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1. Air,

3. Carhonic acid gas, 2. Water,

4. Unknown bodies. These shall form the subject of the four following Sections.

[Thomson's Chemistry.

SECTION 11.

Atmospheric Air. The word Arr seems to have been used at first to denote the atmosphere' in general; but philosophers afterwards restricted it to the elastic fluid, which constitutes the greatest and the most important part of the atmosphere, excluding the water and the other foreign bodies which are occasionally found mixed with it. For many years all permanently elastic fluids were considered as air, from whatever combinations they were extricated, and supposed to possess exactly the same properties with the air of the atmosphere. It is true, indeed, that Van Helmont suspected that elastic fluids possessed different properties; and that Boyle ascertained that all elastic fluids are not capable of supporting combus. tion like air. But it was not'till the discoveries of Cavendish and Priestley had demonstrated the peculiar properties of a variety of elastic fluids, that philosophers became sensible that there existed various species of them. In consequence of this discovery, the word air became generic, and was applied by Priestley, and the British and Swedish philosophers in general, to all permanently elastic fluids, while the air of the atmosphere was distinguished by the epithets of common or atmospheric air : but Macquer thought proper to apply the term gas, first employed by Van Helmont, to all permanently elastic fluids except common air, and to confine the terò air to this last fluid. This innovation was scarcely necessary ; but as it has now been generally adopted, it will be proper to follow it. By the word air, then, in this Section, is meant only common air, or the fluid which forms by far the greatest part of the atmosphere.

The foreign bodies which are mixed or united with air in the at. mosphere are so minute in quantity compared to it, that they have no very sensible influence on its properties. may

therefore consider atmospheric air, when in its usual state of dryness, as sufficiently pure for examination.

We

1, Air is an elastic fluid, invisible indeed, but easily recogniz. ed by its properties. Its specific gravity, according to the expe. riments of Sir George Shuckburgh, when the barometer is at 30 inches, and the thermometer between 50° and 60°, is usually reck. oned 1.000 : It is 816 times lighter than water. One hundred cubic inches of air weigh 31 grains troy.

But as air is an elastic fluid, and compressed at the surface of the earth by the whole weight of the incumbent atmosphere, its density diminishes according to its height above the surface of the earth. From the experiments of Paschal, Deluc, General Roy, &c. it has been ascertained, that the density diminishes in the ra. tio of the compression. Consequently the density decreases in a geometrical progression, while the heights increase in an arithme. tical progression.

Buguer had suspected, from his observations made on the Andes, that at considerable heights the density of the air is no longer proportional to the compressing force *; but the experi. ments of Saussure junior, made upon Mount Rose, have demon. strated the contrary t..

2. Although the sky is well known to have a blue colour, yet it cannot be doubted that air itself is altogether colourless and in. visible. The blue colour of the sky is occasioned by the vapours which are always mixed with air, and which have the property of reflecting the blue rays more copiously than any other. This has been proved by the experiments which Saussure made with his cyanometer at different heights above the surface of the earth. This instrument consisted of a circular band of paper,

divided into fiíty-one parts, each of which was painted with a different shade of blue ; beginning with the deepest mixed with black, to the lightest mixed with white. lIe found that the colour of the sky always corresponded with the deepest shade of blue the higher the observer is placed above the surface ; consequently, at a cer. tain height, the blue will disappear altogether, and the sky appear black; that is to say, will reflect no light at all. The colour be. comes always lighter in proportion to the vapours mixed with the air. Hence it is evidently owing to them I.

• Mem. Par. 1753. p. 515. + Jour. de Phys. xxxvi. 98.
Saussure, Voyages dans les Alpes, iv. 288.

3. For many ages air was considered as an element or simple substance. For the knowledge of its component parts, we are indebted to the labours of those philosophers in whose hands che. mistry advanced with such rapidity during the last forty years of the 18th century. The first step was made by Dr. Priestley in 1774, by the discovery of oxygen gas. This gas, according to the prevailing theory of the time, he considered as air totally de. prived of phlogiston; azotic gas, on the other hand, was air sa. turated with phlogiston. Hence he considered common air as oxygen gas combined with an indefinite portion of phlogistou, varying in purity according to that portion ; being always the purer the smaller a quantity of phlogiston it contained.

While Dr. Priestley was making experiments on oxygen gas, Scheele proceeded on the analysis of air in a different manner. He observed that the liquid sulphurets, phosphorus, and various other bodies, when confined along with air, have the property of diminishing its balk; and this diminution always amounts to a certain proportion, which he found to be between a third and a fourth part of the whole. The residuum was unfit for supportivg flame, and was not diminished by any of the processes which diminish common air. To this residuum he gave the name of foul air, From these experiments, he concluded that air is a compound of two different elastic fluids : namely, foul air, which constitutes more than two.thirds of the whole, and another air, which is alone capable of supporting flame and animal life. This last air he extricated from nitre by heat, from the black oxide of man. ganese, and from other substances, and gave it the name of empya real air. He showed that a mixture of two parts of foul air and one part of empyreal air, possesses the proportion of common. air*.

