the gas evolving from a fish-tail burner forms a mass of some thickness. The oxygen of the air can combine only with the outer surface of this mass, leaving the inside of the flame untouched; just as a tightly closed book, when immersed in water, will be soaking wet on the outside and comparatively dry inside. That gas inside the flame has not enough oxygen to convert all the carbon into carbonic oxide, and the result is that much of the carbon remains unchanged, glowing with the heat of the ardent reaction, and depositing as soot when it passes from the flame. The fish-tail flame (Fig. 39) may be quaintly likened to a chemical factory.

Fig. 38.-Russian alcohol blast lamp

Fig. 39.-Fish-tail flame.

Zone a is the reception room, when the crude material that is to be worked up the gas from the mains-comes in; zone b is the workshop where the chemical reaction is carried on with the utmost vigor-where the hydrocarbon of which the gas is composed is broken into carbon and hydrogen, and these in turn combine with the oxygen of the air to form carbon dioxide and water; while zone c is the shipping-room, whence the finished products just mentioned are sent away. Since the soot is dirty and inconvenient in cooking, efforts were made to so construct a burner that the gas would have contact with the air (or its oxygen, for air is composed of nitrogen, oxygen, and some carbon dioxide), in parts other than the outer surface, and this is done by mixing the gas with air before burning it in the Bunsen burner. Aside from the great advantage of freedom from soot, the flame of the Bunsen burner is superior to the ordinary flame by reason of its greater heat. In it all the carbon of the gas becomes combined with the oxygen of the air, and it is self-evident the more chemical reaction occurs in a certain space (as in the mass of gas issuing from a burner), the greater will be the heat produced. With this purpose in view the Bunsen burner is constructed with a few modifications, according to the purpose adapted. In its simplest form it consists of an upright metallic tube with orifices at the bottom, and as the gas rushes upward through the tube, it mixes with air that enters through the bottom orifices, and when emerging from the top of the tube and ignited, it contains sufficient air thoroughly to combine with all the carbon, thus producing carbon dioxide and water, and no unconsumed carbon or soot. Such a burner (shown in Fig. 40) gives a flame in which three zones can be noted on close scrutiny. The top of zone "a" is called the reducing flame, while the top of zone "b" is the oxidizing

Fig. 40.-Bunsen burner.

flame. This long-accepted view of the gas flame has lately been submitted to criticism, it having been shown that in zone "a" all of the carbon has been changed to carbon monoxide, and that the changing of the flame from a luminous to a non-luminous one can be accomplished as well with carbon dioxide, steam or even nitrogen.

In using such a Bunsen burner the flame sometimes "strikes back," that is, the gas which is ignited at the top of the tube finally catches fire at the bottom of the tube, where the unmixed gas emerges, thus

producing a sooty flame, and at the same time heating the brass tube, sometimes to a dangerous extent. Such a striking back can be avoided only by having a good current of gas turned on. It can usually be detected from a distance by the peculiar odor of acetylene generated in this incomplete combustion.

The Scimatco burner is an improved form of Bunsen burner in which according to the inventor a maximum quantity of gas is produced in a minimum volume of flame; the result is a very hot flame of uniform temperature throughout (Fig. 42).

For the purpose of more extended heating, larger varieties of the socalled gas-stove are obtainable, the most practical of which, from the

pharmacist's standpoint, being the Fletcher radial burner shown in Fig. 43, in which the gas is mixed with the air in a horizontal tube, and emerges through radiating slits cut into a central dome, thus making a flat, clean, and intensely hot flame.

In the old-fashioned form of gas-stove the flame used was a luminous one, and the blackening of the utensils through soot was avoided by placing between the utensil and the flame a wire gauze.

The advantage a Bunsen burner possesses over the ordinary fish-tail burner is due to the fact that the gas before ignition is mixed with sufficient air to produce a complete combustion, and this is of benefit-first, in producing a smokeless flame, and, second, in giving a more intense heat than is produced by the same gas when burned in a fish-tail burner.

SCIMATC

That more heat is produced is selfevident when we consider that the heat generated is in direct proportion to the amount of the chemical reaction, and since in the fish-tail burner all the carbon is not consumed, there is, therefore, in such a flame less heat to a given area than will be produced in the same area of a Bunsen burner.

PAT JAN 1979

Fig. 42.-The Scimatco burner.

