomposed, the carbon is retained to form the various parts of the plant or tree, and the oxygen is restored to the atmosphere during vegetable respiration. Thus the animal and vegetable kingdoms appear to be constantly contributing to each other's benefit-that which would be destructive to animals is the food of vegetables, and the oxygen which vegetables expire is essential to animal life.

a

"Nitrogen gas, which has already been introduced in speaking of the composition of the air, was discovered in the year 1772, by Dr. Rutherford, of Edinburgh, maternal uncle of the late Sir Walter Scott. In 1775 Lavoisier found that it was a constituent part of atmospheric air, and gave it the name of azote, from the Greek negative particle and an life, because animals on being immersed in it are suffocated; but as this property belongs to some other gases, it is now commonly called nitrogen, being the base of nitric acid. This elementary body is not so universally distributed as oxygen. It occurs, however, very abundantly in the animal kingdom; it forms an essential part of all animal matter, excepting fat, oils, and a peculiar substance called cholesterine. In the vegetable kingdom it is found very sparingly, few vegetable substances contain it, and where it does exist in any of them, its quantity is comparatively very small. In the mineral kingdom it is also very scarce, being confined to saltpetre, coal, and a few other minerals.

"As nitrogen gas constitutes four fifths of atmospheric air, it may be readily obtained by any process that will separate the oxygen from it; this may be effected by burning phosphorus in a confined portion of air, or by putting a piece of phosphorus in a vessel full of air, at the temperature of 60°, for about 24 hours; the oxygen combines with the phosphorus, leaving nitrogen and a small quantity of carbonic

acid: the latter may be separated by passing the mixed gases through a solution of potash or lime-water, which readily absorbs carbonic acid, but has no effect upon nitrogen. Nitrogen may also be obtained by exposing muscular flesh to the action of dilute nitric acid and heat, in a glass retort.

66

Nitrogen gas is a permanent, colourless gas, possessing neither taste nor smellits specific gravity is .972-100 cubic inches weigh 30.15 grains. One hundred volumes of water absorb about one volume and a half of this gas; it does not support combustion, neither is it inflammable. Animals cannot live in it; their death, however, is not occasioned by any direct poisonous qualities of the gas, but merely from the exclusion of oxygen.

"The active properties of atmospheric air have been considered as principally dependent upon oxygen, the nitrogen being thought to possess merely negative qualities, and to act as a diluent of the oxygen; it is highly probable, however, that it performs an important office in the process of respiration, and that animals are indebted to its presence for the conversion of vegetable into animal matter its precise operation is not at present understood. Nitrogen combines with many of the other elementary bodies with oxygen it forms nitric acid—with hydrogen it constitutes ammonia.

"Carbonic acid gas, already noticed as existing in atmospheric air, was first discovered in 1757, by Dr. Black, and called by him fixed air. It is a compound gas, consisting of 6 parts of carbon and 16 of oxygen. It is found combined with lime, constituting limestone, chalk, and marble, in the proportion of 22 of carbonic acid and 28 of lime. It is also found in a free state, or mixed with air, in coal-mines, commonly called the choak damp, and in vaults, wells, and caverns; it is given out abundantly during the fermentation of beer and wine-during the combustion of all bodies containing carbon-and also during respiration; spring and river waters always contain a portion of this gas, which gives to them an agreeable taste. It may be conveniently obtained for experiment by the action of muriatic acid on marble or chalk. It is always gaseous under the ordinary temperature and pressure of the atmosphere, but may be brought into a liquid state by the pressure of 36 atmospheres. Its specific gravity in the gaseous state, at the temperature of 60° Far., and 30 inches barometrical pressure, is 1.524. Water absorbs its own bulk of this gas under the pressure of the atmosphere; with double the pressure it absorbs twice its bulk, and with treble its pressure three times its bulk. This gas is unfit for the support of combustion; a candle will not burn in an atmosphere containing one-fifth of its bulk. Neither is it fit for respiration-when animals attempt to inhale it in a pure state, it occasions a spasm of the glottis, and suffocation ensues; but when it is diluted with atmospheric air it may be respired; its effects, however, are not the less fatal; it is an insidious poison, and its narcotic properties render it extremely dangerous, as it may be breathed by its unconscious victim until stupor and insensibility come on, and all means of escape are useless. Many lives have been lost by persons imprudently sleeping in apartments with burning charcoal or coke, without proper ventilation. Before entering a well or a vault that has been long closed, it is advisable to ascertain whether this gas be present in any injurious quantity; this may be generally done by introducing a lighted candle-if it is extinguished by the air, on no account should persons be suffered to enter until means have been employed to remove it; this may be effected by ventilation, or by drawing it out in buckets, which may be easily done, being a heavy gas; or by throwing into the place a quantity of fresh-slacked quick-lime, which speedily absorbs the carbonic acid."

After shewing, experimentally, the properties of this gas, Mr. W. proceeded to the consideration of water.

"This liquid, so essential to animal and vegetable life, occupies a considerable portion of the surface of the globe, besides being combined in various proportions with its solid contents. It is never found, however, in nature in an absolutely pure state, being a solvent of most substances, it usually contains small quantities of those bodies with which it comes in contact during its passage through different strata of the earth, forming spring and river waters. When the quantity of substances held in solution in water gives it a sensible taste or smell, it is then termed a mineral water. Rain and snow water collected at a distance from towns and smoke, are sufficiently pure for all ordinary purposes, but perfectly pure water can only be obtained by distillation in glass vessels. Water, until about the year 1781, had been considered as one of the elements; some near approaches towards the discovery of