Ueber bakteriologische Forschung: Vortrag in der 1. allgemeinen Sitzung des X. internationalen medicinischen Congresses am 4. August 1890

(On Bacteriological Research: Lecture at the 1st General Session of the 10th International Medical Congress on August 4, 1890)

Lecture by Robert Koch, here Google-translated to English. The original lecture text is available at https://archive.org/details/b22305270/page/3/mode/1up.

When I received the honorable commission to deliver one of the lectures for the international congress, I was faced with the choice of choosing the topic for this lecture from the science with which I now primarily concern myself, namely hygiene, or from bacteriology, to which I was able to devote myself almost exclusively for many years.

I opted for the latter because I assume that bacteriology still commands the most widespread interest, and so I will attempt to outline for you, in brief, the current state of bacteriological research, at least in some of its more important areas. Admittedly, I will offer nothing new to those familiar with bacteriology. However, so as not to appear entirely empty-handed to them either, I intend to weave into my presentation some facts discovered during my ongoing studies on tuberculosis that have not yet been made public.

Bacteriology, at least as far as it is relevant to us physicians, is a very young science. Just fifteen years ago, little more was known than that peculiar, foreign-like structures appear in the blood in cases of anthrax and relapsing fever, and that so-called vibrios [a type of bacteria] occasionally occur in wound infections. No proof had yet been provided that these substances could be the cause of those diseases, and with the exception of a few researchers, such findings were regarded more as curiosities than as potential pathogens. This was hardly surprising, since it had not even been proven that these were independent organisms specific to these diseases. Bacteria indistinguishable from anthrax bacilli had been found in putrefying fluids, particularly in the blood of suffocated animals. Some researchers refused to consider them living organisms at all, instead viewing them as crystalline structures. Bacteria identical to relapsing fever spirochetes were said to be found in swamp water and dental mucosa, and bacteria identical to the micrococci of wound infection diseases were allegedly found in healthy blood and healthy tissues.

With the available experimental and optical aids, no further progress could be made, and this would likely have remained the case for quite some time had it not been for the emergence of new research methods that suddenly brought about entirely different conditions and opened the way for further penetration into this obscure area. With the aid of improved lens systems and their appropriate application, supported by the use of aniline dyes, even the smallest bacteria became clearly visible and morphologically distinguishable from other microorganisms. At the same time, the use of nutrient substrates, which could be converted into liquid or solid forms as needed, made it possible to separate individual microbes and obtain pure cultures, from which the specific characteristics of each individual species could be determined with complete certainty.

The capabilities of these new tools soon became apparent. A number of new, well-characterized species of pathogenic microorganisms were discovered, and, of particular importance, the causal link between these and the associated diseases was also demonstrated. Since all the pathogens found belonged to the group of bacteria, this must have given the impression that actual infectious diseases were caused exclusively by specific and distinct types of bacteria, and one could even hope that the causative agents for all infectious diseases would be found in the not-too-distant future.

This expectation, however, has not been fulfilled, and the further development of bacterial research has taken several unexpected turns in other respects as well. If I first consider the positive results of bacteriological research, I would like to highlight the following points.

It is now considered completely proven that bacteria, like higher plant organisms, constitute distinct, though sometimes difficult to define, species. The opinion, tenaciously held until just a few years ago and still advocated by some researchers, that bacteria are mutable in a way unlike all other living beings and can assume sometimes these morphological or biological properties, sometimes others entirely different from them, and that at most only a few species can be assumed; or that bacteria are not independent organisms at all, but rather belong to the evolutionary cycle of molds or, as some believed, of lower algae; furthermore, the view, which further undermines their independence, that they are descendants of animal cells, e.g., blood cells; All these views are untenable in light of the overwhelming number of observations collected, which without exception indicate that we are dealing here with well-characterized species.

If we consider the fact that some bacterial infectious diseases, such as leprosy and phthisis [tuberculosis], were already described in their unmistakable characteristics by the earliest medical writers, we could even conclude that pathogenic bacteria tend to retain their properties over long periods rather than rapidly changing them, as is generally assumed given the mutable nature of some epidemic diseases. Within certain limits, however, deviations from the usual type of species can occur in bacteria, and especially in pathogenic bacteria. However, bacteria do not differ in this respect in the slightest from higher plants, in which numerous changes, mostly attributable to external influences, can be found, which at most lead us to speak of varieties, but to let the species as such remain.

