1000

blown out into a bulb at the lower end, to cause it to float; the under part is loaded with quicksilver or shot, to give it a vertical position. The narrow tube serves to denote the Fig. 5. depth to which it sinks in any liquid, by means of a scale of degrees, with which it is furnished. There are various instruments of this kind, especially adapted for determining the density of spirits, brandy, oil, lye, syrup, &c. If a hydrometer for weighing spirits is put into water, it sinks only to the lowest point on the scale, 0° (Fig. 5, a); but in the strongest alcohol, which is much lighter than water, it sinks to the highest point, 100. A scale for testing lye (Fig. 5, b) must, on the contrary, have the 0° point at the

100

top of the scale, to which it would sink in pure water; for, lye being heavier than water, the instrument would be more or less buoyed up in it, according to its strength. In hydrometers for lighter liquids, the degrees proceed from the bottom to the top; in those for heavier liquids, from the top downwards. In most of these scales, the degrees are arbitrary; and, in order to convert them into the corresponding specific numbers, tables, constructed for the purpose, must be referred to.

Experiment. Pour brandy into a cylindrical jar, and observe the degree which it marks on the hydrometer; then Fig. 6. put it in a warm place, and, when lukewarm, again

-a

note the degree, which will be higher than before, as the heat has expanded the liquid, made it lighter, and, consequently, apparently stronger than it really is (Chap. III.). The specific gravity of all bodies, when warmed, is less than when cold. On this account, in determining the density of bodies, regard should be paid to their temperature, and it has been agreed to consider 60° F. (15.5° C.) (Chap. III.) as the mean temperature.

In the more accurate hydrometers, the mercury serving as the counterpoise has been ingeniously contrived also to indicate the degree of heat of the liquid, by connecting with it a graduated tube. The small scale, a (Fig. 6), denotes the temperature, the long scale, b, the density. The small scale is frequently

so constructed, that the degrees correspond to those on the long scale, and in order to guard against error it is only necessary to add the degrees below the mean temperature to the density, or to subtract from the density those above.

=

Gold is nineteen times, and silver ten times, heavier than water; gold alloyed with silver must, therefore, have a less specific weight than pure gold. The specific weight of brass is only 8. Alcohol and ether are lighter in proportion to their purity and strength, while lye, syrup, the acids, &c., increase in density according to their purity. Hence it is evident how important it is, in many cases, to know the specific gravity of a body in order to judge of its quality.

НЕАТ.

We have already seen that heat is often given out during chemical action. When chemical force is exerted, heat is generally, if not always, produced; and, on the other hand, many chemical changes will not take place without the assistance of heat.

The artificial heat which is employed in chemical operations and in ordinary life is obtained from chemical action. Coal, wood, spirit, oils, and all the other kinds of fuel undergo a chemical.change when they burn, which will be fully explained hereafter. They combine with the oxygen of the air, and in doing so chemical force is consumed and changed into heat.

Sources of heat for chemical operations. If the student is fortunate enough to have gas at his command, he will seldom need any other fuel. The gas may be brought from the nearest burner, from which the jet has been screwed off, by means of an india-rubber tube. For most purposes, the burner known as the Bunsen burner (Fig. 7) is the best. The gas becomes mixed with air in this burner before burning, and burns therefore with a smokeless flame, which gives very little light, but an intense heat. Little iron caps, a (Fig. 7), are sold with each burner, which fit on it, and convert the single flame into a ring of

Fig. 7.

a

smaller jets. A small argand gas-burner with a copper chimney is also very convenient where gentle heat is required.

If gas cannot be obtained, a small spirit-lamp, such as shown in Fig. 8, may be used for ordinary purposes, and methylated spirit, which is very cheap, may be burned in it. More powerful spirit-lamps are, however, sold, and small oillamps may sometimes be employed for the sake of economy when the heat has to be employed for a long time. Furnaces are scarcely necessary to the beginner in chemistry. The few crucible operations he has to perform may be done in a common fire.

Fig. 8.

EFFECTS OF HEAT.

Expansion of Liquids. Experiment 1.-Counterpoise a flask that is, place it on one of the pans of a balance, and equipoise it by weights or shot put into the opposite pan; then fill it with water, and note the weight of the latter. Warm the flask on a tripod over a spirit-lamp or gas-burner, moving it round gently at first, that the flask may heat gradually. The water will soon rise, and part of it run over. When it begins to boil, remove the lamp, and let the vessel cool, and the water will then sink lower than it stood before. How much has been displaced is found by its loss in weight; it will amount

to about of the first weight.

