The simple substances were said very lately to amount to more than fifty in number, but since the truly interesting and very important discoveries of Sir Humphrey Davy, and other eminent chemists, it is scarcely possible to say what substances are not compound bodies. But it will be most. conducive to science to consider all those substances as simple, which no mode of decompounding has yet been discovered. Simple substances naturally divide themselves into two classes. Those which belong to the first class are of too subtile a nature to be confined in any of the vessels which we possess. They do not sensibly affect the most delicate balance, and they have received therefore the name of imponderable bodies. The second class of bodies may be confined in proper vessels, may be exhibited in a separate state, and their weight and other properties may be determined. They have received the name of ponderable bodies..The imponderable bodies at present supposed to exist are four, light, heat or caloric, electricity, and magnetism. The first three are intimately connected with chemistry, but mag-. netism has with it no known connexion.

QUESTIONS.-1. What is the object of chemistry? 2. How does the science divide itself? 3. What is meant by simple substances? 4. What is the difference between decomposition and division? 5. How are compound bodies formed? 6. What is attraction and its most obvious instances? 7. Define attraction of cohesion and attraction of composition. 8. What are the results of each of these kinds of attraction? 9. What example is given to illustrate chemical affinity or attraction? 10. How may you decompose the body thus formed? 11. Define the chemical term precipitate. 12. What is said of the number of simple substances? 13. Into what two classes are they divided? 14. What is stated as the ground of this division? 15. What are the four imponderable bodies?

LESSON 60.

Caloric,

Chem'ically, when a mere mixture of two or more substances is made, they are said to be mechanically united; but when each or either substance forms a component or constituent part of the product, the substances have formed a chemical union.

HEAT is a well known sensation which we perceive on touching any substance whose temperature is superior to

that of the human body. Chemists have agreed to call the matter of heat caloric, in order to distinguish it from the sensation which this matter produces. Caloric has a tendency to diffuse itself equally among all substances that come in contact with it. If the hand be put upon a hot body, part of the caloric leaves the hot body, and enters the hand; this produces the sensation of heat. On the contrary if the hand be put upon a cold body, part of the caloric contained in the hand leaves the hand to unite with the cold body; this produces the sensation of cold. If you pour warm water into one basin, cold water into a second, and a mixture of hot and cold water into a third; then put the one hand into the cold water and the other into the warm, for two minutes, and after that put both hands into the lukewarm water, to the one hand it will feel cold and to the other hot. Persons ascending from the burning shores of Vera Cruz, on the road to the mountain land of Mexico, will feel the climate become colder, and will put on their great coats, and yet they will meet people descending complaining of the heat. Cold therefore is nothing but a negative quality, simply implying the absence of the usual quantity of caloric.

Caloric is uniform in its nature; but there exist in all bodies two portions, very distinct from each other. The one is called sensible heat, or free caloric; the other latent heat, or combined caloric. Sensible caloric is the matter of heat disengaged from other bodies, or, if united, not chemically united with them. Latent caloric is that portion of the matter of heat which makes no sensible addition to the temperature of the bodies in which it exists. Wrought iron, though quite cold, contains a large portion of latent caloric; and if it be briskly hammered for some time on an anvil, it will become red hot by the action of this species of caloric, which by the percussion of hammering is now evolved and forced out as sensible heat.

Caloric pervades all bodies; and this is not the case with any other substance with which we are acquainted. It combines with different substances, however, in very different proportions; and for this reason, one body is said to have a greater capacity for caloric than another. When gaseous substances become liquid, or liquid substances solid, by this change of state they lose in a great measure their capacity for caloric. During the slaking of quick-lime, the caloric

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 instru ment 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,

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. lf 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 gråduated?

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 different 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