The latter alternative, however, would be a mere compromise; as by admitting it we immediately violate, to some extent, the grand condition of procuring an unmixed solution of sugar and

water.

Having thus sketched, the theoretical indications which science proclaims as necessary to be carried out, in the manufacture of sugar from raw juices, I will now offer such a general summary of the method followed in the sugar-producing colonies, as shall enable the reader to appreciate the extent to which the above theoretical conditions are violated.

It must here be premised, that, although the plan of colonial sugar manufacture for all countries is essentially, up to a certain stage, identical, yet, when various colonies are compared as to the respective process of sugar extraction followed by each,-a casual observer might imagine the existence of distinctions as essential, which in reality are merely collateral; and which involve no difference of principle whatever. As it would be exceedingly inimical to exposition of principles, to break in upon the current of observation, for the purpose of announcing mere collateral discrepancies between the machinery, or the processes of different colonies-I will here observe that, in describing the general operation of colonial sugar manufac ture, in reference to the fulfilment or the violation of theoretic indications, I shall select, as typical of colonial operations in general, the process now usually followed in the West Indian islands.

Much has been written, and with great justice, on the very imperfect expression of juice from the canes by the process of mill crushing. Although experiments have demonstrated the sugar-cane to be made up of 90 per cent. on an average of juice, and 10 per cent. of woody fibre, it appears that the average amount of juice expressed by the mill is not more than 50 per cent.

The proper method of obviating this great loss of raw material is altogether a matter for the consideration of the engineer, and does not come within the sphere of chemical comment.

The juice as it comes from the mill is with as little delay as possible treated with lime, as follows, in order to effect a partial purification:

The overseer* commences his operations by putting into a series of wine glasses some of the juice to be defecated. He then adds to each in succession a portion of lime,-either previously mixed up

"The man at the clarifier first raises by heat the cane juice to a temperature of about 180, at which time a dark scum forms at the top; he then throws into the heated juice a small portion of cream, or milk of lime. After waiting two or three minutes, until the scum again forms over the surface, he dips out a wineglassful, and if he sees the mucilage form in well-defined flakes and rise to the top, leaving a clear liquor of a pale amber, or Madeira wine colour, he is satisfied; if not, he adds more lime; but if he finds that the mucilage will not coagulate. thoroughly without such an amount of lime as would deepen the colour_naturally, he stops, and trusts to the skimming."-Moody. †

This is the name of a gentleman from the West Indies, who examined Dr. Scoffern's sheets and remarked upon them.-ED.

with water, or with syrup, to the consistence of gruel or thin pap. Immediately the contact is effected between the lime and the cane juice, a discoloration of the latter ensues; the amount of discoloration varying (cæteris paribus)—in direct proportion to the quantity of lime employed. Having added a different quantity of lime agent to each of the glasses, the operator judges by the resulting tint, which - result is the best, and he is guided accordingly by this evidence, as to the quantity of lime he shall add to the general stock of juice to be defecated.

*

The amount of lime being determined, it is added to the juice in a copper, or an iron vessel, hung over a fire. Sometimes this vessel is the last of a range hung over one long flue,; sometimes, on the contrary, it is heated by a separate fire. In either case its contents are heated to about the temperature of 180° Fahr., when a thick crust of impurities forms upon the surface of the liquid and begins to crack. The fire is now damped, and the crust removed by skimming; occasionally, however, the clear liquid is drawn off by a racking-cock from underneath. In order that the full defecating agency of lime shall be exerted, it is necessary to apply a greater heat than 180° Fahr.; the liquid, in fact, should be brought to the boiling point. Here, however, there is a difficulty:-immediately on the commencement of ebullition, the supernatant crust becomes broken into fragments, and mechanically incorporated with the fluid. so intimately, that it can no longer be removed by skimming, but requires a filtration process to be had recourse to.

The process of clearing or defecation having been effected, and the crust separated by skimming, racking, or filtration, the process of boiling is commenced. A series of copper or iron pans, diminishing in size as they approach the fire gate, and usually four or five in number, are hung over one common flue, or rather fire-place, in which the canes, after their juice has been expressed, are burnt as fuel. This fuel generates a very powerful blast of flame, which not only plays under each of the series of pans, but may be even seen to escape from the chimney.

