What the 'vital principle' may be, neither chemists nor physiologists can tell us with any great degree of clearness; but it is this vital principle, whatever it may be, which prevents decay in a living organic substance, however complex. When life departs, the onslaught begins; the defender has been removed, and a number of assailants make their appearance. _Air_, _heat_, and _moisture_ are the principal of these; they attack the dead organism, and gradually convert it into wholly different and inorganic compounds, such as water, carbonic acid, ammonia, phosphuretted hydrogen, and many others. What, then, would result if these disturbers could be warded off, one or all? It is now pretty well ascertained, that if any one of the three--air, heat, moisture--be absent, the decay is either greatly retarded or indefinitely postponed; and we shall find that in all antiseptic or preserving processes, the fundamental principle has simply such an object in view.
Sometimes the operation of natural causes leads to the preservation of dead animal substances for a great length of time, by excluding one out of the above three disturbing influences. If heat be so deficient that the animal juices become wholly frozen up, the substance is almost proof against decay. Thus, about seventy years ago, a huge animal was found imbedded in the ice in Siberia: from a comparison of its skeleton with those of existing species, Cuvier inferred that this animal must have been antediluvian; and yet, so completely had the cold prevented putrefaction, that dogs willingly ate of the still existing flesh. At St Petersburg, when winter is approaching, the fish in the markets become almost like blocks of ice, so completely are they frozen; and in this state they will remain sound for a lengthened period. Dead poultry, and other articles of animal food, are similarly kept fresh throughout the winter in many rigorous climates, simply by the powerlessness of the attacking agents, when heat is not one of the number. And that which nature effects on a large scale, may reasonably be imitated by man on a more limited one. It is customary to pack many kinds of provisions in ice or snow, either for keeping them in storehouses, or for sending them to market. Thus it is with the tubs of poultry, of veal, and of other kinds of meat, which, killed in the country districts of Russia in autumn, are packed in snow to keep cool till sold at market; and thus it is with much of the salmon sent from Scotland to London. Since the supply of excellent ice from Wenham Lake, commenced about nineteen years ago, has become so abundant and so cheap, it is worth a thought whether the preservative powers of cold might not advantageously be made more available in this country than they have yet been. In the United States, housewives use very convenient refrigerators or ice-boxes, provided with perforated shelves, under which ice is set, and upon which various provisions are placed: a large uncooked joint of meat is sometimes kept in one of these boxes for weeks. Among the celebrities of the Crystal Palace, many will recollect Masters's elegant ice-making machine, in which, by combining chemical action with centrifugal motion, ice can be made in a few minutes, let the heat of the weather be what it may. This machine, and the portable refrigerators manufactured by the Wenham Company, together with our familiar, old-fashioned ice-houses, might supply us with much more preservative power, in respect to articles of food, than we have hitherto practically adopted.