Figs. 62 and 63.) The deep sand-bath consists of an iron pot or basin containing sufficient dry fine sand so that, if desired, the retort or flask may be entirely surrounded by the same. The best shallow sand-baths are made of Russian sheet-iron, and are well adapted for heating flasks and beakers, which require only sufficient sand to form a good bed of support, since an excessive amount would involve a waste of heat.

For use in a laboratory where steam is available, a permanent sand-bath may be provided as shown in Fig. 64. It is constructed from an ordinary galvanized-iron sink and large gas-pipe, about three-quarters to one inch in diameter, arranged horizontally in folds, the ends of the pipe being introduced through holes of appropriate size drilled in the end of the vessel. Sand to the depth of two or three inches may be poured over the pipes, which will form an excellent bed for flasks, dishes, and beakers.

Other apparatus for the use of heat above that of boiling-water, yet avoiding contact with flame direct, are oil-baths, saline solution baths, glycerin baths, or paraffin baths; these are constructed like water-baths, and readily furnish temperatures ranging from 100° to 300° C. (212° to 644° F.).

For all operations requiring a degree of heat below that of boilingwater, water-baths will be found indispensable; they may be made with either a round or flat bottom, as shown in Figs. 65 and 66, and

provided with a set of concentric rings to adapt them for use with dishes or flasks of various sizes. Water-baths made of extra heavy tin will last a long time (provided they be dried properly after use), and do not cost much, while copper is far more expensive, but, on the other hand, resists the action of heat and water better than tinned iron. Ast long as the vapor of boiling water is allowed to escape freely, no amount of heat applied to the vessel can possibly increase the heat of the water above that of boiling, and, as some heat-power is lost during transmission from the water-bath to the vessel resting upon it, the liquid contained in such vessel will always be found a few degrees lower in heat than the water in the bath; under no circumstances can aqueous liquids be made to boil in dishes placed in water-baths.

The name vapor-bath is in the majority of cases more appropriate than water-bath, since the vessel heated by it does not, as a rule, come in contact with the water for any length of time, but derives its heat from the vapor or steam rising from the water and not confined by pressure.

To avoid frequent refilling and consequent interruption in longcontinued operations, water-baths are often provided with a constant supply attachment as shown in Fig. 67, which also serves to keep

a constant water-bath is that suggested by Dr. B. F. Davenport, of Boston, and shown in Fig. 68. It consists of a copper box, A, 10 or 15 inches square, the top being a brass plate inch thick, to enable it to bear considerable weight without yielding. From the point B projects a inch brass tube, B C, which turns up at a right angle. At E is a stopcock which is connected by a thick rubber tube with the glass tube, D F, the latter being fastened against the adjoining wall. Connected with C by a rubber tube-joint is a inch block tin tube of 20 feet length, which extends up the wall, to which it is fastened for 10 feet to the point T, whence it returns and ends just over the top of the glass tube at D. The bath is filled with water (preferably distilled) to just the level, B..b. The steam generated by the constant boiling is condensed in the tube, CT D, either before or after reaching the top, T, and returns to the bath at Cor at D, where it drops into the glass water-gauge, D F. Having once been filled, the water need not be replenished for years, and there being no outlet for the steam, except into the condensing tube, the air surrounding the water-bath will be kept constantly dry-a very

desirable point in the evaporation of liquids. If the water-bath is desired for use at fixed temperatures a thermometer may be intro

FIG. 68.

T

duced through a cork fitted to a tube inserted in the cover of the bath.

The boiling-point of a liquid is that at which the elasticity of its vapor overcomes the pressure of the surrounding atmosphere, or, in other words, beyond which the liquid cannot continue as a liquid without increased presNormal atmospheric

sure.

pressure, 15 pounds to the square inch, which is equal to the pressure of a column of mercury 760 Mm. (29.87+ inches) in height, is always assumed when referring to the boiling-point of a liquid, for any modification of the former will change the latter; thus water, which ordinarily boils at 100° C. (212° F.), has been known to boil at 84° C. (183.2°F.) on Mont Blanc, in Switzerland, and even at 35° C. (95° F.) in a vacuum apparatus; while, under greatly increased pressure, as in Papin's digester, it has been heated to 160° C. (320° F.) without boiling. There exists also a great variability in the boiling-points of different liquids under normal conditions; for, while official ether boils at about 37° C. (98.6° F.), chloroform requires a temperature of 60.5° C. (140.9° F.), alcohol 78° C. (172.4° F.), glycerin 165° C. (329° F.), and mercury about 357° C. (674.6° F.).

