From considering the properties of hard and soft water, we pass by a natural transition to the employment of that liquid in culinary operations. In practice, nothing can at first sight appear more simple than the operation, of boiling, whether it be confined to the mere heating of a liquid, or extended to the preparation of an article of food; yet it is one which involves chemical principles of a very high order, and which is by no means so simple a matter as it may be regarded at a cursory glance.

To trace the steps of the process from its commencement, let us imagine a vessel of water placed over the fire, and receiving constantly a supply of heat from that source; the effect is, that its temperature gradually rises from about 50° or 60°, the usual warmth of ordinary water, to 212°, the point at which boiling takes place; but before it reaches that height, a number of bubbles may be observed foaming on the sides of the vessel; these gradually increase in size, and when they become sufficiently buoyant, quit their position, rise to the surface, and escape; they consist of air previously dissolved in the water, and which is expelled by the increased heat. "Water which has been boiled and allowed to become cold, without much exposure to the air, fails to reabsorb the quantity it previously contained, and consequently has its character somewhat altered. Thus, it freezes more readily than water which has not been boiled, in consequence of the air not having to be expelled in the act of solidifying, as is usually the case: hence, the ice from boiled water is free from those numerous air bubbles which are always to be observed in common ice. It possesses also a mawkish, unpleasant taste, and is totally unable to preserve the life of any aquatic animal. The presence of this minute quantity of air in ordinary water, is very essential to its utility. Faraday found that water, totally destitute of air, does not boil in the usual mode, but when heated to the boiling point, it at once, with an instantaneous and violent explosion, passes into the form of steam. This strange fact, which shows upon what small, and. apparently trivial circumstances, the comfort - nay, we may truly say - the existence of man depends, is strikingly shown by a very ingenious experiment, devised by that celebrated chemist. He took a piece of Wenham Lake ice, (which, from peculiar local causes, such as being formed from spring water, is totally destitute of air,.) and melted it under a covering of sweet oil; this prevented the absorption of any air during the liquefaction; on continuing the heat, the water rose in temperature, and on reaching the boiling point, suddenly burst into steam, with an explosive power sufficiently great to scatter the glass vessel in which the experiment was made into fragments; and had it not been for a protecting covering of wire gauze, very serious effects might have ensued.

From the precipitation of the dissolved chalk present in most kinds of water, a cloudiness or slight turbidity is always to be observed in boiled water.

After the escape of the air, bubbles of steam, at first very small in size, are formed at the bottom of the vessel, those formed at first are at once cooled from the whole water not being of an equal temperature, and are condensed before they reach the surface: this very rapid and successive condensation of numerous small bubbles gives rise to that peculiar vibration which occasions what is termed the singing of the teakettle, and which, as is well known, is indicative of its approach to the boiling point; when the whole water is uniformly heated, this effect no longer occurs, but the bubbles of steam rise to the surface and escape. After having been heated to 212°, the temperature of water no longer rises; it is not possible, under ordinary circumstances, to increase the temperature in the slightest degree; for all the extra heat that is given to boiling water merely produces increased quantity of steam, by which it is carried off, without affecting the heat of the remaining* water. This is a matter of considerable practical importance in cookery; and it should be always borne in mind, that the most gentle simmer is as efficacious in cooking as the most violent boiling; for the degree of heat in both cases is precisely the same, so that after having once raised the water to the boiling point, the most moderate fire is sufficient in ordinary cases to keep it there; by attention to this point, a most enormous saving may often be effected in the consumption of fuel, although this is a consideration that will be more fully entered into in a subsequent chapter.

