A popular form of this instrument is the so-called "self-registering thermometer," in which a portion of the mercury rises to the greatest height in the operation, and does not return with the rest of the mercury in contracting, thus leaving a register of the patient's temperature for examination by the physician on his return visit.

This break in the mercury column must be remedied before the thermometer is used for another operation, and this can be done very simply by grasping the thermometer by the upper end, raising same above the shoulder, and giving it a downward circular sweep. This, repeated several times, will remedy the break. The same treatment applies to a chemical thermometer when such a break in the filament occurs. In some cases, however, the break is so persistent as to resist these efforts, when it can be brought down by gently warming the thermometer until the mercury rises to the broken part and forces same up, and then shaking the mercury down as above mentioned.

Other Varieties of Thermometers.-In pharmacy the thermometers above mentioned are practically the only ones employed, and to such thermometers the measure of the temperature is limited to minus 40° C., the freezing-point of mercury, and to plus 360° C., the boiling-point of mercury. In technic the measurement of higher temperatures is often desired, and for such purposes modifications called "pyrometers" are employed. The simplest form of pyrometer is the iron bar mentioned on p. 76. In another form the bulb and tube are made of finest porcelain, filled with air, which expands on heating, and the expansion is measured by the pressure of the air on a mercury column.

A more modern form of pyrometer is one in which electricity is employed, this acting on the principle that certain substances on heating show more resistance to electricity than when cold; indeed, in exact proportion to the increase of temperature. Therefore in such an instrument a current of electricity is conducted through the metal, which acts as the thermometer, and the temperature is estimated from the reading of the resistance gauge, or ammeter.

CHAPTER V

APPLICATIONS OF HEAT

IN studying the dozen or more pharmaceutic processes involving the use of heat, we note that they can be best understood when presented in the following tabulated form:

I. Chemical change in the heated product:

1. Substance is inorganic, is strongly heated, and gives off gas.

2. Substance is organic.

(a) CO, eliminated, calcination.

(b) Gas eliminated with a flame, ignition. (c) Gas eliminated with crackling sound, deflagration.

(a) Strong heat without access of air, carbonization.

(b) Strong heat with access of air, incineration.

(c) Moderate heat only, torrefaction.

II. Physical change produced in heated product:

1. Solid temporarily changed to liquid, fusion.

2. Solid or liquid temporarily changed to gas, vaporization.

The modifications of vaporization can best be tabulated as follows:

Discussing the several processes individually we find the following points of interest and importance.

Calcination is the process of strongly heating a carbonate, whereby the gas, carbon dioxide, is driven off and the oxide of the metal remains. Note the similarity between the words "calcination" and "calx." Calx is the Latin name for lime, calcium oxide, CaO, which is made by strongly heating calcium carbonate, CaCO,, when carbon dioxide, CO2, passes off. By the identical process-calcination-the oxides of magnesium and zinc are produced, as the following comparison shows:

The results of above subtraction-the remaining oxides-are said to be "calcined."

While the largest operations of calcination are performed in huge plants of masonry called lime-kilns, in the laboratory the substance to be calcined is usually placed in an evaporating dish (see p. 94) or in a crucible, the source of heat being a blast lamp.

Crucibles are utensils of various sizes, shapes, and materials, according to the objects for which intended. The name crucible dates from the middle ages, when the alchemists (the forerunners of the chemists), in invoking Divine aid in their operations, marked utensils of this character with the sign of the cross (Latin, "crux").

The modern crucibles are made of the following materials:

Fig. 47.-Hessian crucible.

Fire-clay or Hessian crucibles (Fig. 47) are very cheap and convenient apparatus for conducting high heat experiments. They usually come nested, in sizes ranging from 15 to 500 Cc. Their chief drawback is that they are poorly annealed, and are therefore liable to break. Moreover, they are too porous to hold water.

Graphite crucibles are more expensive, but are durable. They are commonly called "black-lead crucibles," and are of the same material

of which the "lead" of pencils and the cake stove-blacking is composed. They are used more for chemical than for pharmaceutic work, as is also that form of crucible, "the cupel," which is made of bone-ash and used exclusively in metallurgy.

Porcelain crucibles are the most popular of all forms of crucibles, they being comparatively durable, easy to clean, and moderate in price. They can be obtained in all sizes, but the most convenient are those of 15 to 30 Cc. capacity (Fig. 48).

Platinum crucibles are too expensive to be of ordinary pharmaceutic use, but the purchase of a platinum crucible is by no means a bad investment, as when the instrument has become worn out, the platinum of which it is composed can be sold at a price but little below its original cost. The most convenient size of platinum crucible is one of 10 to 15 Cc. capacity. They are of great value, because durable and because possessing powers of resistance against chemicals far greater than other metals. This great advantage must not be abused, however, for it should be clearly remembered that sulphides, hypophosphites, potassium and lead salts affect platinum, ruining the crucible in which they are heated.

Fig. 51.-Heating crucible: a, Crucible; b, triangle; c, tripod; d, Bunsen burner.

