poisons (Behring's hypersensibility); while, on the other hand, marked immunity may exist without the presence of antitoxins in the blood.

As the result of such observations, a distinction has been made between tissue-immunity and serum-immunity (antitoxin-immunity). The latter is a transitory condition, dependent upon alterations in the blood-mixture from the presence of the circulating toxins; the first is a permanent condition, dependent upon changes in the tissues, upon the activity of the cells, which have become insusceptible to the poisons. Tissue-immunity, or histogenic immunity, is not to be referred to the presence of antitoxins. Fowl, which are highly immune to tetanus, possess little if any antitoxin; but their blood becomes at once antitoxic after injection of tetanus-toxin.

Recently, Behring has returned to his original view, and believes that acquired toxin-immunity, active as well as passive, is always hematogenous—that is, dependent upon the antitoxic activity of the serum. As histogenic he considers only the natural immunity to the bacterial poisons. Finally, the antitoxins alone are as incapable as the other anti-bodies or as phagocytosis of explaining all of the manifestations of immunity.

We have in the foregoing presented, as objectively as possible, the facts that investigation in the domain of immunity have developed in great abundance. They do not permit of the establishment of a single, universally applicable theory of immunity. They rather render it probable that there is no such unity, but that immunity is eventually not a simple and indivisible process; dependent in all cases upon one and the same basis, but, apparently, variable and complex in its nature, dependent in one instance upon this, in another instance upon that, cause, and more frequently due to several in combination.

RELATIONS BETWEEN IMMUNITY AND CURE.-In the case of scarlet fever, measles, and other like diseases, recovery from an attack is attended with immunity. If in the case of other diseases-as, for instance, pneumonia, erysipelas, etc.-recovery from one attack rather predisposes to subsequent attack, this does not exclude the fact that at the time of recovery immunity existed, a so-called temporary immunity, which disappears in the course of a few days or weeks. It is noteworthy in this connection that the dem

onstration was first made experimentally for pneumonia, later for typhoid fever, diphtheria, and cholera, that the blood of individuals convalescent from these diseases exhibits in many instances transitory immunizing activities with relation to the respective bacteria. It appears from this as if recovery from these diseases also is attended with immunity, but that this-through causes not yet made clear-is a quickly passing one.

It has been demonstrated experimentally with certainty that immunization may also lead to cure. By means of serum-immunization it is possible, if the serum is derived from animals highly enough immunized, to induce curative results, even when the treatment is begun a certain time after infection has taken place. These facts are most conclusively demonstrable experimentally in the case of tetanus. With large amounts of serum from animals immunized to tetanus, it is possible to save mice and guinea-pigs that already exhibit distinct tetanic manifestations. We shall refer more fully to these relations in the special section. (See Diphtheria and Tetanus.)

According to the foregoing, two facts have been demonstrated—in the first place, that immunity exists at the time of recovery in the sequence of toxic diseases in man, and, in the second place, that it is possible experimentally to cure infection by the timely establishment of immunity. From this the conclusion may, with all probability, be drawn that also in human beings the connection between recovery and immunity consists in the bringing about of recovery through the development of immunity; recovery from the given disease immunizes the organism, and cure is effected in consequence of immunization naturally induced, and especially the crisis appears as the expression of cure through the sudden setting in of immunity. Upon this knowledge is based recent therapeutic effort, which is known as serum-therapy or immunization-therapy. The object to be attained is the immunization of the diseased organism after infection has taken place-that is, the effecting of a cure in the same way as nature brings about recovery in cases of infectious disease pursuing a favorable course. This mode of therapy is naturally specific, as immunity also is specific. Much more serum, however, is required to effect a cure than to induce immunity; or, what amounts to the same thing, the serum of much more highly immunized animals.