This plan of hanging many consecutive boiling pans over one common flue is in itself most improper. It was first introduced with special reference to the peculiar kind, and the limited amount of fuel at the operator's service, and would appear to be persisted in chiefly in deference to old opinions and customs.

The pans, too, are usually so deep, that great violence is done to the rule, that the amount of evaporation, other things being equal, is in proportion to the extent of surface of the liquid to be evaporated.

The cane juice having entered the first of these evaporating pans, the process of evaporation begins. At this stage, the juice is merely brought to a slight simmer, the heat applied being usually insufficient

"Almost always-I never saw the other done except with steam clarifiers in St. Croix, where they first take off the crust, then add lime afterwards, boil and skim in the clarifier."-Moody.

"The arrangement of the fire and the size of the pans depend on the principle that the juice must simmer to allow of efficient skimming. If it boils, the skimmer cannot catch the scum.”—Moody.

to cause it to boil rapidly. On the surface of this and every other pan in the series, a scum arises, which from time to time is removed by the process of skimming, and put aside for the purpose of yielding rum hereafter.

After the evaporation in the first pan of the series has proceeded to the desired extent, an attendant ladles its contents into the nextin which, and in every subsequent one, until the last two, the process of skimming is repeated. Eventually the juice (now a syrup) is ladled into the teache, or last boiling pan, wherein it is at length brought to that degree of concentration judged most proper to admit of subsequent crystallization.

It would be impossible by mere description to convey an idea of the manifestations of the proper degree of boiling having been achieved. The peculiar sound which the syrup emits when dropped from the ladle into the general contents;-the resistance it offers on being stirred; the peculiar appearance of its bubbles;-all afford good indications to the practised boiler; but the evidence most generally followed is that which also the refiner avails himself of at home, namely, the proof of touch. A drop of the syrup being placed between the thumb and forefinger, and the two separated, a thread of syrup is formed, of varying length, and varying tenacity, according as the syrup has been more or less boiled. In this thread, also, crys tals are occasionally seen, the presence of which affords valuable evidence. In the process of vacuum-pan boiling, these crystals are the operator's surest guide.

From the last, in the series of evaporating pans, the teache, or tayche, as it is called, the inspissated juice is ladled into shallow wooden vessels termed coolers; seldom more than eighteen inches deep; where it is allowed to accrete into a semi-crystalline mass.

These shallow coolers have been loudly and justly reprehended, as most inimical to the formation of crystalline sugar; and certain it is, that a chemist, if made to draw an inference from their appearance and necessary effect, without any collateral guide,—would be constrained to infer, that in the West Indian sugar-manufacturing operation-perfect crystallization was a result to be avoided!

In these shallow coolers the accreted mass is allowed to remain, until it has acquired sufficient consistency to admit of its being dug out, and carried away, in buckets, to the curing-house, without leaking entirely away. In this curing-house it is put into casks with perforated bottoms, each hole being loosely stopped by the stem of a plaintain leaf; and through which the uncrystallized portions of the mass, at least in part, leak into the molasses tank.

This is the ordinary plan followed; but it is subjected to many modifications, in different places.

As might have been inferred from a consideration of the plan of curing the sugar just described, the badly crystallized mass yields up its non-crystallized portion with great difficulty. The process of curing or drainage occupies, in general, many weeks; and, even at the

"In the last two teaches the scum is brushed back into the preceding one, the liquor being too sweet to lose.”—Moody.

expiration of that long time, is so incomplete, that it is not unusual for some 20 per cent. of the weight of a hogshead of sugar to leak into the hold of the ship on its way to Europe, and to be pumped into the sea. In a recent case which came under my notice, 25 per cent. had thus been lost, and the master of a trading vessel* informed Dr. Evans, as I am told by this gentleman, that his ship was often one and a half foot deeper in the water off Barbadoes than when arrived in the British Channel.