The lamp heats the bottom of the glass vessel, which in its turn communicates heat to the water. The heat expands the water, consequently it occupies a greater space than before, and part of it must run over. Hence it follows that warm water must be lighter than cold water. If a pitcher filled with two pounds of ice-cold water be afterwards filled with boiling water, it will weigh about an ounce and a half less.

The same occurs with all other liquids, and, indeed, also with solids and gases; hence, it may be stated as a natural law, that all bodies expand by heat, and contract on cooling. But the amount of expansion is very different in different bodies at the same temperature; alcohol, for example, expands two and a half times more, mercury two and a half times less, than water. When fluids are to be bought and

sold by measure, an advantageous application may be made of this principle. If a hundred measures of brandy or alcohol are purchased in hot, and sold in cold weather, there would be a loss of four or five measures; therefore we should gain by buying in winter and selling in summer.

Experiment 2.-In order to observe more accurately the expansion of water by heat, adapt to a flask a cork, Fig. 9. rendered so soft by gently striking it with a piece of wood that it may be exactly fitted to the opening by mere pressure; perforate the cork with a round file, and make the hole just large enough to admit a glass tube tightly. Fill the flask with water, so that, when the cork is firmly pushed in, the water shall stand at about a (Fig. 9), and heat it as in the former experiment.

The water, which in the former experiment was displaced from the flask, now rises in the tube, and this higher in proportion to the smallness of its bore. By this means very slight changes of space are rendered visible, and these deviations may be applied to the measurement of heat. This is done by particular instruments called thermometers.

Thermometer.-Water might be employed for measuring heat, by marking the boiling and freezing points, and graduating the intervening space; but mercury is far better adapted to the purpose, as it boils and freezes at greater extremes of temperature, and becomes hot and cold more quickly; hence it more quickly denotes the variations of heat and cold: moreover it expands more uniformly than

water.

The vessel containing the mercury may also be regarded as consisting of a flask and tube, but which, instead of being joined by a cork, are composed of one entire piece. Having introduced into it a sufficient quantity of mercury, and sealed the open end by fusion, it is immersed in melting ice, and the point to which the quicksilver falls is marked freezing point; that to which it rises in boiling water, boiling point. The space between these two points can now be divided into degrees, to form the scale. The degrees below the freezing point must be of the same dimensions as those above. There are several scales in use, though it is to be regretted that

more than one has been adopted. The most common are the three following:-Reaumur's (R.), divided into eighty degrees; the centigrade of Celsius (C.), into one hundred; and Fahrenheit's (F.), into one hundred and eighty degrees. The difference between these can be easily seen in the annexed figure. According to R. water freezes at 0° and boils at 80°; according to C. it freezes at 0°, and boils at 100°; according to F. it freezes at +32°, and boils at 212°.

Fig. 10.

Fahrenheit, a philosophical-instrument maker, commenced reckoning very strangely, not at the freezing point, but at 32° below it. His scale is still in common use in England, and the high numbers found in English works are thus accounted for. In Germany, Reaumur's thermometer is used, except for scientific purposes, when the Centigrade, in common use in France also, is employed. In order to compare these thermometers with each other, it need only be remembered that 4° R. are as large as 5° C. or 9° F. In reducing the degrees of Fahrenheit's scale above 32° to those of the Centigrade, the number of degrees above or below 32° must be mulFig. 11. tiplied by 5 and the product divided by 9. And

to reduce those of Fahrenheit to Reaumur, the number of degrees above or below 32° must be multiplied by 4 and divided by 9. To the degrees above 0°, the sign is prefixed or understood; to those below, the sign-is invariably

added.

A cylindrical thermometer, graduated to about 600° F. (315° C.), is best suited for chemical experiments, as shown in the annexed figure, because it can be easily adapted to a perforated cork, and then fitted to a flask, in which liquids are to be heated to a certain temperature. The degrees above the boiling point are to be divided off at distances equal to those below. Mercury freezes at -38° F. (-39° C.). In the northern regions of the earth a degree of cold of 58° F. (-50° C.)