which is evolved escapes from the water in consequence of its changing from a liquid to a solid form by its union with the lime, When solid bodies become liquid or gaseous, their capacity for caloric is proportionately increased. If you place a glass of water in a mixture of equal quantities of snow and salt, during their conversion to a liquid, the water will be frozen in consequence of parting with its caloric to supply the increased capacity of the mixture.

The portion of caloric necessary to raise a body to any given temperature is called its specific caloric.

The instrument in common use for measuring the temperature of bodies is called a Thermometer. It consists of a glass tube containing a portion of mercury, with a graduated scale annexed to it. It is constructed in the following manner. A small bulb is blown on the end of the tube, and this bulb and a part of the tube are to be filled with mercury which is to be heated till it boils. This ebullition forces out the air and the tube is hermetically sealed while the mercury is boiling. The next object is to construct the scale. It is found by experiment, that melting snow or freezing water is always at the same temperature. If, therefore, a thermometer be immersed in the one or the other, the mercury will always stand at the same point. It has been observed, too, that water boils under the same pressure of the atmosphere at the same temperature. A thermometer, therefore, immersed in boiling water, will uniformly stand at the same point. Here, then, are two fixed points, from which a scale may be constructed, by dividing the intermediate space into equal parts, and carrying the same divisions as far above and below the two fixed points as may be wanted. When a thermometer is brought in contact with any substance, the mercury expands or contracts till it acquires the same temperature; and the height at which the mercury stands in the tube, indicates the exact temperature of the substance to which it has been applied. It will not show the absolute caloric in substances; for it cannot measure that portion which is latent, or chemically combined with any body.

Caloric is the cause of fluidity in all substances capable of becoming fluid, from the heaviest metal to the lightest gas. It insinuates itself among their particles and invariably separates them in some measure from each other. Thus ice is converted into water, and by a further portion of caloric,

ATMOSPHERIC AIR.

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into steam. We have reason to believe that every solid substance on the face of the earth might be converted to a fluid, or even to a vapour or gas, were it submitted to the action of a very high temperature in peculiar circumstances. Some bodies give out their superabundant caloric much sooner than others. Iron is a quicker conductor of caloric than glass, and glass than wood. If you take a piece of iron in one hand, and a piece of wood in the other, the iron feels cold, the wood warmer, though the thermometer shows that their temperature is the same. Substances usually become

more dense by the loss of caloric; but the freezing of water is a striking exception to this general law of nature, and is a memorable instance of the wisdom and provident care of the Almighty, when he established the laws of the universe.

QUESTIONS.-1. What is heat? 2. Why is the matter of heat called caloric? 3. How are sensations of heat and cold produced? 4. What is cold? 5. What is sensible caloric? 6. Latent caloric? 7. What experiment illustrates this? 8. Why is one body said to have a greater capacity for caloric than another? 9. How do bodies lose their capacity for caloric? 10. Why is caloric evolved during the slaking of quick-lime? 11. When is a capacity for caloric increased. 12. Describe the experiment. 13. What is specific caloric? 14. Of what use is a thermometer? 15. Of what does it consist? 16. How is it constructed? 17. How is caloric the cause of fluidity? 18. What is said of conductors of caloric? 19. To what general law of nature is the freezing of water an exception? 20. What are the different kinds of thermometers? (See Appendix.) 21. How is each graduated?

LESSON 61.

Atmospheric Air.

Gas. When solid substances are rendered permanently aëriform by heat, the air, thus produced, is called a gas. All the gases are compounds of solid matter and caloric. It is caloric which separates the particles, and gives to the whole a gaseous form. The permanency of the gases appears to be owing to the strength of the affinity existing between caloric and their bases, which affinity resists every reduction of temperature.

THE atmosphere, which was formerly supposed to be a simple fluid, is composed of two distinct substances, termed oxygen gas and nitrogen gas. It is not a chemical compound, but a mere mixture of those gaseous substances in

the proportion of 21 of the former and 79 of the latter. It contains also about one part in every thousand of carbonic acid gas, a considerable portion of water in a state of elastic vapour, and several adventitious substances.

Oxygen is an element or simple substance generally diffused through nature, though like caloric it does not exist by itself. It takes its name from two Greek words, signifying that which produces or generates acids, because one of its general properties is to form acids by combining with dif ferent substances, which are called the bases of the several acids. Its different combinations are essential to animal life and combustion. Acted upon, or combined with caloric, it becomes oxygen gas, which is distinguished from all other gaseous matter by several important properties. Inflammable substances burn in it under the same circumstances as in common air, but with infinitely greater vividness. If a taper, the flame of which has been extinguished, the wick only remaining ignited, be plunged into a bottle filled with it, the flame will instantly be re-kindled, and will be very brilliant, and accompanied by a crackling noise. If a steel wire, or thin file, having a sharp point, armed with a bit of wood in a state of inflammation, be introduced into a jar filled with the gas, the steel will take fire, and its combustion will continue, producing a most brilliant phenomenon. Oxygen gas is a little heavier than atmospheric air, and from its being absolutely necessary to the support of animal life, it has been called vital air.

Nitrogen is a substance diffused through nature, and particularly in animal bodies. It is not to be found in a solid or liquid state; but combined with caloric, it forms nitrogen, or azotic gas, in which no animal can breathe, or any combustible burn. It is uninflammable and somewhat lighter than atmospheric air, and though, by itself, it is so noxious to animals, it answers an important end when mixed with oxygen gas in atmospheric air. Were it not for this large quantity of nitrogen in the atmosphere, the stimulating power of the oxygen would cause the blood to flow with too great rapidity through the vessels; the consequence of which would be, that the life of man would not be protracted to the length that it now is. The vermilion colour of the blood is owing to the inhalation of oxygen gas. When the dark purple blood of the veins arrives at the lungs, it imbibes the

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 sinall, 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 ni trogen 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'e-jen,) 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

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