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All that pours profuse
From things, perpetual, the vast ocean joins
Of air sublime ; which if to things again
Paid not, thus ballancing the loss sustain'd,
All into air would dissipate and die.
Hence, born from things, to things air still returns
Ceaseless, as prove their fluctuating forms.

Good.

But these opinions continued in the state of vague conjectures, till the matter was explained by the sagacity of Hales, and of those philosophers who followed his illustrious career.

It was not till the time of Bacon, who first taught mankind to investigate natural phenomena, that the atmosphere began to be investigated with precision. Galileo introduced the study by pointing out its weight; a' subject which was soon after investigated completely by Torricelli, Paschal, &c. Its density and elasticity were ascertained by Boyle and the Florence Academicians. Ma. riotte measured its dilatability ; Hooke, Newton, Boyle,' Der. ham, pointed out its relation to light, to sound, and to electri. city. Newton explained the effect produced upon it by moisture; from which Halley attempted to explain the changes in its weight indicated by the barometer. But a complete enumeration of the discoveries made upon the atmosphere in general belongs to pneumatics; a science which treats professedly of the mechanical pro. perties of air.

The knowledge of the component parts of the atmosphere did not keep pace with the investigation of its mechanical properties. The opinions of the earlier chemists concerning it are too vague and absurd to merit any particular notice. Boyle, however, and his contemporaries, put it beyond doubt that the atmosphere con. tained two distinct substances. 1. An elastic fluid distinguished by the name of air. 2. Water in a state of vapour. Besides these two bodies, it was supposed that the atmosphere contained a great variety of other substances, which were continually mix. ing with it from the earth, and which often altered its properties, and rendered it noxious or fatal. Since the discovery of carbonic acid gas by Dr. Black, it has been ascertained that this elastic fluid always constitutes a part of the atmosphere. The constituent parts of the atmosphere therefore are,

1. Air,

3. Carbonic 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 Arş 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 ascer. tained 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 term air to this last fluid. This innovationwas 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 meent 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. We may therefore consider atmospheric air, when in its usual state of dryness, as sufficiently pure for examination.

*

1, Air is an elastic fluid, invisible indeed, but easily recognized 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 Muid, 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 to

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 fifty-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. lle 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 cousidered 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 bulk ; 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 supporting 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 empyreal 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. Ac. cording to him, therefore, air is a compound of two parts of azotic and one part of oxygen gas.

le 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 substance 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 diminu. tion 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

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