empirical method, though hard on the patients, resulted in the course of
five thousand years in the discovery of a number of useful remedies. But
the modern medicine man when he knows the cause of the disease is
usually able to devise ways of counteracting it directly. For instance,
he knows, thanks to Pasteur and Metchnikoff, that the cause of wound
infection is the bacterial enemies of man which swarm by the million
into any breach in his protective armor, the skin. Now when a breach is
made in a line of intrenchments the defenders rush troops to the
threatened spot for two purposes, constructive and destructive,
engineers and warriors, the former to build up the rampart with
sandbags, the latter to kill the enemy. So when the human body is
invaded the blood brings to the breach two kinds of defenders. One is
the serum which neutralizes the bacterial poison and by coagulating
forms a new skin or scab over the exposed flesh. The other is the
phagocytes or white corpuscles, the free lances of our corporeal
militia, which attack and kill the invading bacteria. The aim of the
physician then is to aid these defenders as much as possible without
interfering with them. Therefore the antiseptic he is seeking is one
that will assist the serum in protecting and repairing the broken
tissues and will kill the hostile bacteria without killing the friendly
phagocytes. Carbolic acid, the most familiar of the coal-tar
antiseptics, will destroy the bacteria when it is diluted with 250 parts
of water, but unfortunately it puts a stop to the fighting activities of
the phagocytes when it is only half that strength, or one to 500, so it
cannot destroy the infection without hindering the healing.

In this search for substances that would attack a specific disease germ
one of the leading investigators was Prof. Paul Ehrlich, a German
physician of the Hebrew race. He found that the aniline dyes were useful
for staining slides under the microscope, for they would pick out