The foul air of Scheele was the same with the phlogisticated air of Priestley, or with what is now known by the name of azotic gas. His empyreal air is the same with the dephlogisticated air of Priestley, or with what is at present called oxygen gas. According to him, therefore, air is a compound of two parts of azotic and one part of oxygen gas. lle accounted for the diminu. tion of air by the liquid sulphurets and other similar bodies by

* Scheele on Air and Fire, p. 7, &c. Engl. Transl.

his theory of the composition of caloric, which he considered as a compound of phlogiston and oxygen gas. According to him, the phlogiston of the sulphuret combines with the oxygen of the air, and passes through the vessels in the state of caloric, while the azotic gas, which has no affinity for caloric, is left behind.

While Scheele was occupied with his experiments on air, Lavoisier was assiduously employed on the same subject, and was led by a dif. ferent road to precisely the same conclusion as Scheele. By oxidizing mercury in a vessel filled with common air, and heated to the boil. ing point of mercury, he abstracted the greater part of its oxygen gas; and by heating the red oxide thus formed; he reconverted it into mercury, while at the same time a quantity of oxygen gas was extricated. The residuum in the first experiment possessed the properties of azotic gas; but when the oxygen gas extricated from the mercury was added to it, the mixture assumed again the properties of common air. Hence he concluded that air is com. posed of azotic gas and oxygen; and from a variety of experi. ments he determined the proportions to be 73 parts of azotic gas and 27 parts of oxygen gas. He demonstrated, too, that when air is diminished by liquid sulphurets, metals, &c. the oxygen gas which is abstracted combines with the sulphurets, &c. and con. verts them into acids or oxides according to their respective na. ture.

Air, then, is a compound of oxygen and azotic gas : but it becomes a question of considerable consequence to determine the proportion of these two ingredients, and to ascertain whether that proportion is in every case the same. Since azotic gas, one of the component parts of that fluid, cannot be separated by any sub. stance with which chemists are acquainted, the analysis of air can only be attempted by exposing it to the action of those bodies which have the property of absorbing its oxygen. By these bo. dies the oxygen gas is separated, and the azotic gas is left behind, and the proportion of oxygen may be ascertained by the diminution of bulk; which being once known, it is easy to ascertain the proportion of azotic gas, and thus to determine the exact relative quantity of the component parts of air.

After the composition of the atmosphere was known to philosophers, it was taken for granted that the proportion of its oxygen varies at different times and in different places; and that upon this variation depended the purity or noxious qualities of air. Hence it became an object of the greatest importance to get possession of a method to determine readily the quantity of oxygen in a giren portion of air. Accordingly various methods were pro. posed, all of them depending upon the property which many bo. dies possess, of absorbing the oxygen of the air without acting upon its azote. These bodies were mixed with a certain known quantity of atmospheric air in graduated glass vessels inverted over water, and the proportion of oxygen was determined by the diminution of bulk. These instruments received the name of eudiome. ters, because they were considered as measures of the purity of air. The eudiometers proposed by different chemists may be re. duced to five.

1. The first endiometer was made in consequence of Dr.Priest. ley's discovery, that when nitrous gas is mixed with air over wa. ter, the bulk of the mixture diminishes rapidly, in consequence of the combination of the gas with the oxygen of the air and the ab. sorption of the nitric acid thus formed by the water. When ni. trous gas is mixed with azotic gas, no diminution at all takes place. When it is mixed with oxygen gas iu proper proportions, the absorption is complete. Hence it is evident, that in all cases of a mixture of these two gases the diminution will be proportional to the quantity of the oxygen. Of course it will indicate the proportion of oxygen in air; and by mixing it with different portions of air, will indicate the different quantities of oxygen which they contain, provided the component parts of air be susceptible of va. riation. Dr. Priestley's method was to mix together equal bulks of air and nitrous gas in a low jar, and to transfer the mixture into a narrow graduated glass tube about three feet long, in order to mea. sure the diminution of bulk. He expressed this diminution by the number of hundred parts remaining. Thus, suppose he had mixed together equal parts of nitrous gas aod air, the sum total of this mixture was 200 (or 2.00): suppose the residuum when mea. sured in the graduated tube to amount to 104 (or 1.04), and of course that 96 parts of the whole had disappeared, he denoted the purity of the air thus tried by 104. A more convenient instrument was invented by Dr. Falconer of Batb; and Fontana greatly ims

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