becomes turbid, and throws up more scum on the further application of heat.*

With regard to filtration through animal charcoal, it can never be profitably applied to the treatment of raw juices, or those as have merely been exposed to the process of defecation, without subsequent concentration. It has been demonstrated, that this agent produces its maximum effect on sugar solutions of about the density of 28° Beaumé; hence, if employed at all, it should be at the interval between the last boiling-pan and the teache. Numerous experiments, however, have convinced me that animal charcoal should never be employed in the colonies for the purpose of making any but absolutely white sugar. The beautiful straw-colored tinge, so admired by grocers, and which all sugars by a proper system of defecation can be made to assume, animal charcoal has a tendency to destroy-imparting a disagreeable neutral tint in its place.

The expense of using animal charcoal, too, in the West Indies, being somewhat about £2 per ton of sugar, is in itself a most serious obstacle to its general adoption.

The last essential improvement introduced into the colonial sugar manufacture is that of the vacuum pan, an instrument which merits a full description.

At the ordinary level of the sea the atmosphere exerts a pressure of 15 lbs. on every square inch; and, whilst exposed to this pressure, water boils at a temperature of 2120 Fahr.t

If, however, by means of the air-pump or otherwise, a portion of the atmospheric pressure be removed from the water's surface, then the degree of heat necessary to effect ebullition is reduced-reduced, too, in a known and definite ratio, so that for every pound of atmospheric pressure taken off, a proportionate diminution of the boiling temperature is accomplished.

Not the most perfect vacuum which we are capable of forming, is sufficient to cause water to boil at ordinary atmospheric temperatures without the application of any extraneous heat-simply because water is not a fluid of sufficient volatility. If ether, however, which is a far more volatile liquid, be exposed to the same treatment, it boils with violence: water under similar circumstances would merely be rapidly given off in the form of vapor.

The rationale, and also the laboratory practice of increased evaporation under diminished pressure, has already been explained. It now remains to be stated, that the vacuum pan is merely an instru

"The sliminess which affects the bag filters is a great disadvantage to charcoal filters. I have known the charcoal frequently clogged; and, when washed with care and placed in open casks for collection for re-burning, ferment to such a degree as to char the casks, and reduce the value of the charcoal by a considerable production of white ash."-Moody.

+ In a metallic vessel. Gay-Lussac has proved that water boils at 214° in one of glass; owing, apparently, to its adhering to glass more powerfully than to a metal.

ment which unites to the principle of evaporating under diminished pressure, the application of a certain-but comparatively smallamount of artificial heat.

To the Honorable Mr. Howard we are indebted for the invention of this most useful instrument, which has already effected such improvements in the home refinery process, and which is destined before long to extend its ameliorating influence to the colonies. abroad.

The vacuum pan may be described as composed of two copper segments of spheres joined together at the edges. The lower hemisphere is imbedded into a steam-jacket or space, into which steam, of a varying pressure up to 3 lbs. to the inch, can be forced; and in order to increase the area of heating steam surface beyond the amount furnished by the lower segment of the pan, there passes internally a coil of copper pipe, through which a current of steam may be made to rush. It is obvious that any liquid put into a vessel of this kind, will be exposed to so large an amount of heating surface that it must soon arrive at its boiling point; but the vacuum producing part of the apparatus has yet to be described. Attached to the pan at its upper part is a pipe of communication, with a cylindrical vessel called a condenser, and which is exactly similar to the condensing apparatus in a low-pressure steam engine; consisting either of a means of injecting a gush of cold water through a series of minute holes, which plan is called that of direct condensation, or else of a series of small tubes exposed to the external agency of water-a plan denominated that of indirect or tubular condensation. Beyond this condenser, and communicating with it, is a powerful air-pump. The accompanying wood-cut, however, represents a condenser (7) of a different and more effective kind. It is the external condensing system of Messrs. Pontifex.

