sive: it is less porous, is infusible, and has the great merit of bearing great changes in temperature without risk of fracture. Porcelain or wedgwood crucibles are fragile, and have to be very gradually cooled to prevent breakage. Fletcher's gas crucible furnace (see Fig. 95) is very useful in this connection. Of the metals used in making crucibles,

platinum is superior to any its well-known power of resisting fusion, its cleanliness, and its non-liability to be acted upon by most chemical substances render it invaluable to the chemist, notwithstanding its costliness (see Fig. 96).

The following processes require the application of high heat:

1. Ignition, in the sense in which it is used in the Pharmacopoeia and by chemists generally, is the process of strongly heating solid or semisolid substances, the residue left at the conclusion of the process being the object sought. The U.S. 1890 quantitative tests for potassium bitartrate, sodium benzoate, and purified antimony sulphide afford examples of the use of this process.

2. Fusion is the process of liquefying solid bodies by the application of heat without the use of a solvent: the melting of wax, and the preparation of moulded silver nitrate, are familiar examples of this process.

3. Calcination is the process of separating volatile substances from fixed inorganic matter by the application of heat without fusion: its principal application in pharmacy is in the expulsion of water and carbonic acid from carbonates, as shown in the processes for making magnesia, lime, etc.

4. Deflagration is the process of heating one inorganic substance with another capable of yielding oxygen (usually a nitrate or a chlorate); decomposition ensues, accompanied by a violent, noisy, or sudden combustion. Deflagration is used in making some of the salts of antimony and arsenic, and in some qualitative analytical examinations.

5. Carbonization is the process of heating organic substances without exposure to air until the volatile products are driven off, and the residue assumes the black color characteristic of free carbon or charcoal. The manufacture of bone-black and wood charcoal affords good illustrations.

6. Torrefaction (known also as roasting) is the process whereby organic substances have some of their constituents modified by the application of a degree of heat somewhat less than that necessary to carbonize them. The most familiar example of this process is the roasting of coffee. Rhubarb in coarse, dry powder, when subjected to this process, loses its cathartic properties, but retains its astringent qualities, and is known as Torrefied Rhubarb.

7. Incineration is the process of heating strongly, organic substances with access of air until all the carbon is consumed, the ashes which remain being the object sought. The process is frequently used in analysis to determine the amount of fixed matter in an organic substance. 8. Sublimation is the process of separating a volatile solid substance from one which is not volatile by the application of heat. A special chapter on this subject will be found in the succeeding pages.

OPERATIONS REQUIRING HEAT IN WHICH LOWER TEMPERATURES ARE USED.

In this class of operations will be found the most important of those requiring the application of heat; almost all medicinal substances have their properties altered by the action of heat, and many cases are met with where it is necessary to moderate carefully the heat in order to prevent the decomposition or destruction of the active agent; for the purpose of controlling heat various baths are used, as the sand-bath, oilbath, solution-bath, steam-bath, water-bath, etc.

FIG. 97.

The sand-bath is usually an iron vessel of hemispherical or other convenient shape, containing dry, clean sand (see Fig. 97); the vessel to be heated is embedded in the sand, and the bath is then heated to the required degree. The object of this form of bath is to equalize the temperature, and to prevent a too sudden rise or fall of heat whereby unequal expansion or contraction might cause fracture to a glass or porcelain vessel being heated. Iron-wire clippings have sometimes been substituted for sand, with doubtful advantage, however.

Sand-bath.

The practical error usually made by inexperienced operators in the use of the sand-bath is in permitting too large a body of sand to rest between the bottom of the vessel to be heated and the flame; this results in an unnecessary waste of heat.

The oil-bath is designed to furnish a regulated temperature below 260° C. (500° F.). A fixed oil is the medium usually employed for communicating the heat, but one of the best substitutes for oil is petrolatum. Most fixed oils, when heated above 177° C. (350° F.), evolve disagreeable fumes.

In fractional distillation on a large scale, oil-baths are often used to control temperature, and the fumes arising from the heated oil are carried off by a pipe to the chimney.

The glycerin-bath.-In order to avoid the disagreeable odors arising from hot oil, glycerin is sometimes substituted. Acrolein, an acrid, volatile product, however, is produced if glycerin is heated nearly to

boiling. A temperature of 250° C. (482° F.) can be maintained in a glycerin-bath without much inconvenience.

Salt-water baths are sometimes used in special operations; their principle of action depends on the fact that the boiling-point of a liquid is raised in proportion to the quantity of fixed salt dissolved in it. Water, as is well known, boils at 100° C. (212° F.), but if common salt is dissolved in water until it ceases to take up any more, and a saturated solution is produced, it is found that this solution does not boil until the temperature of 108.4° C. (227.1° F.) is reached. The following table shows the boiling-point of certain saturated solutions as determined by Legrand and others:

Table of Boiling-Points of Saturated Solutions of various Salts.

