result, hot water being a menstruum scarcely less eligible for effecting the solution; for albumen, when coagulated by heat, is no longer soluble in water.

Thus, at a first glance, the problem appears solved, even as regards the large scale; but a slight analysis of facts soon demonstrates the contrary. The first difficulty is one that would scarcely be imagined à priori. It is difficult, if not impossible, to use any slicing machine that shall not very speedily become blunted by the hard siliceous covering of the canes. Once blunted, the first object of the operation is lost; instead of a clean cut we have a bruise, and the saccharine cellular juice mingles with the sap: the grand objection to the usual squeezing of the mill obtains, without any of the mill's advantages. Then, how are sliced canes to be stove-dried in large quantities? Where is the necessary amount of hot water to come from? Where the fuel necessary to evaporate so dilute a solution as must result if the sugar be thoroughly washed out?

All these are practical questions, which the planter would do well to answer to himself, before making arrangements for the carrying out of this very philosophical, but impracticable scheme.

Discarding the first of the two problems as incapable of a practioable solution, the second presents itself to our consideration; but as a preliminary, a few words concerning the mill and its operation will be desirable.

The sugar-mill consists of a series of cylindrical rollers, usually three, between which the canes are pressed; the result of this operation is obviously to extrude not only the sugar-containing liquid in the hexagonal cells-but also the complex vegetable juice of the vascular tissue, and also a portion of wax, which is secreted by certain little glands on the periphery of the cane nodules.

Hence, cane juice, or the fluid of expression, is a fluid of very complex nature; being made up of a great number of mineral salts, and

of so many vegetable principles, that no perfectly trustworthy analysis of it is as yet recorded.

In a treatise which aims solely at being a guide to practice, it would savour of pedantry to expatiate on analyses which do not further the object to be kept in view. Without, therefore, entering minutely into the chemistry of cane juice, it will be sufficient for all practical purposes to consider, first, that it is made up of sugar, water, and impurities; and, secondly, that the prevailing or typical impurity is albumen.

Such is the fluid on which practical necessities oblige us to operate: and now the second problem is fairly before us for solution;namely, to extract all succulent matter from the canes, and to effect a separation between the sugar and its accompanying impurities.

This separation must be effected beyond a certain extent, or the sugar existing in the cane juice obstinately refuses to crystallize on being evaporated; a circumstance not peculiar to sugar alone, but of almost universal occurrence in all parallel cases.

Thus, the juice of limes and lemons contains a large amount of citric acid; a body which, though easily crystallizable out of an aqueous solution, obstinately refuses to crystallize until a great part of its associated vegetable impurities is removed. The method of this removal I need not describe, as it does not in the least resemble any of the processes which will effect the purification of cane juice. The object to be gained, however, in either case, is identical.

Considering that the leading impurity in cane juice is albumen, and considering that albumen coagulates by heat-it might have been theoretically inferred, that a mere heating of the cane juice to a temperature sufficiently elevated to coagulate the albumen, would have left the sugar in a solution of sufficient purity to admit of crystallization: experiments, however, have demonstrated that such is not the fact, and have proved the necessity of adding to the juice some material endowed with a chemical potency of effecting a greater separation of impurities than is possible by heat alone. The usual agent employed for this purpose is lime, the mode of operation of which will be fully detailed hereafter.

We now arrive at a most important division of our subject; we have to examine closely into the changes which sugar is made to undergo by the combined agency of heat, impurities, and lime.

This will be best accomplished by leaving for a period the consideration of cane juice, and by substituting for it a solution of pure sugar and pure water. This pure solution will be the starting point of all remarks on the more complex case, and will enable the following important agencies to be contemplated in detail; whereas in the actual colonial operation on cane juice they operate simultaneously. We shall have to study

1. The changes effected on solutions of sugar in water by heat alone.

2. The changes effected on the same by the united agencies of heat and lime.

3. The changes effected on solutions of sugar, water, and impurities, by heat and lime, (the colonial operation.)

THIS important investigation will be approached with the greatest advantage by an examination of the phenomena attendant on the crystallization, from a menstruum of some body which is not capable of decomposition by the agency of heat.

For this purpose no substance is better than common nitre. If a portion of this substance, dissolved in water, were given to an operator with the object of evaporating away all the water of solution, and leaving the whole of the salt unchanged, this could easily be effected. The operator would simply have to apply heat to the solution, and the desired result would speedily be achieved. Whatever the amount of heat applied, no injury would occur to the salt, which would be found gradually incrusting the evaporating dish; and, by carrying on the process of evaporation to a sufficient extent, the whole of the nitre would be left dry. Under the circumstances, however, of rapid evaporation detailed, the salt would assume an imperfectly crystalline state; indeed, the chances are that no crystals would be visible. Were the object to obtain the nitre in perfect crystals, the evaporation should be modified thus:-The evaporation should be stopped short at a certain point, and the hot liquid allowed to cool-the result of which cooling would be the formation of well-defined crystals. A portion of the liquor of solution, however, would still remain uncrystallized, until drawn off and subjected to a process of re-evapora tion; when another crop of crystals would be formed, and another quantity of uncrystallized but crystallizable liquor would remain.

Upon the latter, the processes of reboiling and crystallizing might be repeated, until the total expenditure of the liquor of drainage; and with the result of obtaining literally the whole of the nitre employed--and crystallized, too; up to the period when the dimi nished amount of liquid to be evaporated and drained, furnished so small a mass, that the gradual cooling and perfect drainage, so essential to the production of good crystals, were conditions no longer under control.