Recent experiments of Dönitz with tetanus-antitoxin show, further, in a most conclusive manner, that the amount of serum necessary for curative purposes is the greater the longer the period of time that has elapsed between the intoxication and the institution of serum-therapy. Eight minutes after tetanus-intoxication, according to Dönitz, six times as much serum is required in order to save the animal as when the serum is injected immediately after the poison; after an hour the curative dose is twenty-four times the original dose; and so on, until finally a period is reached at which it is entirely impossible to save the animal, even with the largest amount of the most active serum. For this reason it is, above all things, essential in obtaining serum for therapeutic purposes to establish in the animals yielding the blood (generally horses) as high a degree of immunity as possible. The higher the degree of immunization established, the smaller the amount of serum required to effect cure. In making the degree of immunity as high as possible it must be borne in mind that the immunizing process pursues a wave-like course (Brieger and Ehrlich). Immediately after introduction of the next higher toxic dose the immunizing power of the blood-serum diminishes. It remains for a few days at the lower level, gradually rising again until it reaches the maximum. From this point it sinks again, and it finally reaches a level at which it persists for weeks. The most favorable time for injecting new toxin into the animals in order further to fortify their immunity is when the strength of the serum is highest-that is, when the anti-bodies are present in the body in largest number.

The efforts in the domain of curative serum-therapy have already yielded material practical results in the case of diphtheria. In that of tetanus success is as yet doubtful. We shall refer fully in the special section in the discussion of these two diseases to the mode of obtaining and of estimating and to the dosage of the curative serum.

Naturally, immunization is not the only manner in which therapeutic attack upon bacterial diseases is to be made. An infectious disease may be terminated by reason of the death of the bacteria in the body. It would be possible to effect cure in this way if the infectious agent could be destroyed within the body by means of internal disinfection. In spite of the large number of antiseptics at our command,

and in spite of their promptness in destroying bacteria in test-tubes, they are not applicable to the living body, because they either fail or, employed in the necessary strength, destroy not only the bacteria, but also the cells of the body. The possibility, however, that a disinfectant may yet be found that will destroy only the bacteria without affecting the tissues, must remain an open one.

IV. METHODS OF CULTURE AND OF EXAMINATION.

STERILIZATION.

In order to follow the individual varieties of bacteria perfectly in the course of their development, and in order to employ them in animal experimentation, it is necessary to obtain them in pure culture. In the making of such pure cultures the most scrupulous care must be observed to exclude the large number of bacteria that are everywhere present. The instruments employed in the various manipulations, the nutrient media, and the vessels that serve as the field of development for the bacteria, must be absolutely germ-free-sterile. To accomplish sterilization of these, ordinary antiseptic measures can not be employed, as the addition of substances capable of destroying the germs or of inhibiting their activity would naturally render the nutrient media unsuitable for culture-purposes. For this reason heat exclusively is employed for the sterilization of all materials used in the culture of bacteria, and both dry heat, as well as moist heat, in the form of live steam.

Dry Heat. As dry heat penetrates but slowly into the interior of objects, it is employed principally for the sterilization of articles of small volume only; thus, platinumneedles are sterilized directly by exposure to the flame of a spirit-lamp or of a Bunsen burner, and other instruments by being moved to and fro for about a minute immediately above the flame. Articles made of glass and other substances that tolerate high temperatures are placed in a double-walled sheet-iron receptacle covered with asbestos (drying chamber), which is heated to between 150° C. (302° F.) and 170° C. (338° F.) by means of a gas-flame burning beneath it. (Fig. 9.) After exposure for half an hour to air thus heated to from 150° C. (302° F.) to 170° C.

(338° F.), even the most resistant spores are destroyed. It suffices, further, in this mode of sterilization, to heat the drying chamber in which the instruments to be sterilized are placed, until a thermometer introduced from the top indicates a temperature of 170° C. (338° F.); then the supply of gas is cut off, and after the apparatus has completely cooled, the now sterile contents are removed.

Live Steam.-Most substances, however, that are employed in bacteriologic investigations, especially the nutrient media, do not bear sterilization by means of such high degrees of heat as have been mentioned; they are, therefore, rendered germ-free by means of live steam. For this purpose they are introduced into a cylindric apparatus, made of galvanized iron or of copper, and covered with felt or with asbestos. (Koch's steam-chamber, Figs. 10, 11.) This vessel is divided by means of a perforated shelf or a wire grating into an upper larger and a lower smaller