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vital air of the atmosphere, which changes its dark colour to a brilliant red, rendering it the spur to the action of the heart and arteries, the source of animal heat, and the cause of sensibility, irritability, and motion. With regard to the nitrogen that is combined with atmospheric air, the greatest part of it is thrown out of the lungs at every respiration, and it rises above the head, that a fresh portion of air may be taken in, and that the same air may not be repeatedly breathed. The leaves of trees and other vegetables give out during the day a large portion of oxygen gas, which, uniting with the nitrogen thrown off by animal respiration, keeps up the equilibrium, and preserves the purity of the atmosphere. In the dark, plants absorb oxygen, but the proportion is small, compared to what they exhale by day.

QUESTIONS.-1. Of what is atmospheric air composed? 2. What is the proportion of each, and what other substances does it contain? 3. What is oxygen? 4. Why is it thus named? 5. How does it become oxygen gas? 6. What are some of its important properties? 7. Why has it been called vital air? 8. What is nitrogen, and how does it form nitrogen or azotic gas? 9. What are some of its properties? 10. What important end does it answer, and how? 11. How is the vermilion colour of the blood produced? 12. What becomes of the nitrogen that is thrown out of the lungs?-why? 13. What tends to preserve the purity of the atmosphere? [NOTE. Nitrogen (pronounced Ni'trō-jěn,) is called azote by the French chemists on account of its being so destructive of life. Oxygen, (pronounced ox'ē-jén,) besides producing most of the acids, is necessary also to the production of the alkalies.]

LESSON 62.

Water.

Cal'cine, to burn in the fire to a calx ;-calx is a substance easily reduced to powder. Efferves'cence, an intense motion which takes place in certain bodies, occasioned by the sudden escape of a gaseous substance.

WATER was formerly considered as a simple substance, and chemical philosophers were for a long time unwilling to allow of its being otherwise. Its compound nature, however, has been fully proved. It is composed of eighty-eight parts by weight of oxygen, and twelve of hydrogen, in every hundred parts of the fluid. It is found in four states, namely, solid or ice; liquid or water; vapour or steam; and in a

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state of composition with other bodies. Its most simple state is that of ice, and the difference between liquid water or vapour and ice, is merely that water contains a larger portion of caloric than ice, and that vapour is combined with a still greater quantity than water. However long we boil a fluid in an open vessel, we cannot make it in the smallest degree hotter than its boiling point, for the vapour absorbs the caloric, and carries it off as fast as it is produced. It is owing to this, that all evaporation produces cold. An animal might be frozen to death in the midst of summer, by repeatedly sprinkling ether upon him, for its evaporation would shortly carry off the whole of his vital heat. Water thrown on burning bodies acts in the same way; it becomes, in an instant, converted into vapour, and by thus depriving them of a large portion of their caloric, the fire, as we term it, is extinguished. Vapour occupies a space eight hundred times greater than it does when in the form of water, and the expansive force of steam is found by experiment to be much greater than that of gunpowder. There is reason to believe that, in time, steam may be applied to many useful purposes of which at present we have no idea.

Hydrogen is the base of the gas which was formerly called inflammable air, and when in the aëriform state, it is the lightest of all ponderable things. If you put a quantity of filings of zinc into a vessel which has a glass tube adapted to it, and then pour upon them sulphuric acid (oil of vitriol) diluted with six or eight times its quantity of water; an effervescence will immediately take place, the oxygen of it will become united to the metal, and the hydrogen gas will be disengaged, and may be conveyed by the glass tube into any proper receiver. While it is rushing through the tube, it may be kindled with a taper, and it will burn with a long flame like a candle. In the burning of the gas, the hydrogen unites with the oxygen of the atmosphere, and the result of the combination is flame and water. It has been supposed that the torrents of rain, which generally accompany thunder storms, may arise from a sudden combustion of hydrogen and oxygen gases by means of lightning. Hydrogen gas is only one fourteenth of the weight of atmospheric air, and occupies a space fifteen hundred times greater than it possessed in its aqueous combination. It is continually emanating from vegetable and animal matters during their

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decay, and is evolved from various mines, volcanoes, and other natural sources. From its great levity it has general

ly been used to fill air-balloons.

Water is said to be in a state of composition with other bodies, because in many cases it becomes one of their com ponent parts. It is combined in a state of solidity in marble, in crystals, in spars, in gems, and in many alkaline, earthy,' and metallic salts, both natural and artificial, to all of which substances it imparts hardness, and to most of them transparency. Near the poles water is eternally solid; there it is similar to the hardest rocks, and may be formed by the chisel of the statuary, like stone. It becomes still more solid in the composition called mortar, and in cements, having parted with more of its caloric in that combination than it does in the act of freezing. If you take some ground plaster of Paris, fresh calcined, and mix it with a little water, the affinity of the plaster for the water is so great, that in a few minutes the whole will be converted to a solid.