Thick liquids, which do not readily permit the escape of steam or the rapid motion between the particles of fluid, may, however, be readily heated at the part exposed to the fire to a much higher degree, while those portions not immediately in contact with the heat are much colder; from this cause they are very apt to be charred, and if articles of food, they are totally spoiled. To avoid this effect, recourse may be had to the bain marie, which is simply the same contrivance that may be observed in a carpenter's glue-pot, applied to the preparation of articles of food, - being merely an inner vessel to contain the substance to be heated; this is placed in an outer one, the space between the two containing water. On placing this contrivance on the fire, it is obvious that the substance in the inner vessel being heated solely by the boiling water, cannot possibly become burnt. This most useful contrivance is adopted in all first-class kitchens, and is equally indispensable in the chemist's laboratory; by its aid, soups, gravies, etc, can be kept hot any length of time without risk, preserves made without burning, etc. The chief precautions required in its use are, that the inner vessel should be thin and formed of metal, so as to allow the rapid transmission of heat from boiling water, and care should be taken that the outer vessel does not boil dry. One serious disadvantage attends its use as ordinarily employed: it is, that it is impossible to heat substances in it to the boiling point, for the water itself is only at that temperature, and the substance in the inner vessel is always a few degrees below. This evil, however, may be entirely obviated, by using a solution in the outer vessel, which boils at a higher temperature than 212°, and which will therefore raise the inner vessel and its contents to that point; thus, if the water be made to dissolve as much common salt as it is capable of doing, it will not boil until it is heated to 224°; or if it is saturated with sal-ammoniac or nitre, the heat will rise 12° or 14° higher. We need scarcely say that the first of these substances will be found a very useful and economical addition to the bain marie. When chemists require a still higher temperature, they have recourse to a bath of olive oil, which is capable of bearing a degree of heat as high as 500°; but its extreme danger over an open fire entirely precludes its use in any culinary operation.

The mode of conducting the operation of boiling should not be uniform, but vary with the different purposes required. Thus, in the case of meat, a temperature of 212° hardens, instead of softening, two of the substances which it contains; namely, the fibrine, or material forming the chief part of the muscular fibre, and the albumen, or portion which is analogous to white of egg; if, on the contrary, meat is cooked by means of water at a lower temperature, the most nutritious parts are dissolved out, and the solid food left comparatively innutritious. The celebrated German chemist, Liebig, proposes the following plan:- he recommends that a piece of meat of considerable size should be taken and plunged into perfectly boiling water, over a good fire; that the water should be kept boiling for a few minutes, and then a portion of cold water, equal in quantity to about one-half of the boiling water, should be thrown in: this will reduce the temperature to about 160°, at which point the meat should be kept until thoroughly done; which, however, takes a much longer time than the ordinary mode.

The object of this mode of proceeding is, in the first instance, to harden the exterior of the meat, converting it into a sort of crust, which prevents the escape of the nutritious juices into the water, while the long continuance of the gentle heat afterwards cooks the interior without hardening either the albumen or the fibrine. Of course, where the object of boiling is to make soup, the opposite plan must be had recourse to; the meat should be in small instead of large pieces, put on in cold water and very slowly heated, so that all the soluble parts may be dissolved before the fibre is hardened by the action of boiling water.

In boiling eggs, the effect of heat in hardening the albumen is well known; by being suddenly plunged into boiling water, the outside is hardened to the greatest degree of which it is capable, and is thereby rendered exceedingly difficult of digestion, while the inside is barely warmed; if, on the contrary, they are placed in cold water, which is then raised to the boiling point, removed from the fire, and allowed' to stand about a minute (or two, if required to be well done,) it will be found that, instead of having an almost leathery consistence, the white will be uniformly partially hardened, and will furnish a much more pleasant and digestible article of diet; the improvement, in fact, is so great, that common eggs cooked in this manner very nearly approach new laid ones in quality. If the operation of boiling has to be performed on any substance containing starchy matters - as potatoes, rice, flour, etc, then the heat must, on the contrary, be raised to a sufficient degree to burst the little grains of which the starch consists, and liberate the interior nutritious portions, before it can become fit food for man; uncooked starch not being readily or easily digested. And even in the case of those "Vegetable-feeding animals whose power of digesting such substances surpasses that of man, there is the greatest advantage to be derived from the use of cooked food, as the most intelligent and scientific farmers at the present day well know; and we would strongly urge on those of our readers who keep pigs to try the experiment of baking the potatoes they give them; for this process, like boiling, has the effect of bursting the starch grains; they will find the effect to be, that the food will go much further, all of it being digested, and that the quality of the flesh will be very materially improved.