The water-bath is one of the most useful of all the forms of pharmaceutical apparatus for regulating temperature, and the frequency with which it is directed to be used in works of authority indicates its importance as a necessary implement in the equipment of every pharmaceutical laboratory. Almost all the water-baths used by pharmacists are extemporized, and these are generally crude and inconvenient; two dishes usually suffice, one of them somewhat larger

FIG. 98.

Water-bath.

in diameter than the other. Water is poured into the larger dish, and the other dish, containing the liquid to be heated, is placed in the water and the heat applied; the room is soon filled with the escaping steam, and in winter the condensation of the moisture upon the windows is alone a sufficient inconvenience to render it undesirable. Fig. 98 shows a tinned copper water-bath in which

this annoyance is overcome. The water-level has at its lowest point a piece of block-tin tube soldered in; this extends half-way up the glass tube in the inside, whilst a perforated cork at the upper end of the glass tube permits the insertion of another piece of block-tin tube; the upper

tube connects with the cold-water faucet and terminates in the smokeflue or with the outside air; the vapor arising from the boiling water either passes off into the chimney, as shown by the arrow, or is condensed, the loss being supplied by a small stream of water from the cold-water faucet, shown by the arrow pointing downward; the lower block-tin tube acts as an overflow, the excess of water being carried off by a rubber tube into the sink; all possibility of the water-bath "boiling dry" is thus obviated. Vapors from the liquid in the water-bath may be carried off by a hood (see Fig. 129). A simple water-bath may be made by encasing a tinned-copper round-bottomed dish in one of larger diameter having a flat bottom. Water is poured in through a tubulure in the top, and it is replenished as required. Fig. 99 shows a similar water-bath, a porcelain evaporating dish taking the place of the copper

FIG. 99.

one. It is useful where a metallic dish would be acted on by the substance to be

FIG. 100.

Water-bath (porcelain dish).

Water-bath (copper ring).

heated. A water-bath intended for the smaller operations of analytical chemistry is shown in Fig. 100. The different sizes of the rings render it convenient for vessels of various shapes and sizes. It will be necessary to allude frequently hereafter to the uses and modifications of the water-bath.

THE USE OF STEAM IN PHARMACEUTICAL OPERATIONS.

The scope of this work will not permit of any extended consideration of the use of steam in technical pharmacy, yet it is of vital interest to be acquainted not only with the theories underlying its employment, but also with the apparatus used in its practical application.

When water is heated to the boiling-point and steam is produced, a certain amount of heat is absorbed (or apparently lost): this has been termed latent heat. When steam comes in contact with surfaces having less heat than itself, it is condensed, water is produced, and the latent heat becomes sensible (or reappears), thus proving the well-established physical law that when a liquid assumes the gaseous state, a certain fixed and definite amount of heat disappears; and, conversely, when a gas or vapor becomes a liquid, heat to a corresponding extent is evolved. Watts has illustrated this as follows: "When water at 0° C. is mixed with an equal weight of water at 100° C., the whole is found to have the mean of the two temperatures, or 50° C. On the other hand, 1 part by weight of steam at 100° C., when condensed in cold water, is found to be capable of raising 5.4 parts of the latter from the freezing-point to the boiling

point, or through a range of 100° C. Now, 100 x 5.4-540; that is to say, steam at 100° C. in becoming water at 100° C. parts with enough heat to raise a weight of water equal to its own (if it were possible) 540° of the Centigrade thermometer, or 540 times its own weight of water one degree of the same." When water passes into steam the same quantity of sensible heat becomes latent. A consideration of these facts in physics leads to the practical application of steam as a transmitter of heat, whereby heat from any source may be absorbed by steam and carried through suitable pipes to the vessel designed to be heated. If this vessel is filled with a cold liquid, the latent heat of the steam is rapidly communicated to the liquid, the steam is condensed, and the result is this most convenient and economical method of producing a temperature which is capable of being regulated with great exactness. Steam-baths may be divided into two classes: 1, those in which steam is used without pressure; and, 2, those in which steam is used under

pressure.

1. The use of Steam without Pressure.-In many cases open steam, as it is termed, is used (see Fig. 101). The pipe which conveys the steam from the boiler is conducted to the bottom of a hemispherical kettle, and the liquid to be heated is poured into a dish of larger diameter, which is placed upon the top; the steam is turned on, and as it condenses is carried off by the drip-pipe. A temperature of about 100° C. (212° F.) can usually be maintained by this method.

Sometimes the steam-pipe is conducted directly from the top into the liquid to be heated. A steam-distributor, as shown in Fig. 102, may be used at the end of the pipe near the bottom of the kettle; it is made by screwing a cross upon the end of the pipe, and an elbow to each arm

of the cross; the steam issues usually with some force from each elbow and effectually stirs up the liquid, and rapidly produces a uniform temperature in it. The principal disadvantages about using steam in this way are the noise at first produced by the contact of the hot steam with the cold liquid, and the increase in bulk of the liquid through the condensation of the steam.

2. The use of Steam under Pressure.-This is by far the most convenient method of using steam practically as a means of transmitting heat. It has been stated that steam produced in open and unconfined vessels, with the ordinary pressure of the atmosphere, has the temper