In order to expedite this process of curing, recourse is had on some estates to the expensive contrivance termed a pneumatic chest. This instrument consists of a chest of iron, or copper, supplied with a false bottom, either of finely-perforated plate or cane-wicker work ;-on which the sugar to be acted upon, for the purpose of drawing off its molasses, is put. Under this false bottom is a space which communicates with a powerful air-pump; by the action of which, a partial vacuum can be produced, the tendency of which is to draw the more liquid portions of the mass through the false bottom.

The effect of such a contrivance as this, when made to act upon a badly crystallized sugar, need not be indicated. Not only are the uncrystallized portions drawn into the reservoir, but also a large amount of the small ill-developed crystals.

A pneumatic chest, to be really useful, should be employed upon a well-crystallized sugar-a material which, as a general rule, drains perfectly well of itself, without any mechanical aid whatever.

The process of sugar manufacture here described is, as was previously remarked, the typical one of the West Indies; it has, however, been modified in various ways. Thus, as regards the boiling range, instead of pouring the juice from one pan to the next in order, by the process of dipping, the pans in some ranges have been furnished with valves, to admit of the passage of the fluid towards the teache. Ranges of this kind have been frequently heated by steam.

Several modifications (some, unquestionably, improvements) have been made on the teache, chiefly with the view of reducing the period during which the concentrated syrup is allowed to remain exposed to the agency of the fire. One of these modifications consists in an addition to the teache, of an internal hollow core exactly fitting it, and supplied at its under part with a valve, opening inwards. This core, technically called a "skipper," being dropped from a crane into the teache, the contents of the latter open the valve, and rushing at one gush into the core, may be removed bodily, by raising the core through the medium of the crane.

Another good modification of the teache has been introduced by the French into some of their colonies. It consists in altering the form of the teache into the shape of a coal-scuttle, the lip of which rests in such a manner on a pivot, that, at the proper time, the whole teache may be raised by leverage, and its contents poured out. This kind of teache is called a bascule.

Captain Fowlis-who estimated the loss from this cause at from 3s. 6d. to 48. the cwt., or 31. 68. to 47. the ton.

Amongst the essential modifications which have been attempted from time to time on the colonial manufacture of sugar, with variable amounts of success, may be enumerated the following:

If the impurities which are so inimical to the crystallization of sugar out of the crude juice, were merely of a mechanical nature, a process of mechanical filtration would be reasonable enough; but if those impurities be really of a chemical nature, then such mechanical filtration is entirely out of place. Accordingly, the process of preliminary filtration is spoken of with universal discontent by all who have tried it. Not only is it totally inefficacious in effecting the end desired, but it is productive of much positive harm. It has already been remarked, that raw vegetable sugar-containing juices are most susceptible of fermentation; hence the operation of filtration, even if productive of benefit in this stage, should, to be useful, be most rapidly conducted. Now this is impossible,-for chemists very well know that raw vegetable juices in general, even although thin and limpid to the view, pass through filtering tissues most tardily. Add to this the amount of porous surface, moistened with fermentable liquid, exposed during the operation to atmospheric influences; and it will be readily understood that preliminary filtration is most fatal to the interest of the colonial sugar-maker. I would, by no means, extend this remark to a rough process of straining, at this stage, for the purpose of removing broken pieces of cane, fragments of leaves, and other mechanical impurities, which might somewhat inconvenience future operations. Such an operation, however, is not one of filtration, but of straining.

Should the process of mechanical filtration be executed after defecation? As a general rule, doubtless this question should be answered affirmatively, as being a step in the right direction; but so long as lime is used as a defecator, the process of filtration will be deprived of half its value. Not only is the flocculent scum developed by lime most unfavorable to the process of rapid filtration, but the advantage gained is more specious than real, inasmuch as so many impurities still remains in juices defecated by lime, that, although the act of filtration may have yielded a liquor of great brightness, it

* The processes of claying and liquoring, under whatever modification, are here purposely omitted, as not being improvements of the sugar manufacture—but merely an extension of that manufacture beyond the usual colonial limits, into the art of the refiner.