Appliances necessary to the successful handling of crucibles are crucible tongs (Fig. 50), which are used for lifting crucibles and triangles on which they rest while being heated. These, as shown in cut, primarily consist of iron wire covered by clay piping, hence obtainable at small cost. The triangle rests on or is supported by a tripod (Fig. 51) or else on the ring of a retort stand. Ignition is the process of heating inorganic matter strongly, whereby certain portions are driven off in gaseous forms (usually with formation of a flame), leaving the non-volatile residue. This process is used in pharmacy in some analyses, for example: the United States Pharmacopoeia of 1890 directed in a test for sulphurated antimony, that the same, on being heated, should lose its sulphur by ignition, leaving a residue consisting chiefly of antimony oxide. Likewise ignition is

used in the burning of iron pyrites, when the sulphur which it contains combines with the oxygen of the air to form the gas sulphurous oxide, leaving a residue (ferric oxide) behind.

The ignitions of pharmacy are usually performed on platinum foil, in crucibles, or in evaporating dishes, or even on the end of a spatula or knife-blade. Thus the presence of calcium sulphate in a sample of precipitated sulphur is easily detected by igniting a bit of the suspected chemical on the blade of a spatula, when it leaves a residue only in case calcium sulphate is present.

Deflagration is the process of strongly heating a substance readily yielding oxygen, when this element is evolved with a characteristic crackling sound. This phenomenon occurs when potassium nitrate or potassium chlorate is heated, and the process is quite different from that of "decrepitation," which will be explained in the chapter on Crystallization.

Deflagration is used in no process of pharmacy, but it is of value in the testing of some official chemicals, such as potassium nitrate. To produce deflagration the salt is sprinkled on hot coals, or is heated on platinum foil or is placed in a deflagrating spoon (Fig. 52).

Carbonization is the process of strongly heating an organic substance (that is, a substance containing carbon) in a vessel protected from access of air. Under this treatment the hydrogen and oxygen usually found in organic substances associated with carbon pass off in the form of water, leaving behind the carbon or charcoal.

This process is rarely employed in pharmacy, and when utilized may be performed in a rather tightly covered crucible. The typical example of carbonization is the manufacture of charcoal,-"the charcoal burning, "-an industry of the southern United States.

If carbonization is carried on in a retort and the volatile products collected, the process is called destructive distillation. By such destructive distillation of wood, wood-alcohol, acetone, and acetic acid are now made.

Fig. 52.Deflagration spoon.

Incineration is the process of strongly heating an organic substance with access of air, whereupon the carbon, as well as the hydrogen and oxygen, is dissipated, as oxida tion products, and nothing is left but inorganic salts found in the original substance, and commonly called ashes.

Incineration is like carbonization, save that the former process is performed in a utensil where the air cannot get at the organic substance, while in incineration the air has full access. This makes the important difference that in carbonization the residue, after heating, is carbon or charcoal and inorganic salts; while in incineration the carbon combines with the oxygen of the air to form the gas, carbon dioxide, CO2, which passes off, leaving behind only a small residue (ashes) consisting of inorganic salts. Such an operation is every open fire; such is the process of cremation. Its use in modern pharmacy is limited to chemical examinations, the amount of ash yielded by drugs being in some cases a valuable criterion of quality. Such ash estimations are performed by carefully drying a crucible by heating in Bunsen burner for fifteen

minutes, weighing same after cooling in a desiccator (p. 109), carefully placing the drug in the crucible, and weighing, incinerating same, and finally weighing the remaining ash.

In alchemistic times the ashes from certain plants were supposed to be of great value in medicine. Thus the ashes of wormwood, which modern chemistry has shown to consist chiefly of potassium carbonate, was highly esteemed under the name of "sal absinthii," or "salt of wormwood," two synonyms which still cling to the commercial

salt.

Torrefaction is the process of more moderately heating organic matter with access of air, in which case the active principles of the substance are not eliminated, but are merely modified. The process is what is commonly called "roasting," and a familiar illustration is the roasting of coffee, peanuts, etc. In pharmacy it is an obsolete process, although formerly "torrefied rhubarb" enjoyed considerable vogue.

The operation is performed by placing the drug in an evaporating dish, watch-glass, or crucible, and exposing it to a steady moderate heat; best applied by means of a drying oven (p. 109).

Fig. 53. Melting point apparatus: a, Flask; b, thermom

eter; c, substance in

Fusion is the process of converting a solid into a liquid by means of heat without the aid of a solvent, the object usually being to enable the straining of the fluid mass or the molding of it to some special shape at the moment of solidification. The process is so well known that it is scarcely necessary to do more than refer to the well-known operation of melting wax as an illustration of fusion. This, therefore, is an official fused substance, as is also the official molded silver nitrate.

Substances are fused in any appropriate vessel, -crucible, test-tube, or dish, and the melted mass strained or poured into molds as desired, it being hardly necessary to state that since the liquid returns to solid form as soon as the heated mass reverts to ordinary temperature, the fused condition is merely temporary.

The melting-point of a substance is estimated by placing the powdered mass in a very small tube, attaching this tube to a thermometer, and immersing the thermometer and tube into concentrated sulphuric acid contained in a beaker or flask (Fig. 53). Heat is then applied to the acid, and the moment the substance fuses, the thermometer is read. Vaporization is a general term meaning the change of a solid or a liquid into a vapor; the latter term meaning the gaseous form of a substance, which at ordinary temperatures is either a solid or a liquid.

tube.

Liquids and solids are held together by the force of cohesion exerted by their molecules. In gases, the molecules, instead of being held together, actually repel each other; the force of cohesion is practically overcome. To overcome this requires much energy, and it is into such energy that that surplus heat of vaporization is converted. Steam, when placed in a position in which it can gradually assume normal temperature, reconverts the energy necessary to keep its molecules