GERMICIDES, agents used to destroy or to hinder the growth of microscopical forms of plant and animal life.
Germicides may be grouped under three gen eral heads, those that act mechanically, those that destroy life by physical means and those whose action is chemical. Inasmuch as each particular germ is an individual with its own particular characters, definite methods for its killing must be devised. Thus it is well known that quinine, for instance, is very active in de stroying animal parasites, such as the malarial organism, but it is practically of no service in combating a large number of vegetable forms; and vice-versa, many substances which are capable of destroying plant germs are in efficient when applied to animals.
Of the physical agents, heat, light, cold and electricity, heat is the most satisfactory. High degree of temperature will destroy all forms of parasitic life. The animal forms succumb very readily to the influence of heat, and most of the plant parasites are destroyed by it; but whereas heat may be applied with great suc cess as a germicide in general disinfection, it naturally cannot be used as a general agent on the body. The red-hot iron in the form of a galvanocautery, or a heated wire, makes a most efficient form of cautery to destroy the poison of dog-bite, or to destroy localized forms of tuberculosis, etc., but heat is thus limited in its application.
Cold was formerly regarded as a powerful germicide; but now there is a tendency to em phasize other factors than cold itself as potent Cold— which kills the bacteria of yellow fever, but not those of smallpox or typhoid—may even act as a preservative of germ life, as is established by the high germicidal content of frozen food stuffs, after months and weeks in cold storage. Recent investigation has led to the conclusion that the degree of cold, time of freezing, crystallization and external pressure, and the composition in which the freezing oc curs, all have an influence on the germicidal potency of cold.
Light, especially sunlight, is a very efficient germicide, but the exposure must be continued for an appreciable length of time. Sunlight is nature's great germicide. Within recent years the light given by the R6ntgen ray (X-ray), by radium, polonium, thorium and similar agents, has been used with great success in the treatment of certain parasitic skin diseases, but whether the effects are due to any ger micidal action of the light, or to a normal tissue stimulation, is not decided. It seems from experiments thus far recorded that these forms of light are not definite germicides.
Various colored lights, particularly red and am ber, are known to restrict the growth of certain forms of bacteria. They do not, however, de stroy them. This principle is made use of in smallpox hospitals and similar institutions, but it does not seem that the results are sufficiently striking to base any general deductions thereon. Electricity is not an efficient germicide. The passage of electrical currents through water does not necessarily kill the bacteria contained therein, notwithstanding the many claims made by enterprising manufacturers of electrified water, said to be made germ-free by the electri cal current.
Chemical germicides are numerous, both for external and internal use, although intracel lular germicides, or those that can be used within the tissues of the body, are much to be desired. The list of chemical germicides is enormous. Thousands of different agents have been used and these exhibit varying degrees of germicidal activity. As has already been said, each form of germ possesses its own powers of resistance, and each germicide its ability to kill in varying degrees of strength. The germicides in popular use are chlorinated lime, carbolic acid, creosote, alcohol, boracic acid, ammonia, formaldehyde, hydrogen peroxide, iodine and its preparations, mercury and its preparations, volatile oils of cinnamon, mustard, peppermint, turpentine, pen nyroyal, oxygen, quinine, salicylic acid and its derivatives. Of these for external use, for use in closets, in bedding, for linen, etc., carbolic acid in the percentage of 1 to 50 of water, for maldehyde in percentage of a teaspoonful of the 40 per cent solution of the gas to a quart, bichloride of mercury in the proportion of one part to 1,000, are the most practical and con venient germicides. So far as is now known, quinine is about the only efficient chemical sub stance that can be used as an intracellular ger micide. It has the singular property of poison ing the malarial parasites within the red blood cell without poisoning the blood-cell itself, a selective property which most poisonous agents lack.
The great germicide of the human body, and the one that protects it in its various strug gles with different forms of parasites, is the blood-serum. This is a very efficient germicide, a full consideration of the action of which will be taken up under the heading of IMMUNITY. Consult Buck,