ix INDEX showing the subjects required for the "Chemical" branch of the Matriculation Examination, and the page and paragraph where Phosphorus. Combining proportions by weight and by General nature of acids, bases, and salts The atmosphere—its constitution; effects of 194-198 XVI. XV. 327-339 Chemical peculiarities of natural waters, such as rain-, river-, spring-, and sea-water Carbonic acid........ 199-202 340-349 XVII. ELEMENTS OF HEAT AND NON-METALLIC CHEMISTRY. PART I. НЕАТ. CHAPTER I. GENERAL NOTIONS. § 1. HEAT is that physical force which causes, or tends to cause, change in the temperature of bodies. § 2. All known matter contains more or less heat. It receives heat when it becomes warmer; it gives heat out when it becomes colder, in all cases unless change of physical state takes place. There is therefore no such thing as cold, except as the comparative absence of heat. § 3. When two bodies of different temperature are in contact with one another, heat always passes from the warmer to the colder body, until the temperature of the two becomes the same. § 4. A body is, in familiar language, called hot or warm according as it gives more or less heat to the hand, or other part of the body in contact with it. It is said to be cool or cold according B as the hand gives less or more heat to it. Thus, in fig. 1, the direction of the arrows may represent the direction in which the heat moves (to or from the hand), and the length of the arrows may represent the relative quantity of heat which passes. Hence all bodies feel hot or warm whose temperature is higher than that of the blood; they feel cool or cold when their temperature is lower than that of the blood. § 5. The sense of heat is conveyed to the brain by the same nerves as convey the sense of touch; so that most parts of the surface of the body, except the hair and nails, are sensitive to change of temperature-that is, to gain or loss of heat. If only a small part of the body be subjected to violent change of temperature, the sensation produced may be the same, whether that change be a gain or loss of heat. § 6. The action of heat on inanimate matter is recognized through the eye chiefly:-(a) by the way in which bodies are affected by it, in regard to size, shape, and physical state (change of solids, liquids, and gases into one another); (b) by the changes it causes in the luminosity, colour, and other optical properties of the substances upon which it acts; (c) by its influence upon magnetism and electricity; (d) by its effect upon the chemical constitution of bodies. Of these visible effects of heat we have only to consider (a) and (d); the latter can be discussed only after gaining some idea of chemical change (Part II.). § 7. Little is known concerning the nature of heat. It is, with great reason, supposed to consist of vibrations in some unknown medium or æther, which is imagined to pervade all space and all matter. According to this supposition, one body is hotter than another when the vibrations of the æther of the first are more violent than those of that of the second. One body gives heat to another by communicating its ætherial vibrations to the æther of the second, and so on. It is, however, simpler, and will be amply sufficient for our purpose to conceive heat as something without appreciable weight or mass, and whose loss or gain by bodies produces certain sensational, physical, and chemical effects. CHAPTER II. SOURCES OF HEAT. § 8. Strictly speaking, all known bodies are possible sources of heat, because they all contain heat, and will give up part of that heat to bodies of a lower temperature. But certain bodies, and bodies under certain conditions, are so much hotter than neighbouring bodies and give heat to the latter so habitually, that they are looked upon as special sources of heat. § 9. The Sun is the chief source whence the earth receives heat. It is calculated that the earth receives every year from the sun enough heat to melt a coating of ice enclosing the earth of a thickness of 34 yards. Probably also the fixed stars give a little heat to the earth; but neither their heat, nor that, if any, given to the earth by the moon, has been determined, being too small for measurement. § 10. The earth itself is, in all probability, very much hotter towards the interior than it is on the surface. That such is the case is rendered probable by the occasional breaking forth of * What is usually called "the dynamical theory of heat," supposes that the particles of the matter itself are in vibration. This hypothesis, alone, appears incompetent to account for the transmission of heat through space void of ponderable matter, as in the case of solar heat. |