128 GENERAL PRINCIPLES respiration, which have the best tendency to promote health, enjoyment, and cheapness of living; and it sets him on his guard against many unseen evils, to which those who are ignorant of its laws are continually exposed. In a word, from a speculative science, chemistry, since the middle of the eighteenth century, has become eminently and extensively a practical one. From an obscure, humble, and uninteresting place among the objects of study, it has risen to a high and dignified station; and instead of merely gratifying curio sity, or furnishing amusement, it promises a degree of utility, of which no one can calculate the consequences or see the end. QUESTIONS.-1. What does chemistry do for the husbandman? 2. For the manufacturer? 3. For the mechanic arts? 4. For the physician? 5. For the student of natural history? 6. For the public economist? 7. For the philanthropist? 8. For the domestic eco nomist? LESSON 59. General Principles of Chemistry. THE object of chemistry is to ascertain the ingredients of which bodies are composed,-to examine the compounds formed by those ingredients,-and to investigate the nature of the power which produces these combinations. The science therefore naturally divides itself into three parts: a description of the component parts of bodies, or of elementary or simple substances as they are called,-a description of the compound bodies formed by the union of simple substances, and an account of the nature of the power which produces these combinations. This power is known in chemistry by the name of affinity, or chemical attraction. By simple substances is not meant what the ancient philosophers called elements of bodies, as fire, air, earth, and water, nor particles of matter incapable of farther diminu tion or division. They signify merely bodies that have never been decomposed, or formed by art. The simple substances of which a body is composed are called the constituent parts of that body; and, in decomposing it, we separate its con stituent parts. If, on the contrary, we divide a body by cutting it to pieces, or even by grinding it to the finest powder, each of these small particles will consist of a portion of the several constituent parts of the whole body: these are called the integrant parts. Compound bodies are formed by the combination of two or more simple substances with each other. Attraction is that unknown force which causes bodies to approach each other. Its most obvious instances are, the gravitation of bodies to the earth; that of the planets towards each other, and the attractions of electricity and magnetism. But that attraction, which comes under the more immediate cognizance of chemists, subsists between the particles of bodies; and when it operates between particles of the same species, it is called the attraction of cohesion, or the attraction of aggregation; but when between the particles of dif ferent substances, it is called the attraction of composition, chemical attraction, or chemical affinity. The attraction of cohesion, then, is the power which unites the integrant particles of a body: the attraction of composition that which combines the constituent particles. When particles are united by the attraction of cohesion, the result of such a union is a body of the same kind as the particles of which it is formed; but the attraction of composition, by combining particles of a dissimilar nature, produces compound bodies quite different from any of their constituents. If, for instance, you pour upon a piece of copper, placed in a glass vessel, some of the liquid called nitric acid (aqua fortis) for which it has a strong attraction, every particle of the copper will combine with a particle of acid, and together they will form a new body, totally different from either the copper or the nitric acid. If you wish to decompose the compound which you have thus formed, present to it a piece of iron, for which the acid has a stronger affinity than for copper; and the acid will quit the copper to combine with the iron, and the copper will be what the chemists call precipitated, that is to say, it will be thrown down in its separate state, and reappear in its simple form. In order to produce this effect, dip the blade of a knife into the fluid, and when you take it out you will observe that, instead of being wetted with a bluish liquid like that contained in the glass, it will be covered with a thin coat of copper. 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 magnetism 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 calorie 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 If 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. 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, |