It can happen that a bacterial species, under unfavorable nutritional conditions, produces stunted forms, and that individual, conspicuous properties, or those of interest to us from a medical standpoint, but perhaps of little importance to the overall growth of the plant [here “plant” is used to refer to the bacteria, which at the time were considered a type of single-celled plants]—for example, the formation of a pigment, the ability to grow in the living animal body, or the production of certain toxins—may temporarily or, as far as current experience allows, even completely disappear. However, these are always only fluctuations that remain within certain limits and never deviate so far from the average of the species type that one would need to assume a transition to a new or already known species, for example, from anthrax bacillus to hay bacillus.

Since bacteria, due to their small size, lack the comprehensive morphological characteristics used for systematic classification found in higher plants, we are all the more dependent on identifying species by refraining from relying on individual characteristics, especially when it is impossible to know in advance whether these characteristics are fixed or variable. Instead, we must meticulously collect as many characteristics as possible, even those that may initially seem insignificant—both morphological and biological—and only then determine the species based on the overall picture thus obtained. In this respect, one cannot be too thorough, and many misunderstandings and contradictions encountered in bacteriology can be traced back to the unfortunately still insufficient adherence to this principle.

A very characteristic example of the difficulty involved in species identification is provided by the typhus bacillus. If one encounters the same bacteria in the mesenteric glands, spleen, or liver of a typhoid corpse, then there will likely never be any doubt that one is dealing with genuine typhoid bacilli, since no other bacteria that could be confused with them have ever been observed in these locations.

However, the situation is quite different when it comes to detecting typhoid bacilli in intestinal contents, soil, water, or airborne dust. Numerous bacilli very similar to them are found there, which only a highly experienced bacteriologist, and even then not with absolute certainty, can distinguish from typhoid bacilli, since there is still a lack of unmistakable and consistent characteristics. The recent claims that typhoid bacilli have been detected in soil, tap water, and foodstuffs can therefore only be accepted with justified skepticism. The situation is similar with diphtheria bacteria.

By a fortunate coincidence, however, certain reliable characteristics were found for some other important pathogenic bacteria, such as tubercle bacilli and cholera bacteria, allowing them to be reliably identified under all, even the most difficult, circumstances. The significant advantages resulting from the reliable diagnosis of pathogens in these cases must be a compelling reason for us to continually search for similar reliable characteristics for typhoid, diphtheria, and other important pathogenic bacteria, despite all previous fruitless efforts. Only then will it be possible to trace these pathogens along their hidden and often convoluted pathways outside the body and thus obtain a solid foundation for effective prophylaxis [measures to reduce the spread of disease].

But how cautious one should be in evaluating the characteristics used to distinguish bacteria, even in well-known species, I learned from the tubercle bacilli. This species of bacteria is known to be so precisely characterized by its reaction to dyes, its vegetation in pure cultures, and its pathogenic properties—by each of these characteristics individually—that confusion with other bacteria seems entirely impossible. And yet, even in this case, one should not rely on a single one of the aforementioned characteristics for species identification, but rather follow the established rule that all available properties must be considered, and only when they all coincide can the identity of the bacteria in question be considered proven.

When I conducted my initial investigations on the tubercle bacilli, I made it a point to proceed strictly according to this rule, and accordingly, tubercle bacilli of the most diverse origins were tested not only for their reactions to dyes, but also for their vegetation in pure cultures and for their pathogenic properties. This was impossible only with regard to chicken tuberculosis, as it was not possible at that time to obtain fresh material from which I could have cultivated pure cultures. However, since all other types of tuberculosis had yielded identical bacilli, and the bacilli of chicken tuberculosis completely matched them in appearance and behavior towards aniline dyes, I believed I could, despite the remaining gap in the investigation, assert their identity.

Later, I received pure cultures from various sources, which supposedly originated from tubercle bacilli, but differed from them in several respects; in particular, the infection experiments carried out on animals by experienced and entirely reliable researchers had also led to divergent results, which are still considered unresolved contradictions. Initially, I thought I was dealing with changes such as are not uncommon in pathogenic bacteria when they are cultivated outside the body, i.e., under more or less unfavorable conditions, for a longer period of time.

