Particular terms have been applied to particular cases of solution.
The solution of a fluid in the atfhos phere is termed spontaneous evaporation. It is promoted by exposing sur face, by frequently renewing the air in contact with the surface, and by increase of temperature.
Some solids have so strong an affinity for water, that they attract it from the at mosphere in sufficient quantity to dissolve them; these are said to deliquesce ; others, on the contrary, retain their water of crys tallization with so weak a force, that the atmosphere attracts it from them, so that they crumble into powder. These are said to effloresce. Both operations are promoted by exposing large surfaces, and by a current of air; but the latter is facili tated by a warm, dry air, and the former by a cold, humid atmosphere.
Solution is also employed to separate substances, (for example, saline bodies), which are soluble in the menstruum, from others which are not. When object is to obtain the soluble substance in a state of purity, the operation is term ed lixiviation, and as small a quantity of the menstruum as is possible is used. When, however, it is employed to free an insoluble substance from soluble impuri ties, it is termed edulcoration, which is best performed by using a very large quantity of the menstruum.
Organic products being generally com posed of heterogeneous substances, are only partially soluble in the different mcnstrua. To the solution of any of these substances, while the others remain un dissolved, the term extraction is applied; and when, by evaporation, the substance extracted is reduced to a solid form, it is termed an extract, which is hard or soft, watery or spiritous, according to the de gree of consistency it acquires, and the nature of the menstruum employed.
Infusion is employed to extract the vir tues of aromatic and volatile substances, which would be dissipated by decoction, and destroyed by maceration, and to se parate substances of easy solution from others which are less soluble. The pro
cess consists in pouring upon the sub stance to be infused, placed in a proper vessel, the menstruum, either hot or cold, according to the direction, covering it up, agitating it frequently, and, after a due time, straining or decanting off the li quor, which is now termed the infusion.
Maceration differs from infusion, in be ing continued for a longer time, and can only be employed for substances which do not easily ferment or spoil.
Digestion, on the other hand, differs from maceration only in the activity of the menstruum being promoted by a gentle degree of heat. It is commonly performed in a glass matrass, which should only be filled one-third, and co vered with a piece of wet bladder, pierc ed with one or more small holes, so that the evaporation of the menstruum may be prevented as much as possible, with out risk of bursting the vessel. The ves sel may be heated, either by means of the sun's rays, of a common fire, or of the sand-bath : and when the last is employ ed, the vessel should not be sunk deeper in the sand than the portion that is filled. Sometimes when the menstruum employ ed is valuable, a distilling apparatus is used to prevent any waste of it. At other times, a blind capital is luted on the ma trass, or a smaller matrass is inverted within a larger one ; and as the vapour which arises is condensed in it, and runs back into the larger, the process in this form has got the name of circulation, up on which we have observed already. Decoction is performed by subjecting the substances operated on, to a degree of heat which is sufficient to convert the menstruum into vapour, and can only be employed with advantage for extracting principles which are not volatile, and from substances whose texture is so dense and compact as to resist the less active methods of solution. When the menstru urn is valuable, that portion of it which is converted into vapour is generally saved by condensing it in a distilling appara tus.