The second general method of preventing fermen tation is desiccation, or the removal of that degree of moisture which is an essential condition to this kind of chemical action. Desiccation takes place in con sequence of the air absorbing the moisture of bodies exposed to its action. It is therefore promoted by the size of surface exposed, by the dry state of the atmosphere, increased temperature, and by the con stant change of the air in contact with the body to be dried, or, in other words, by exposing it to a free current of air. This mode of checking fermen tation is assisted by dividing or cutting the bodies to be dried, especially across the grain, which acts not only by increasing the surface, but perhaps still more by dividing the vessels containing the moisture or fluids, and thus allowing them to be freely acted upon by the air, to which the skin or epidermis, when entire, frequently exposes a very great obstacle. In phar macy, where exsiccation is often necessary, this is well understood, and expressed in the Collegiate direc tions for drying squills, and other succulent roots. In domestic economy, it is also practised in drying artichoke bottoms, and guarded against when vege tables are required to be kept succulent, which are or ought to be trimmed or wounded as little as possible till they are to be used. Animal substances also dry much more slowly so long as the surface is entire ; and hence some country butchers skin veal joint by joint only, as it is required. The influence of extent of surface is a matter of constant observation. A little water, which would have required days to have evaporated out of a wine glass, disappears in a few minutes when spread over a China plate. Also solid substances dry more quickly in proportion to the smallness of their size, as the surface exposed is pro portionably greater. But none of the agents in ac celerating exsiccation has so great an influence as the dry state of the air by which it is effected.
Mr Hearne (Journey to the Northern Ocean) gives an account of the manner in which the Indians of North America preserve, by means of mere exsiccation, the flesh of musk oxen, deer, or any other animal. To prepare meat in this manner, no farther operation is required, than cutting the lean parts of the animal into thin slices, and dry ing it in the sun, or by exposing it to the heat of a fire, when it is reduced to powder by beating it between two stones. Meat prepared in this way is very portable, and always ready for use, and it is very substantial ; for Mr Hearne found, that he could always travel longer without victuals after making a meal of it, than after any other kind of food. 1 he northern Indians dry their meat by the heat of a very slow fire, or by fastening it to the tops of the women's bundles, and allowing it to dry by the sun and wind as they walked along. But the southern In dians expose it to the heat of a very large fire, which, in Mr Hearne's opinion, exhausts its juices, renders it as hard as horn, and gives it a bitter taste, whereas the other is soft and mellow in the mouth, and en tirely free from smoke, Fish is also dried by them in the sun, and pounded for the sake of carriage.
The third general means of preventing fermenta tion is the total exclusion of atmospheric air, or ra ther of oxygen. The truth of this is not so obvious to common observation as that of the others ; for, on the contrary, we every day see substances spoiled by being apparently excluded from the air, but, in reality, by being shut up in confined air ; and we are taught to consider free ventilation as a power ful means of preservation. The manner in which the latter acts we have shown to be by removing moisture, especially when contaminated by the ex halation of the perishable substance. The ra
tionale of the former is more obscure, but has been ingeniously attempted by Gay Lussac and the Edinburgh Review (No. XLV.), in order to explain the processes of M. Appert. Gay Lussac found that neither fresh vegetable juices nor animal mat ter fermented so long as oxygen gas was perfectly ex cluded ; and that the fermentation, in both cases, com menced as soon as any portion of oxygen was admitted. When oxygen gas is confined in contact with a ferment able substance, it is changed into an equal bulk of car bonic acid gas, and all farther action ceases. Methods of preserving fermentable substances, illustrative of this principle, have long been practised imperfect ly by housewives. Nothing can be simpler than Mr Raffald's receipt for preserving green peas, cran berries, currants, &c. " Put them into dry clean bottles, cork them close, and tie them with a blad der; keep them in a cool dry place." A variation of this process was to fill the bottles previously with sulphurous acid vapour, by holding in them for some time a lighted sulphur match. One effect of this is to remove all uncombined oxygen. Other methods of ex eluding air were also employed, as filling up the inter stices with water or melted suet. The success of this process was greatly promoted by subjecting the sub stances to the action of a certain degree of heat, rafter being put into the bottles or jars in which they were to be preserved ; and then we are desired " to set them in a copper of hot water till they are hot quite through," or to " put them in an oven when the bread is drawn, and let them stand till shrunk a quarter part." In these cases, the heat seems to act by coagu lating and rendering insoluble and inactive a kind of gluten which seems to be a principal agent in beginning fermentation. In general, in the old processes, we were directed not to cork or tie up the vessels until they were quite cool, by which it now appears a very great advantage was lost. But Mr Saddington obtained, in 1817, a premium from the Society of Arts for a method of preserving fruit without sug ir, for house or sea stores ; the chief pe culiarity of which consisted in filling the bottles as soon as they are taken out of the bath, with boiling water to within an inch of the cork, and immediate ly corking them very tight and laying them on their sides, that the cork may swell and effectually ex clude the air. Animal substances have also long been preserved by the mere exclusion of air. The most familiar example is the buttering of eggs, which has the effect of closing the pores in the shell by which the communication of the embryo with the external air takes place. It is best performed by rub bing over the shell with butter while it is still warm after being laid ; and an egg in this way retains the curdy milk, and possesses all the properties of a new laid egg for a great length of time; but at whatever period after being laid the egg is but tered over, its progress to .decay seems to be arrested. The same effect is produced, though not so perfectly, by immersing eggs in water. From an experiment of Reaumur s, it appears that the cutting off the access of air to the embryo in the egg, does not kill it or prevent it from being hatched, but, on the contrary, preserves it alive for a much greater time than if it had not been treated in this manner. He covered over eggs with spirit varnish, and he found them capable of producing chickens after two years, when the varnish was care fully removed.