To solve the riddle, attempts were made to transform the common tubercle bacilli into the aforementioned supposed variety through a wide range of influences. They were cultivated for many months at temperatures so high that only meager growth occurred; in other series of experiments, even higher temperatures were repeatedly applied to the cultures for extended periods until they were brought as close as possible to death. Similarly, chemicals, light, and dehydration were applied to the cultures; they were cultivated together with other bacteria in many generations; and inoculated in continuous series onto less susceptible animals. But despite all these interventions, only minor changes in the properties could be achieved, which fell far short of what occurs under the same conditions in other pathogenic bacteria.

It therefore appears that the tubercle bacilli tenaciously retain their characteristics, which is also consistent with the fact that pure cultures of these bacilli, which I have been cultivating in test tubes for more than nine years now, and which have never since been introduced into a living organism, have remained completely unchanged except for a slight decrease in virulence.

When all attempts to find the connection had failed, a chance encounter finally brought the explanation. A year ago, I received some live chickens suffering from tuberculosis, and I used this opportunity to do what had previously been impossible for me: to cultivate cultures directly from the diseased organs of these animals. As the cultures grew, I saw to my surprise that they possessed exactly the appearance and all other characteristics of the enigmatic cultures resembling true tubercle bacilli. It was subsequently discovered that the latter originated from avian tuberculosis, but were considered to be true tubercle bacilli under the assumption that all forms of tuberculosis were identical. I find confirmation of my observation in studies on chicken tuberculosis conducted by Professor Maffucci and recently published. I do not hesitate to consider the bacilli of chicken tuberculosis as a distinct species, but one very closely related to true tubercle bacilli.

This naturally raises the important practical question of whether the bacilli of chicken tuberculosis are also pathogenic to humans. However, this question cannot be answered until this species of bacilli is encountered in humans during continued investigations, or until its absence has been established in a sufficiently long series of all cases. For this, however, one must not, as before, limit oneself to investigations using dye reagents, but must apply the culture method in each individual case.

All recent experiences therefore certainly indicate that it is important to proceed as carefully as possible in separating the bacterial species and to draw the boundaries for the individual species too narrowly rather than too broadly.

In another important fundamental question, the situation has also become significantly clearer and simpler compared to earlier times, namely with regard to proving the causal relationship between pathogenic bacteria and the infectious diseases they cause.

The idea that microorganisms must be the cause of infectious diseases was indeed expressed very early on by some outstanding minds, but the general public was not yet comfortable with it and remained very skeptical of the first discoveries in this field. All the more reason, therefore, to provide irrefutable proof, especially in the early cases, that the microorganisms found in an infectious disease were indeed the cause of that disease. At that time, the objection was still valid that it could be a matter of chance encounter between disease and microorganisms, that the latter played the role not of dangerous parasites, but of harmless parasites which were unable to live in a healthy body, and which found the right conditions for life only in diseased organs. Some acknowledged the pathogenic properties of bacteria, but considered it possible that they initially were harmless organisms which had only transformed into pathogenic bacteria under the influence of the disease process.

If, however, it could be demonstrated: firstly, that the parasite is found in every single case of the disease in question, and indeed under conditions that correspond to the pathological changes and the clinical course of the disease; secondly, that it occurs in no other disease as an accidental and non-pathogenic parasite; and thirdly, that, completely isolated from the body and sufficiently often cultivated in pure cultures, it is capable of reproducing the disease; then it could no longer be an accidental side effect of the disease, but in this case no other relationship between parasite and disease could be conceived than that the parasite is the cause of the disease.

This proof has now been fully established for a number of infectious diseases, including anthrax, tuberculosis, erysipelas, tetanus, and many animal diseases—in fact, for almost all diseases transmissible to animals. Furthermore, it has become apparent that in all cases where the regular and exclusive presence of bacteria has been demonstrated in an infectious disease, the bacteria never proved to be accidental parasites. Rather, they were like bacteria already reliably identified as pathogenic. We are therefore justified in asserting that if only the first two requirements of the proof are met—that is, if the regular and exclusive presence of the parasite has been demonstrated—then the causal link between the parasite and the disease is fully and conclusively proven.

Starting from this premise, we must nevertheless consider a number of diseases to be parasitic, even though it has not yet been possible, or only partially possible, to infect experimental animals and thus provide the third part of the proof. These diseases include abdominal typhus, diphtheria, leprosy, relapsing fever, and Asiatic cholera. I would like to emphasize cholera in particular in this regard, since the view of it as a parasitic disease has been resisted with extraordinary tenacity. Every conceivable effort has been made to deprive the cholera bacteria of their specific character, but they have triumphed over all challenges, and it can now be considered a generally confirmed and firmly established fact that they are the cause of cholera.