Surface Combustion.-Mixtures of gas and air in proper proportions brought into contact with heated porous material will burn with intense heat without giving much flame. Ovens of this kind are now being constructed.

The principle of the blow-pipes is merely an extension of the principle of the Bunsen burner. With the blow-pipe, the air expelled from the lungs is blown through the tube upon the flame, this insuring a complete combustion of the gas. The blow-pipe produces more heat than the

Bunsen burner, for the reason that in blowing air into the flame the flame is rendered much sharper and narrower than in a Bunsen burner, and in this way heat produced by chemical action is concentrated to a more limited area; hence by this instrument we are able to make a very intense heat, albeit in a very narrow area.

The blow-pipe consists of a bent and tapering brass tube ending in a very minute orifice. the larger end connecting with an appropriate mouth

Fig. 44.-Blow-pipes.

piece which is pressed firmly against the lips when the air is energetically blown from the lungs through the pipe into the flame. In the cheaper forms of such blow-pipe, where no mouth-piece is provided, the larger end should be fitted with a piece of rubber tubing to prevent the lips coming in contact with the brass, which causes irritation. The continuous blowing of air through the blow-pipe is merely a matter of practice, the expert

being able to inhale through his nose at the same time that he forces the air from his lungs through his mouth into the blow-pipe, and this must be done if a continuous flame is desired (Fig. 44).

The blow-pipe is used entirely for minute operations, such as small chemical work, or for insignificant cases of soldering in the jewelry business. For large operations in which a concentrated flame of high intensity is desired the blast lamp (Fig. 45) is used, and this is merely a modification of the blow-pipe in that the air is furnished by means of the bellows rather than by the lungs, and thus the instrument can be made larger.

In such a blast lamp a rather large flame of great intensity (1700°C.) can be easily attained, with the sole physical outlay of operating the bellows with the foot When a very intense flame is desired, the same blast can be applied, but instead of illuminating gas, hydrogen is pumped in, and instead of air, oxygen is supplied. This forms the oxyhydrogen. blast lamp, and with this instrument a heat sufficient to melt platinum. (about 2000°C.) can be attained.

Electricity is also used for the production of heat, and in future will play a more important rôle in our domestic economy. Even now electric stoves are used for warming purposes, while the greatest artificial heat is derived by the use of the Moissan electric furnace.

An interesting method of production of heat without flame is shown in the use of thermite. (See p. 490.)

The following table of relative temperatures of the commonly used heating appliances may be of interest.

Baths in pharmacy are devised for uniform distribution or for the limitation of heat. Thus the heat of a Bunsen burner, while very satisfactory, is apt to be more intense on one portion of the utensil being heated than on another, and to this cause is due most of the fractures on heating glass and porcelain utensils.

It is a dictum that, in heating substances in porcelain or glass utensils, the direct heat of a Bunsen burner should never be applied, but the flame should be uniformly distributed in the several ways at our disposal. The simplest form for the uniform distribution of heat is a sheet of asbestos or of tin, by which the cylindric flame of the Bunsen burner is spread out to the entire surface of the interposing medium, and a uniform heat is in turn transmitted from it to the container above.

Wire Gauze.-Far more convenient than these, however, is a piece of gauze of iron or copper wire. Through such gauze the heat more readily penetrates than through asbestos, and at the same time the heat is as uniformly distributed by reason of the fact that the wire of which the gauze is constructed is an admirable conductor of heat, and the moment the flame comes in contact with the gauze, each of its many filaments aid in drawing the heat to every portion of the gauze. This has the effect of chilling the flame itself, hence such flame cannot pass through a wire gauze until the gauze has become heated to a temperature almost equal to the flame.

On this principle is based the construction of the Davy's safety lamp for use in mines, the use of which is of no pharmaceutic importance.

This phenomenon also explains why smoke cannot penetrate through such gauze, hence the value of gauze in the old form of gas-stove.

Sand-baths.-A sand-bath consists of a suitable flat dish in which is put a sufficient quantity of sand, the container of the liquid to be evaporated is placed therein, and the sand is piled around so as to completely surround it. Heat by means of the Bunsen burner or other flame is applied under the dish, and is gradually uniformly transmitted to all portions of the heating substance.

In this form of bath the heat is limited only to the amount produced by the burner, the sole aim being to distribute the heat uniformly.