Some of the coal-tar products, as we see, serve many purposes.  For instance, picric acid appears in three places in this book.  It is a high explosive.  It is a powerful and permanent yellow dye as any one who has touched it knows.  Thirdly it is used as an antiseptic to cover burned skin.  Other coal-tar dyes are used for the same purpose, “malachite green,” “brilliant green,” “crystal violet,” “ethyl violet” and “Victoria blue,” so a patient in a military hospital is decorated like an Easter egg.  During the last five years surgeons have unfortunately had unprecedented opportunities for the study of wounds and fortunately they have been unprecedentedly successful in finding improved methods of treating them.  In former wars a serious wound meant usually death or amputation.  Now nearly ninety per cent. of the wounded are able to continue in the service.  The reason for this improvement is that medicines are now being made to order instead of being gathered “from China to Peru.”  The old herb doctor picked up any strange plant that he could find and tried it on any sick man that would let him.  This 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.