Beyond these general, yet fundamentally important questions, bacteriological research has gained a firm foothold in many other areas and clearly established the relationship between pathogenic bacteria and infectious diseases. However, it would go too far to discuss these in detail, and it may suffice to point out that we are only now able to form accurate ideas about how pathogens behave outside the body in water, soil, and air—ideas that differ considerably from earlier ones derived from uncertain hypotheses. Only now can we obtain reliable information about the extent to which pathogens are to be considered true parasites, that is, those that depend exclusively on the human or animal organism, or whether we are dealing with parasites that also find the conditions for their existence outside the body and only occasionally function as pathogens. These are conditions that are of crucial importance for prophylactic measures in some diseases, particularly tuberculosis.

Furthermore, the manner in which pathogens enter the body has been determined with sufficient accuracy for some pathogenic bacteria to arrive at a more accurate understanding of these processes. Our knowledge of the behavior of pathogenic bacteria within the body is also becoming increasingly comprehensive, and some pathological processes that previously seemed enigmatic are thus brought closer to understanding. This includes the frequent occurrence of combinations of several infectious diseases, one of which is then considered the primary and the other the secondary. The latter then gives the actual disease a different, particularly severe character or follows it as a secondary illness. These are conditions that are observed primarily in smallpox, scarlet fever, diphtheria, cholera, as well as in typhoid fever and tuberculosis.

Furthermore, the results obtained from the investigation of bacteria with regard to their metabolic products should be mentioned here, since some of these have peculiar toxic effects and may influence the symptoms of infectious diseases, perhaps even causing the most important ones. Of particular interest in this respect are the recently discovered toxic proteins, the so-called toxalbumins, which can be obtained from cultures of anthrax, diphtheria, and tetanus bacteria.

The question of the nature of immunity, which also belongs here, has been addressed with great zeal and can only be solved with the aid of bacteriology. It has not yet reached a definitive conclusion, but it is becoming increasingly clear that the prevailing opinion for a time—that immunity involved purely cellular processes, a kind of battle between invading parasites and the phagocytes defending the body—is steadily losing ground, and that chemical processes most likely play the main role here as well.

In this relatively short time, bacteriological research has yielded a wealth of material concerning the biological properties of bacteria, and much of this is also important for the medical aspects of bacteriology. For example, the occurrence of persistent states, which in some bacteria such as anthrax and tetanus bacilli occur in the form of spores and are characterized by an unparalleled resistance to high temperatures and the effects of chemical agents compared to other living organisms. Numerous studies on the influence of heat, cold, drying, chemical substances, light, etc., on non-spore-forming pathogenic bacteria have also yielded some results that can be used prophylactically.

Among these factors, light seems to me to be one of the most important. It has been known for some years that direct sunlight can kill bacteria quite quickly. I can confirm this for tubercle bacilli, which, depending on the thickness of the layer in which they are exposed to sunlight, are killed within a few minutes to a few hours. What seems particularly noteworthy to me, however, is that diffuse daylight, albeit correspondingly more slowly, has the same effect. For the cultures of tubercle bacilli died within 5–7 days when placed close to the window.

For the etiology of infectious diseases, it is also important to consider that all bacteria can only multiply in moist conditions, that is, in the presence of water or other suitable liquids, and that they are unable to migrate from moist surfaces into the air on their own. Consequently, pathogenic bacteria can only enter the air in the form of dust and dust particles, and only those that remain viable for extended periods in a dried state can be carried by air currents. But they are never able to multiply in the air itself, as earlier views of disease-causing agents assumed.

In all the areas discussed so far, bacteriological research has fully fulfilled, and in some cases even surpassed, what it seemed to promise at the time of its initial development. In other areas, however, it has not lived up to the expectations. For example, despite ever-improving staining methods and the use of lens systems with increasingly wider aperture angles, it has not been possible to learn more about the internal structure of bacteria than could be ascertained with the original methods. Only recently have new staining methods seemed to be providing further insights into bacterial structure, insofar as it has become possible to distinguish an inner part, probably to be interpreted as the nucleus, from the outer cytoplasm and to visualize the flagella, the motor organs that apparently extend from the cytoplasm, with a clarity that was previously impossible.