E

Davenport's constant water-bath.

b

The simplest method for determining the boiling-point of a liquid is to introduce some of it into a flask provided with a lateral tube in the neck and a thermometer passing through the cork, as shown in Fig. 69, or into an ordinary Florence flask provided with a doubly

perforated cork, through one orifice of which a thermometer is inserted and through the other a bent glass tube, as represented in Fig. 70. If inflammable or noxious vapors are likely to be evolved, the tube from either flask may be connected with a condenser. It is important that the thermometer should not be immersed in the liquid, but only introduced into the flask so far that the bulb may be enveloped by

Flasks arranged for finding the boiling-point of a liquid.

the vapor of the boiling liquid, as shown in the illustrations. Heat should be carefully applied and gradually increased until the liquid boils actively, at which time the boiling-point will be indicated by the height of the mercurial column in the thermometer. In the case of very accurate determinations, it may be necessary to make corrections for increased or decreased atmospheric pressure, and according to Kopp the correction amounts to 1° C. (1.8° F.) for every 27 millimeters above or below the normal height of the barometer column of mercury. In order to avoid errors, which might arise from the cooling of the long mercurial column outside of the flask, specially constructed thermometers, known as Zincke's thermometers (see page 90) are usually employed for temperatures above 100° C. (212° F.).

Fusible substances, when gradually heated to their melting-point, do not all behave in the same manner; as a general rule, crystallizable bodies become brittle just before melting, while non-crystallizable substances assume a plastic condition. When fusion commences they

combine, as it were, with heat in an intimate manner; that is, they occlude heat, so that the further addition of heat does not cause any rise in temperature until all of the substance has become liquefied. The heat thus disappearing is called the latent heat of fluidity, because it is used to change the solid form of a body into the liquid form without any change in the temperature of the body; thus if crushed ice be heated, the temperature will not vary from 0° C. (32° F.) while the ice is melting, and when completely changed to water, the temperature of the water will also be 0° C. (32° F.), provided the application of heat be not continued beyond fusion. The amount of heat necessary to produce complete fusion varies with different substances; thus in the case of ice it has been ascertained to be 79.25 C. or 142.65 F. degrees; this was determined as follows: Two vessels, containing respectively equal weights of ice and water at 0° C. (32° F.), and each provided with a thermometer, were heated in a bath of water; at the moment when the ice had completely melted the temperature was indicated as still at 0° C. (32° F.), while the temperature of the water in the other vessel had risen from 0° C. (32° F.) to 79.25° C. (142.65° F.). If a pound of ice at 0° C. (32° F.) and a pound of water at 100° C. (212° F.) be mixed so as to avoid loss by evaporation, the result, when all the ice has melted, will be two pounds of water at 10.4° C. (50.7° F.); whereas if a pound of water at 0° C. (32° F.) be mixed with a pound of water at 100° C. (212° F.), the result would be two pounds at 50° C. (122° F.). In the first case, 79.25 C. (142.65 F.) degrees of heat were withdrawn from the boiling water to melt the ice at 0° C. (32° F.) into water at 0° C. (32° F.), but in the second case this was not necessary, and the mixture assumed the mean temperature of the two. The latent heat of fluidity of water being known as 79.25° C., a simple rule can be formulated for ascertaining the amount of ice necessary to reduce any given weight of water at stated temperature to a stated lower temperature, as follows:

Add the desired temperature to 79.25°C. (142.65° F.) and divide the sum into the difference between the stated temperature of the water and the desired temperature-the quotient will be the required proportion of ice as compared with the given weight of water.

Example: How much ice is required to cool 1000 Gm. of water from 100° C. to 25° C.?