† Between the vacuum pan and condenser is a vessel destined to contain any solution which may boil over. This vessel, however, theoretically-may be considered as a mere expansion of the vacuum pan. This is indicated in the dia

The action of this apparatus in the aggregate will be as follows:The pan being filled to the desired extent with sugar solution to be evaporated, steam being let on to the jacket and into the coil, the temperature of the liquid continues to rise. Meantime the air-pump being set to work, a partial vacuum is produced, and the atmospheric pressure exerted upon the syrup in the pan is gradually lowered to such an extent, that the liquid begins to boil.

The vapor resulting from this ebullition passing into the condenser, is exposed to the agency of cold water, and immediately assumes the liquid state; finally this condensed water is drawn off by the air pump, as it is called, although the instrument performs the mixed function of pumping both air and water.

Such are the essential portions of the vacuum-pan, but certain accessory parts are yet to be described. The most important of these is the appendage called the proof-stick, a contrivance by means of which the operator can from time to time remove and examine a portion of the evaporating syrup without the least destruction of the partial vacuum. Attached to the copper segment of the pan is a thermometer, for the purpose of indicating the temperature of the syrup, and also a vacuumgauge, as it is technically called, an instrument on the barometric principle, by referring to which the amount of atmospheric pressure exercised at any period on the evaporating liquid can be at once read off. On the summit of the upper segment of the pan is a man-hole,* supplied with an accurately ground cover, and by the side of it an entrance for each successive charge of liquor, which passes from an adjoining vessel of determinate capacity called the measure.

At the lowest part of the under hemisphere is situated a valve through which the sugar solution, when sufficiently boiled, is allowed to escape into another vessel called the heater, or occasionally the cooler.t

This heater may be compared to the lower segment of a vacuumpan, minus its coil, being a copper pan, imbedded in a steam jacket, by the agency of which a graduated heat may be applied.

The use of this instrument is to allow the conditions of time and prolonged fluidity for the more perfect development of those crystals,

gram by the figure 6. The other portions of the apparatus are as follow: (1) the measure, (2) the man-hole, with ground cover, (3) vacuum-pan, (4) proof-stick, (5) heater, (7) condenser, (8) steam-engine and air-pump, (9) escape valve of the vacuum-pan, through which its contents pass into the heater.

The aperture through which a man enters the vacuum-pan for the purpose of cleaning it.

The indifferent application of the term heater and cooler, the one for the other, is curious, although easy to be explained. So long as sugar solutions were boiled in open pans under the ordinary atmospheric pressure, and at a temperature of 220° and upwards, the vessel in which the boiled liquor was allowed to assume a temperature of 176° might appropriately enough be denominated a cooler. But under the process of vacuum-pan boiling, at a temperature of about 140°, this vessel, in which the latter degree of temperature becomes changed to 176°, as before, is to all intents a heater; still the old name is in many refineries maintained.

the formation of which had been already commenced during the operation of vacuum boiling.

Fuller details of the employment of the vacuum-pan and its accessory, the heater, will be given under the head of "refinery operations," such a general account of these instruments merely being here given, as might suffice for the purpose of investigating the advantages and disadvantages of the colonial operation of vacuum boiling.

It is a subject of much surprise to many persons who have witnessed the results of vacuum-pan boiling at home in refineries, that when used in the colonies it has been productive of such ambiguous results. This surprise will vanish when we consider the conditions under which a vacuum-pan can be profitably worked, and how difficult of attainment these conditions have hitherto been in the colonies. However objectionable in most points of view the ordinary colonial evaporating process may be, it nevertheless is well adapted to the end of removal of impurities by surface skimming-an operation which is totally impracticable when the vacuum-pan is employed. Hence, although this valuable instrument exercises the full amount of its well-known and legitimate influence-although it may effect evaporation at the practical minimum-although an experienced boiler may be present to strike or let off its contents at the proper time; still the result of boiling an impure juice in vacuo will in all cases be-an impure sugar.