In several areas, and precisely where it was least expected, bacteriological research has completely failed us, namely in the investigation of a number of infectious diseases which, due to their pronounced infectivity, seemed to offer particularly easy targets for research. This applies primarily to the entire group of exanthematous infectious diseases, that is, measles, scarlet fever, smallpox, and exanthematous typhus. For not a single one of these has it been possible to find even the slightest clue as to what kind of pathogens might be responsible. Even the vaccine, which is readily available and so easily tested on animals, has stubbornly resisted all efforts to identify its actual agent. The same is true of rabies.

We also know nothing about the pathogens of influenza, whooping cough, trachoma, yellow fever, rinderpest, pneumonia, and many other undeniable infectious diseases. In most of these cases, there has been no lack of skill and perseverance in the application of all available tools, and we can only interpret the negative results of numerous researchers' efforts as indicating that the investigative methods that have proven successful in so many cases are no longer sufficient for these tasks.

I am inclined to believe that the aforementioned diseases are not caused by bacteria at all, but by organized pathogens belonging to entirely different groups of microorganisms. This view is all the more justified given that, as is well known, peculiar parasites belonging to the lowest class of the animal kingdom, the protozoa, have recently been discovered in the blood of some animals, as well as in the blood of people suffering from malaria. However, we have not yet progressed beyond the simple detection of these remarkable and highly important parasites, and we will likely not make further progress until we have succeeded in cultivating these protozoa in a manner similar to bacteria, in artificial culture media or under other, as natural as possible, conditions separate from the body, and in studying their living conditions, their developmental process, and so on. Should this task be accomplished, which there is no reason to doubt, then the study of pathogenic protozoa and related microorganisms will most likely develop into a counterpart to bacteriological research, which will hopefully also provide us with insights into the aforementioned infectious diseases whose etiologies are not yet understood.

Up to this point, I have deliberately left one question unaddressed, even though it is precisely the one most frequently posed to bacteriologists, and not without a certain reproach. I am referring to the question of what purpose all the painstaking work previously devoted to the study of bacteria has served. In fact, such a question should not even be asked, since research pursues its course unwaveringly, unaffected by the consideration of whether its work provides immediate benefit or not. However, I cannot consider this question entirely unjustified in the present case, as very few of those engaged in bacteriological research have completely lost sight of practical objectives.

The practically applicable results of bacteriological research to date are by no means as meager as those questioners believe. I need only remind you of the achievements in the field of disinfection. Previously, there was a complete lack of guidance in this area; people operated entirely in the dark and often wasted large sums on useless disinfection, quite apart from the indirect damage that a failed hygiene practice can cause. Now, however, we have reliable indicators that allow us to test disinfectants for their effectiveness. While there is still much to be done in this field, we can confidently assert that the disinfectants currently in use, insofar as they have passed testing, truly fulfill their purpose.

The practical successes also include the use of bacteriological methods for monitoring water filtration, as these methods are irreplaceable for this specific purpose. Related to this are the insights that bacteriological investigations have provided regarding the filtering properties of the soil and the important conclusions that can be drawn from them for the use of groundwater for water supply and for the proper construction of wells.

In the same way as for water, this method could also be used to control milk, particularly insofar as it is intended for children's nutrition, as well as for examining other foodstuffs and everyday objects suspected of being contaminated. The investigation of air in sewage canals and the correction that has resulted from this to the widely held beliefs about the harmfulness of sewage air, the investigation of air in classrooms, the detection of pathogenic bacteria in foodstuffs, in the soil, etc., are, as cannot be denied, closely connected with practical application. Among the practical successes I would also like to include the diagnosis of isolated cases of Asiatic cholera and the first stages of pulmonary tuberculosis made possible with the help of bacteriology; the former being important for the prophylaxis of cholera, the latter for the early treatment of tuberculosis.

All of these advantages, however, can only be used indirectly in the fight against bacteria. We can hardly compare these indirect ones with directly acting, i.e., therapeutic, agents. The only thing that can be cited in this regard are the successes that Pasteur and others achieved with vaccinations against rabies, anthrax, blackleg, and swine erysipelas. And regarding the rabies vaccine, the only one usable for humans, it could be argued that the cause of rabies is still unknown and probably not even bacterial in nature, and that this vaccination therefore cannot be attributed to bacteriology. Nevertheless, this discovery also grew on bacteriological ground and would probably not have been made without the previous discoveries of vaccinations against pathogenic bacteria.