QUESTIONS.-1. Of what is water composed? 2. In what four states is it found? 3. What is its most simple state? 4. What is the difference between liquid water or vapour and ice? 5. Why cannot water in an open vessel be made hotter than its boiling point? 6. How may an animal be frozen to death in the midst of summer? 7. Why would this happen? 8. Explain the extinguishing of fire by water. 9. What space does vapour occupy? 10. What is said of the expansive force of steam, and its probable application? 11. What is hydrogen, and how may hydrogen gas be obtained? 12. What is the result of kindling hydrogen gas on its rushing from the glass tube? 13. What is its weight and what space does it occupy? 14. In what substances is water combined in a state of solidity? 15. Why does water become solid in mortar and in cements? [NOTE. Hydrogen (pron. Hi'drō-jën,) takes its name from two Greek words signifying to produce water.]

LESSON 63.

The Earths and Alkalies.

The earths are silex, or silica, alumine, glucine, zircon, yttria, magnesia, barytes, strontites, and lime :-the four last mentioned are called alkaline earths.

Stra'ta (plural of stratum) beds, layers.

EARTHS are such incombustible substances as are not ductile, are mostly insoluble in water or oil, and preserve

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their constitution in a strong heat. Notwithstanding the varied appearance of the earth under our feet, and of the mountainous parts of the world, whose diversified strata present to our view substances of every texture and of every shade, the whole is composed of only nine primitive earths; and as three of these occur but seldom, the variety which is produced by the other six becomes the more remarkable. One of the most valuable earths with which we are acquainted is silex or pure flint. It is the most durable article in the state of gravel for the formation of roads; it is a necessary ingredient in earthenware, porcelain, and cements; it is the basis of glass, and of all vitreous substances. It is white, inodorous and insipid in its pure state, and the various colours, which it assumes in different substances, proceed from the different ingredients with which it is mixed. Alumine obtained its name from its being the base of the salt called alum. It is distributed over the earth in the form of clay, and on account of its aptitude for moulding into different forms, and its property of hardening in the fire, is employed for various useful purposes. In making earthenware, a due proportion both of silex and alumine are neces sary; for if alumine alone were used, the ware could not be sufficiently burnt without shrinking too much, and even cracking; and a great excess of silex would lessen the tenacity and render the ware brittle. Lime is never found pure in nature; it is obtained by decomposing calcareous matters by the action of fire, which deprives them of their acid. In its pure state it is used in many of the arts. It is employed by the farmers as a manure; and by bleachers, tanners, iron-masters and others, in their several manufactories, and in medicine. The use of lime in agriculture may be attributed to its property of hastening the dissolution of all animal and vegetable matters, and of imparting to the soil a power of retaining a quantity of moisture necessary for the nourishment and vigorous growth of the plants. Magnesia, besides being the basis of several salts, is of great use in medicine; and is employed by the manufacturers of enamels and porcelain.

The alkalies are distinguished by an acrid and peculiar taste; they change the blue juices of vegetables to a green, and the yellow to a brown, and have the property of rendering oils miscible with water. They form various salts by

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combination with acids, act as powerful caustics when applied to the flesh of animals, and are soluble in water, Potash and soda have been called fixed alkalies, because they will endure a great heat without being volatilized: and yet in a very high temperature they are dissipated in vapour. They were formerly considered to be simple substances, but they are now found to be compounds of metallic substances, called potassium and sodium, with oxygen. They have various uses in surgery and medicine, and are employed in large quantities by the glass-maker, the dyer, the soapmaker, the colour-maker, and by many other manufacturers. Ammonia is so extremely volatile as to exhale at all known temperatures. When combined with carbonic acid, it takes a concrete form, and a beautiful white colour, and is known in commerce by the name of volatile salts. With muriatic acid it forms what is termed sal ammoniac, which is employed in many of our manufactories, particularly by dyers to give a brightness to certain colours. In tinning metals it is of use to cleanse the surfaces, and to prevent them from oxydizing by the heat which is given to them in the operation. Ammonia is furnished from all animal substances by decomposition. The horns of cattle, especially those of deer, yield it in abundance, and it is from this circumstance that a solution of ammonia in water has been called hartshorn.

QUESTIONS.-1. What are earths? 2. What the names of the nine earths? 3. What is said of silex? 4. Of alumine ? 5. Of lime? 6. Of magnesia? 7. How are alkalies distinguished? 8. Why are potash and soda called fixed alkalies? 9. Of what are they compounds? 10. What is said of their uses? 11. From what is ammonia furnished? 12. What is said of its combinations and uses? [NOTE. Besides the nine earths, above enumerated, we have now thorina, which is a rare earthy substance lately discovered. A new alkali, called lithia, has recently been discovered, which, like potash and soda, is found to be a metallic oxyd: its base is called lithium. Three new vegetable alkalies have also been discovered, called morphia, picrotoxine, and vauqueline. Clay, as it exists in soils, is commonly called argillaceous earth; and lime in soils is called calcare. ous earth.]

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