A term generally used to signify the dissipation of the volatile parts of a compound body, whether caused by the action of the sun and atmosphere, or by artificial means; although some authors restrict the use of the word to the former case, and employ the term vaporization in the latter. A distinction is likewise drawn in the case where the volatile parts are the objects of the process, which is then termed distillation; but the fixed parts, or the residuum, are the products sought by evaporation. The vessels are accordingly different, evaporation being commonly carried on in open shallow vessels, and distillation in vessels nearly closed from the external air. The degree of heat must be duly regulated in evaporation. When the fixed and more volatile matters do not differ greatly in their tendency to fly off, the heat must be very carefully adjusted; but in other cases this is less necessary. As evaporation consists in the assumption of the elastic form, its rapidity will be in proportion to the degree of heat and the diminution of the pressure of the atmosphere; a current of air is likewise of service in this process. Dr. Ure, in his Chemical Dictionary, mentions the following method of evaporating liquors, as being practised in some large alum manufactories.

A water-tight stone cistern, about three or four feet broad, two feet deep, and from twenty to forty feet long, is covered over by a low brick arch. At one extremity of this tunnel a grate is built, and at the other a lofty chimney. When the cistern is filled, and a strong fire kindled in the reverberatory grate, the flame and hot air sweep along the surface of the liquor, raise the temperature of the uppermost stratum almost instantly to near the" boiling point, and draw it off in vapour. The Doctor observes, that the great extent, rapidity, and economy of this process recommend it to general adoption on a large scale.

More recently, Mr. Jacob Perkins has obtained a patent for a novel mode of forming steam of very high pressure, by confining the water under mechanical pressure, in a boiler properly constructed for the purpose and intensely heated;

Fig. 1.

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Fig. 2: and subsequently he obtained another patent for an adaptation of that apparatus for the evaporating of water and other fluids. For this purpose the high pressure steam is projected from the generator through pipes which circulate through the evaporating pans or boilers; and such an arrangement is made, that the water produced by the condensation of the steam is returned into the generator by means of valves and a force pump, the steam or water being always under mechanical pressure. The preceding engravings give a sectional elevation and a plan, and the same letters of reference apply to similar parts in each figure, a is the generator; b the forcing pump; c a pipe opening into, and projecting from the upper part of the generator at d, and opening to, and projected from the lower part of it, opposite to e; f is a pipe leading from the pipe c to the forcing pump; g is a valve; h a vessel, containing the liquid to be boiled or evaporated; and j a safety valve. At l, in the plan, is a valve opening into the generator; it is not seen in Fig. 1, but is in a line with the part marked e in that figure.

When steam is admitted from the generator into the pipe c c, it becomes condensed in heating the surrounding fluid in the vessel h, and collects in the form of water at the valve g. Upon raising the handle of the forcing pump, the valve g opens, and the water fills that portion of the pipe marked f, between the pump and the valve g, the pressure in the generator keeping the valve at the opening l shut. When the handle of the force pump is depressed, the valve g shuts, and the water being prevented, in consequence, from returning into the pipe c, necessarily forces open the valve at f, and is returned into the generator; and this operation is of course successively repeated at every stroke of the piston rod of the pump.

Messrs. Beale and Porter's patent method of applying heat for the purposes of evaporation, consists in the use of various fluids as media for the communication of heat, which rise in vapour or boil at different degrees of temperature; so that one substance may be chosen as proper for one process, and another substance or combination of substances may be employed as more suitable for other processes, the nature of these substances being such that, under the ordinary pressure of the atmosphere, each will indicate a known and unvarying degree of heat at its boiling point, which may be communicated to any substance exposed to its action. Amongst the numerous substances suitable for heating media, are the following: Spirits of turpentine, which boils at 316° Fahr., and naphtha, which boils at 400° Fahr.; and by distilling coal tar, and collecting the products at different periods, various other bodies are obtained, which will furnish different degrees of heat ranging between 400° and 700° Fahr.