The reader will have anticipated my coming remark, that this liquor of drainage stands in the same mechanical relation to nitre, that molasses does to sugar. Beyond this mechanical relation, however, the analogy ends—as will be presently made known. If, instead of nitre and water, a solution of pure sugar in pure water be taken, and treated according to the scheme just indicated, the results are as follow:

A portion of sugar crystallizes; but, instead of being white, as it was when dissolved, the crystals will have assumed a yellow tint, and the syrup of drainage will be more or less coloured. If this syrup be collected and evaporated, there will result another produce of crystalline sugar still more yellow than the first, and the liquor of drainage from this second product will also have acquired a much darker colour than its parallel in the first operation. Proceeding in

this way, there is at length a period arrived at, when the liquor of drainage becomes a dark-coloured viscid mass, incapable of crystallizing at all.

Thus, according to testimony of this experiment, it is impossible to extract, by the evaporative process (at least when heat is applied), the total amount of pure sugar dissolved in a quantity of pure water; a portion of such sugar being destroyed, and converted into a dark product.

If this operation had been conducted on a solution of sugar-not in pure water, but admixed with impurities of various kinds, such as coexist with the sugar in cane-juice; had the case been still more involved by the addition of a foreign agent, such as lime, the experimentalist might have imagined the destruction of sugar just indicated to be exclusively due to the agency of the collateral bodies: an explanation which is obviously inapplicable to the conditions just detailed. Indeed, the experiment of Professor Soubeiran, heretofore mentioned, sets all doubt at rest on this point.

The agency of heat alone being proved sufficient to effect a certain destruction upon a solution of pure sugar in water, it is an important point to determine the lowest amount of heat which is thus injurious; and whether a process of evaporation can be devised, which shall not overstep that limit of temperature where the injury first commences.

In the laboratory, a chemist easily solves the problem; indeed, it involves a process which he very constantly applies to effect the evaporation or desiccation of many bodies,-so delicate in their nature, that a slight artificial temperature would subject them to decomposition.

The chemist would proceed by taking advantage of the fact, that to produce evaporation the removal of atmospheric pressure is equivalent in effect to the application of artificial heat. He, therefore, would put under the receiver of an air-pump a shallow saucer, containing oil of vitriol; over which he would place, on a convenient support, a dish containing the sugar solution. A vacuum being now produced, the water of the solution would commence to be evolved as vapor; and this vapor would be immediately absorbed by the oil of vitriol. Thus, without any further working of the pump, a tendency to a vacuum would be kept up, until the sugar would have become dry, crystallized, and chemically unchanged.

I need not say that this process of evaporation is totally unadapted to any commercial case of sugar manufacture. A compromise, however, between two conditions, has been effected by the vacuum pan, (an instrument hereafter to be described,) which enables sugar solutions to be boiled in commercial quantities, under the joint circumstances of a partial vacuum, and a great diminution of temperature. The lowest practical temperature at which I have ever seen a vacuum pan worked is 135°; a temperature which I would, therefore, consider the practical minimum, but which is sufficient to effect a certain amount of destruction on sugar solutions.

Having recognized the fact, that the lowest practicable degree of heat for effecting the evaporation of sugar solutions, is sufficient in

itself to produce a certain amount of destruction-it now remains to be shown how much this amount of destruction is increased, by the concomitant agency of lime, and impurities.

Proceeding in the demonstration systematically, I will assume a portion of white sugar to be dissolved in water, admixed with lime, and then boiled. If this operation be performed, the eye alone will recognize the fact, that a destructive process supervenes to a far greater extent than when a mere solution of sugar and water, without the addition of lime, was employed. Not only does the fluid become dark with increased rapidity, but it exhales a newly developed smell, indicative of some process of decomposition effected upon the sugar. If, moreover, the crystallization of the limed liquid be attempted, a further proof of destruction will be manifest in the increased amount of non-crystallizable material, which leaks away from the crystallized

mass.

These are but rough indications of the injury to which sugar is exposed when solutions containing it are heated in combination with lime-indications which are so visible to all who have seen the operation performed, that there exists not a sugar producer, so far as my experience goes, who does not fully recognize the powerfully destructive agency of this alkaline earth. Nay, in the absence of other testimony, the multitude of contrivances which have, from time to time, been introduced to public notice, with the express intention of either diminishing indefinitely the amount of lime to be used, or of reducing the quantity to some definite standard-would be ample evidence in support of the position, that lime is commonly recognized to be a most destructive agent on sugar.

The minute chemistry of the agency of lime on sugar solutions, would be somewhat out of place here. So much of this agency as is necessary for the guidance of a practical sugar manufacturer has already been given in other parts of this treatise.

Having successively examined the agency of heat on a pure solution of sugar and water,--and on the same with admixture of lime; we have next to investigate the complex changes which occur during the treatment by heat of sugar solutions mixed with vegetable impurities and lime.

That the cases selected may be consecutively demonstrative, I will suppose the experimentalist to contaminate a portion of pure sugar and water with some raw vegetable juice--that of raw parsnips, for example. Thus contaminated, the solution will be amenable to a new series of chemical decompositions of greatly increased complexity; of which the following are the most remarkable, and of greatest practical value, to be well understood, and remembered.

The first great influence exerted on sugar solutions by the presence of raw vegetable juices generally, is that of causing various fermentations. Thus, although solutions of pure sugar in water may be allowed to remain exposed to temperatures most conducive to fermentation for days, and even weeks, without any perceptible effect of this kind, yet the addition of very small quantities of these raw vegetable juices causes them, under the same circumstances, readily