Now, supposing we make another experiment with the test tube, and instead of boiling we add to its contents a few drops of carbolic acid; we find that decomposition is prevented almost as effectually as by the use of the cotton wool. There are many other substances which act like carbolic acid, and they are known by the common name of antiseptics or antiseptic agents. They all act in the same way; and in such cases as the dressing of wounds it is more easy to use this method of excluding bacteria than by the exclusion of the air or by the use of cotton wool. We have here another object for inquiry--viz., the particular property of these different antiseptics, the property which they possess of preventing decomposition. This knowledge is _very_ ancient indeed. We have the best evidence in the skill of the Egyptians in embalming the dead. These substances are obtained from wood or coal, which once was wood. Those woods which do not contain some antiseptic substance, such as a gum or a resin, will rot and decay. I am not sure that we can give a satisfactory reason for this, but it is certain that all these substances act as antiseptics by destroying the living organisms which are the cause of putrefaction. Some are fragrant oils, as, for example, clove, santal, and thyme; others are fragrant gums, such as gum bezoin and myrrh. A large class are the various kinds of turpentine obtained from pine trees. We obtain carbolic acid from the coal tar largely produced in the manufacture of gas. Both wood tar, well known under the name of creosote, and coal tar are powerful antiseptics. It is easy to understand by what means meat and fish are preserved from decomposition when they have been kept in the smoke of a wood fire. The smoke contains creosote in the form of vapor, and the same effect is produced on the meat or fish by the smoke as if they had been dipped in a solution of tar--with this difference, that they are dried by the smoke, whereas moisture favors decomposition very greatly. I can show why a fire from which there is much smoke is better than one which burns with a clear flame, by a simple experiment. Here is a piece of gum benzoin, the substance from which Friar's balsam is made. This will burn, if we light it, just as tar burns, and without much smoke or smell. If, instead of burning it, we put some on a spoon and heat it gently, much more smoke is produced, and a fragrant scent is given off. In the same way we can burn spirit of lavender or eau de Cologne, but we get no scent from them in this way, for the burning destroys the scent. This is a very important fact in the disinfection of the air. The less the flame and the larger the quantity of smoke, the greater the effect produced, so far as disinfection is concerned. As air is a vapor, we must use our disinfectants in the form of vapor, so that the one may mix with the other, just as when we are dealing with fluids we must use a fluid disinfectant.