bacteria are so small and so transparent that they can not be easily, if at all, seen in the midst of animal tissues. This difficulty may be lessened by the use of staining agents, which color the bacteria differently from the animal cells. But it often requires long and tedious trials to find the right dye. The obstacles in the way of the second part of the proposition mentioned are no less appalling. Having found a suspected parasite in the blood or flesh of a patient, we can not accuse the parasite with certainty of being the cause of the disease, unless we can separate it entirely from the fluids and cells of the diseased body without depriving it of its virulence. In some cases it is not easy, if possible, to cultivate the parasite outside of the body; in other instances it can be readily accomplished. Of course, all such attempts require scrupulous care to prevent contamination from other germs that might accidentally be introduced into the same soil. If we can now reproduce the original disease in other animals by infection with these isolated bacteria, the chain of evidence is complete beyond cavil and doubt. But this last step may not be the least difficult, as many diseases of mankind can not be transferred to animals, or only to some few species.
If we apply these rigid requirements, there are not many diseases of man whose bacterial origin is beyond doubt. As the most unequivocal instance, we can mention splenic fever, or anthrax, a disease of domestic animals, which sometimes attacks man, and is then known as malignant pustule. The existence of a parasite in this affection in the form of minute rods and its power of reproducing the disease are among the best-established facts in medicine. It is also known that these rods form seeds, or spores, as they are termed, in their interior, after the death of the patient, which germinate again in proper soil. These spores are the most durable and resisting objects known in animated nature. If kept in the state of spores they possess an absolute immortality; no temperature short of prolonged boiling can destroy them, while they can resist the action of most poisons, even corrosive acids, to a scarcely credible extent.
Another disease, of vastly greater importance to man, has lately been added to the list of scourges of unquestionable bacterial origin. I refer to tuberculosis, or consumption. It is true, this claim is based upon the work of but one investigator—Robert Koch. But whoever reads his original description must admit that no dart of criticism can assail his impenetrable position. Here also a rod-shaped bacillus, extremely minute and delicate, has been found the inevitable companion of the disease. With marvelous patience Koch has succeeded in getting the parasite to grow in pure blood, and freeing it from all associated matter. It must have been a rare emotion that filled the soul of that indefatigable man, when he beheld for the first time, in its isolated state, the fell destroyer of over one eighth of all mankind! None of the animals experimented upon could withstand the concen-