Although bacteriological research in this area has yielded only such insignificant results despite immense effort, I do not believe that this will always remain the case. On the contrary, I am convinced that bacteriology will once again be of paramount importance for therapy. However, I anticipate less therapeutic success for diseases with short incubation periods and rapid progression. In these diseases, such as cholera, the greatest emphasis will always have to be placed on prophylaxis. I am thinking more of diseases with a less rapid course, because these offer far more opportunities for therapeutic intervention. And there is hardly a disease that, partly for this reason and partly because of its significance far surpassing all other infectious diseases, challenges bacteriological research as much as tuberculosis.

Motivated by such thoughts, I began very soon after the discovery of the tubercle bacilli to search for agents that could be used therapeutically against tuberculosis, and I have continued these experiments tirelessly to this day, although frequently interrupted by professional obligations. I am by no means alone in my conviction that there must be a cure for tuberculosis.

Billroth expressed himself unequivocally in this regard in one of his last writings, and it is well known that numerous researchers are pursuing the same goal. However, it seems to me that the latter generally did not take the correct approach in their investigations by beginning their experiments with humans. I attribute to this the fact that everything that was thought to have been discovered in this way, from sodium benzoate to the hot air method, has proven to be an illusion. Experiments should first be conducted not with humans, but with the parasite itself in its pure cultures. Even if methods are found that are able to halt the development of tubercle bacilli in cultures, one should not immediately choose humans as the experimental subject again, but first try to determine whether the observations made in the test tube also apply to the living animal body. Only when the animal experiment has been successful can one proceed to application in humans.

Following these principles, I have over time tested a very large number of substances to determine their effect on tubercle bacilli cultivated in pure cultures. It has been found that quite a few substances are capable of inhibiting the growth of tubercle bacilli even in very small doses. Of course, a remedy need not do more than that. It is not necessary, as is still often mistakenly assumed, for the bacteria to be killed in the body; rather, it is sufficient to prevent their growth and reproduction in order to render them harmless to the body.

Among the most important substances that have proven effective in very small doses in inhibiting growth are a number of essential oils; among the aromatic compounds, naphthylamine, para-toluidine, and xylidine; some of the so-called tar dyes, namely fuchsin, gentian violet, methylene blue, quinoline yellow, aniline yellow, and auramine; and among the metals, mercury in vapor form, silver, and gold compounds. The cyano-gold compounds, in particular, stood out due to their effect, which far surpassed all other substances; even at a dilution of 1 to 2 million, they inhibit the growth of tubercle bacilli.

However, all these substances remained completely ineffective when tested on animals with tuberculosis. Despite this setback, I was not deterred from searching for growth-inhibiting agents and finally discovered substances capable of halting the growth of tubercle bacilli not only in test tubes but also in the animal body. As anyone who has experimented with it knows all too well, all investigations into tuberculosis are very lengthy; thus, my experiments with these substances, although they have occupied me for almost a year, are not yet complete, and I can therefore only report that guinea pigs, which are known to be extraordinarily susceptible to tuberculosis, no longer react to inoculation with the tuberculous virus when exposed to the effects of such a substance, and that in guinea pigs already severely ill with general tuberculosis, the disease process can be brought to a complete standstill without the body being adversely affected by the agent in any other way.

From these experiments, I would like to draw no further conclusions for the time being, other than that the hitherto rightly doubted possibility of rendering pathogenic bacteria harmless in the living body without harming the latter has been proven.

However, should the hopes attached to these experiments be fulfilled, and should it prove possible to first overcome the microscopic, yet hitherto overwhelming, enemy within the human body itself in the case of a bacterial infectious disease, then, I have no doubt, the same will very soon be achieved with other diseases as well. This opens up a very promising field of research with tasks worthy of being the subject of an international competition of the noblest kind. The sole reason I have, contrary to my usual practice, reported on experiments that are not yet completed, was to encourage further experiments in this direction.

And so let me conclude this lecture with the wish that the forces of nations may measure themselves in this field of work and in war against the smallest but most dangerous enemies of humankind, and that in this struggle, for the benefit of all humanity, one nation may always surpass another in its successes.