But to convert a fluid into steam, not only a certain quantity of heat is required, but the heat must also be of a certain intensity; thus, although a pound of steam at 212°, would raise six pounds of water to the boiling point, it would convert no portion of it into steam, as the moment the water had acquired the heat of the steam, it would receive no further portion of heat from it; but if the steam is formed under a pressure exceeding that of the atmosphere, its heat, as indicated by the thermometer, is increased, and consequently it will continue to impart heat to a liquid which has attained the boiling point under a less pressure than the steam employed to heat it. Upon the combination of these two principles, Mr. Stem's apparatus is constructed. Nos. 1, 2, 3, and 4, are four oblong elliptical vessels or stills, two of which are shown in section; the lowermost halves are enclosed in casings a a, forming thereby steam chambers b b; each still has a vertical pipe c c c c, terminated by a double passage cock d d, one passage opening into the pipe e, which leads to the worm tub, whilst the other opens a communication from one still to the steam case of the next still, by means of the curved pipes f f f; that from the still 1 leading to the steam case of still 2, and so on in succession.

The stills are charged from the pipe g, the quantity admitted being regulated by the floats h; each steam case communicates by the pipes l l l l (which are furnished with cocks) with the pipe k proceeding from the boiler. From the lower part of the steam case proceed pipes m m; that from still 1 leads to the cistern, which furnishes the steam boiler with hot water, whilst the others may either communicate with one common main n, leading to a refrigerator, or they may each communicate with a separate refrigerator. From the upper part of each steam case proceeds a pipe (shown at 3 and 4) which communicates with a gauge pipe p p, and terminates in a syphon barometer q q. r r r r are the man holes to each still; s s s s the discharge pipes to the stills, the steam cases being emptied by opening cocks in the pipes m m leading to the main n. The operation is as follows: - The stills being charged, and the cocks d being open to e, the steam is admitted to each case by the pipes f f leading from the steam pipe k, and is rapidly condensed in the steam case, the air escaping by a pipe not shown in our drawing.

When the liquor in the stills has nearly attained the boiling point, the steam is shut off from all the cases except that of 1, and the cocks d axe opened to the pipes f, and the main n, being cleared of the condensed water, the cocks on m of 2, 3, and 4 are closed. The steam from the boilers (which is under considerable pressure) continues to flow into the case 1, and by the heat given out to the liquor in the still, causes it to boil; the vapour passes into the steam case of 2, and the liquor in 2 condenses the steam in 1, until a common temperature is attained; then the steam from 1 being no longer condensed, and continuing still to receive heat from the boiler, its pressure, and consequently its temper-ature, increases, and it again gives out heat to the liquid in 2, which causes it to boil. The vapour from 2 then passes into the steam case of 3, where the same process ensues, and which is subsequently repeated under the still 4, the steam from which passes by the pipe e to the condenser.

As soon as the liquids in 2, 3, and 4 begin to boil, the cock on m must be partially opened to allow the condensed spirit to pass by the pipe n to the refrigerator; yet always retaining a certain portion in the steam case, the height of which may be ascertained by the gauge pipe p, whilst the barometer q q will indicate the pressure of the steam in each steam case. The proper pressures will be best obtained by observation, as it will vary in different liquors during the distillation. The person conducting the process must, therefore, pay great attention to the barometer; and to enable him to do this with facility, the gauge pipes and barometer are all ranged in a cluster at the centre of the apparatus. By this mode of distillation, it will be seen that the latent heat of three-fourths of the liquid evaporated is saved, which produces a corresponding saving in the article of condensing water.

In Mr. Williams's apparatus for distillation, for which he obtained a patent about the year 1821, the improvements projected are comprised under the following heads; viz. an enlarged capacity of the still head, to cause a separation of the aqueous vapour by condensation, previous to its passing over the neck of the still into the spirit condenser; in the employment of numerous small vertical tubes, surrounded with cold water, to increase and accelerate the condensation; in the adaptation of a peculiarly constructed " cooling worm," by which it is conceived the quantity of spirit will be increased, by preventing evaporation in its progress to, and when in the receiver; and in the employment of refrigerating saline mixtures, for the more effectual cooling of the spirit in warm climates, or in warm weather. In the body of the still (that part where the vapour is generated) there is no improvement proposed, but an enlarged capacity of its globular head, to cause the watery particles to fall back into the still; this part of the apparatus we have omitted in our diagram, as it requires no additional explanation; the engraving, therefore, relates wholly to the apparatus for condensation, a is the termination of the neck of the still, which conveys the vapour into the "upper drum " b, whence it is divided among a number of small vertical tubes c, which the patentee says, should not exceed three-fourths of an inch in their interior diameter.

As the tub inclosing this apparatus is filled with cold water, the condensation immediately commences in the upper drum, and is completed in its subsequent progress through the vertical tubes, and the "lower drum" d. From thence the fluid runs down a central neck e into the trap f, from the upper part of which trap it enters the cooling worm b. It is evident that the trap f is, in working, always partly filled with liquid; and the neck e being immersed therein, any vapour which may have escaped condensation can pass no further. The trap f has a funnel-shaped bottom, from which a pipe h passes through the coils of the worm, and through the side of the tub, where it is furnished with a cock for the purpose of drawing off any impure spirit which may be separated from the wash in the first stage of the process, and to discharge what may remain in the trap when the process is over. To the trap f is also attached another pipe i, called the safety pipe, for the purpose of allowing the egress and ingress of atmospheric air from and to the condenser, to prevent both pressure and vacuum therein.