This fact is at length becoming so well recognized, that a gentleman of much practical experience, as an engineer in the West Indian colonies, informs me of a resolve he had made, never again to set up a vacuum-pan on any West Indian property, (save a few exceptional estates on which the juice was remarkably pure,) except under a guarantee that the juice should have been submitted to charcoal filtration-this being the only means at that period known to him, as capable of effecting the necessary amount of defecation.

It may very safely be asserted that the great utility of the vacuumpan has yet to be demonstrated to colonial sugar growers. Hitherto, even on estates where it has given a qualified satisfaction, the true genius of the instrument has been altogether misunderstood. Instead of aiming at the production of a well crystallized result, mixed with a thin syrup of drainage, or molassess, admitting of easy removal, and then leaving the sugar almost dry, the general aim of the colonial sugar-maker has been to produce, by high or stiff boiling, the maximum amount of semi-crystalline produce. If this kind of material were a marketable commodity in its present state, the endeavor of aiming at its maximum produce would be intelligible; but as it requires the expensive process of liquoring to render it fit for the market, the process of stiff boiling is in contravention of all proprieties.*

* Dr. Evans informs me that in the island of Java there are used vacuum-pans having an escape aperture in the side, through which the solid concrete is shovelled by a man sent into the pan for that purpose, after each boiling operation. A patentee, moreover, actually proposes to grind this kind of concrete into grains by a mill !

The method in which the process of liquoring is commonly practised in the West Indies is fearfully wasteful, and in other respects open to the greatest censure.

Under the definition of liquoring, the principle of that operation has been explained. I will now offer, in anticipation of another part of this treatise, a concise explanation of the mode of conducting it in refineries, in order to demonstrate most powerfully the destructive mode followed in the colonies.

Under the proper conditions of temperature, hereafter to be mentioned, the refiner puts his boiled and crystalline syrup into moulds, supported on their apices, and the hole of each apex stopped with a pledget of brown paper. Here the mass is allowed to cool; and, when cold, the plug in the apex being withdrawn, each mould is supported on a corresponding earthen pot. A portion of syrup, technically known as green syrup, more or less colored, now drains away, and the cone of sugar is left comparatively dry. The sugar forming the base or face of each cone being now removed by a revolving cutter, termed the facing machine, the sugar so removed is mixed with water to the consistence of a thin magma (technically named clay), and reimposed on the base of the cone. This is the operation termed in refineries claying. After some hours the operation of liquoring commences by pouring on the smooth surface, or face left by the subsidence of the clay, a concentrated aqueous solution of sugar. The result of this operation is, that the coloring matters of the sugar are totally washed into the pot below, and a loaf of white sugar is formed.

As conducted in refineries, the operation of liquoring is most philosophical, and most efficient-an operation without which, or the equivalent one of prolonged claying, a thoroughly white sugar cannot be made. Its success depends so entirely on the purity and saturation of the magma liquor, or aqueous solution of sugar, that the preparation of the latter is a matter to the intelligent refiner of greatest solicitude. If the magma liquor be colored, it is evident the sugar which it is employed to wash cannot be colorless. If the magma liquor, also, be not fully saturated, it will become so during percolation, at the expense of the sugar which it is intended to cleanse,and the loaves will be partially dissolved. This is too evident for further comment. What, then, would be thought of the refiner, who, in violation of the obvious principles just laid down, should attempt to liquor by means of water? And yet this is the kind of liquoring very frequently performed in some of the West Indian islands where vacuum-pans are used. The concentration having been carried on, as I have already remarked, to a higher extent than propriety warrants, the mass is cooled, thrown into a pneumatic chest, and affused with water by means of a garden-pot! The air pump is now put vigorously to work; a partial vacuum is produced underneath, and the water of affusion, carrying with it many impurities, and much sugar also, is drawn into the cistern below. It is a fortunate circumstance that the crystals or grains of vacuum boiling are usually large, and, therefore, less easily soluble than they would