By this arrangement it was expected that the maximum degree of heat would be always and altogether independent of accident or want of skill, so that no injury from burning could possibly arise, except through the employment of an improper medium, which, as fluids may be chosen whose boiling points vary between the range of 200° and 700° Fahr., need never occur. The mode of applying this principle to boiling or distilling, is, by using a double vessel, having one part placed within the other, so as to leave a small intermediate space. Into this space the substance intended to form the medium must be introduced, in sufficient quantity to cover the flat bottom of the outer vessel, to such a depth as to secure it from injury by means of the fire. When this fluid is made to boil, it will give off vapour of the same temperature, which, as it comes in contact with the surface of the inner vessel, will part with its heat thereto, and, resuming the fluid form, will fall again to the bottom of the vessel, to be again vaporized; and so on, in a constant alternation of evaporation and condensation.

To keep up a communication between the fluid medium and the atmosphere, and avoid thereby all tendency to rupture or explosion, a tube, open at both ends, is introduced into the intermediate space between the vessels. Should there be gross mismanagement of the fire, some portion of the vapour would be forced up this tube; it is therefore made to pass through a condenser, the action of which will return the fluid to the double vessel, so that little or no waste of the fluid medium will be sustained. This mode of evaporating has, we understand, been applied with great advantage in the refining of sugar, - a substance so liable to injury from an excess of heat, that the most complex and expensive plans have been resorted to in order to avoid the danger of burning. The plan has likewise been successfully adopted in several large medicinal laboratories, and is equally valuable to distillers, dyers, and, in short, almost every process in the arts where a steady uniform temperature is of importance

The diagram here given will show the great simplicity of the plan, and the little chance there is of injury to the apparatus from the fire, as it comes in contact only with a surface always protected by a fluid which rapidly absorbs and carries off the caloric. The engraving represents the apparatus as adapted to the purpose of sugar refining, a the evaporating pan; b b the outer pan, containing c, the fluid medium, represented by dotted lines, and which, for the purposes here described, is a modified product, by distillation of coal tar, which furnishes vapour at the fixed degree of 350° Fahr, d the breathing pipe, which, in the event of injudicious firing, will serve as an outlet and condenser for such portion of the vapour as may not otherwise be condensed by the comparatively cold surface of the evaporating pan a; e an ordinary furnace; f the ash pit.

Air, it is well known, has a great affinity for vapour, and large quantities of water are evaporated by the process of nature, even at the temperature of the atmosphere, wherever a large surface is exposed to the action of the air. Taking advantage of this fact, Mr.Cleland has contrived a new and singularly elegant method of evaporating the aqueous parts of syrups and saline solutions. The principle of the invention consists in continually exposing a thin film (if the expression may be allowed) of the liquid to the joint action of heat and air, and by that means effecting a rapid evaporation. The apparatus consists of a convoluted worm of great length, heated by steam in the interior, which is made to revolve horizontally upon its axis, partly immersed in the liquid under evaporation, which is thereby constantly taken up by it in the thinnest possible stratum; and being in contact with the hot surface of the metal, the aqueous portion of the matter is quickly formed into steam, and carried off by the surrounding air. a is the boiler or vessel affording steam, which may therefore be imagined as set over a furnace; b is a shallow vessel, containing the syrup to be concentrated, and so placed upon the boiler as to form the top or cover to it; c c is the worm, supported by stays upon an axis d d, which has a cavity at each end communicating with the worm.

One end of d is supported in n stuffing-box e upon a hollow arm f, which communicates with the boiler, and is pierced with numerous small holes in that part which turns in the stuffing-box; the other end of the axis d is supported by a solid arm g, and is open at the extremity for the emission of the steam after it has passed through the numerous coils of the worm. The axis may be turned by a winch h, or by a pulley k, or receive its motion from any convenient prime mover. By this excellent arrangement it will be seen that the steam in the boiler acts upon the bottom of the evaporating pan, and raises the temperature of its contents; at the same time it passes by the hollow arm f through the small apertures in the axis d into the worm c; herein it traverses through all the turns, and escapes finally at the opposite end of the axis into the atmosphere. The lower part of the worm reaches to but, a small depth in the syrup, and by turning the worm, every portion of it becomes covered with the liquid, and lying in contact with an extensive heated surface, vapour is given off, which is quickly absorbed by the surrounding atmosphere.

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