It is a consequence of this complicated structure that organic compounds are unstable in their character, and liable to decomposition, or, in other words, to resolve themselves into simpler compounds. An inorganic substance, on the contrary, however complex its formula may appear, is actually built up of binary compounds, the simplest that can be formed. But in the organic substance the carbon and hydrogen have a strong tendency to form carbonic acid; the hydrogen and oxygen to form water; the hydrogen and nitrogen to form ammonia; or, the hydrogen and the sulphur to form sulphuretted hydrogen, &c. In popular language, these changes are expressed by such terms as _decay_ and _putrefaction_. Liebig, however, has given precision to them by limiting the term _decay_ to the decomposition of moist organic matter freely exposed to the air, the oxygen of which gradually burns and destroys it without sensible elevation of temperature.[1] The term _putrefaction_ is limited to changes which occur in and beneath the surface of water, the effect being a mere transposition of elements or metamorphosis of the organic body.[2] The conversion of sugar into alcohol and carbonic acid is a simple illustration of the term. The contact of oxygen is, however, first necessary to the change, which, when once begun, is continued without the aid of any other external substance, except perhaps water, or its elements. Every instance of putrefaction begins with decay; and if the decay, or its cause, viz., the absorption of oxygen, be prevented, no putrefaction occurs. In short, if the access of oxygen be prevented, there is no decay; if the access of water be prevented, there is no putrefaction. The exclusion of air and moisture forms the basis of some of the best methods of preserving food. There are certain substances named _Antiseptics_ (from ἀντὶ, against, and σήπομας, to putrefy), from their property (exerted, however, very unequally) of preventing the putrefaction of organic substances. Thus, alcohol, and common salt in certain proportions, check all putrefaction and all the processes of fermentation by depriving the putrefying body of water. Nitre, vinegar, spices, and sugar are also antiseptics. The antiseptic effect of a very low temperature is caused by the solidification of the water and other juices, which, in their usual fluid state, allow the molecules to move freely on one another. We will first notice the various methods of preserving animal food. These are: 1, by drying; 2, by cold; 3, by salting and by sugar; 4, by smoking; 5, by vinegar; 6, by parboiling and excluding air; 7, by potting; 8, by alcohol.