The first characteristic of salvarsan which we should bear in mind especially, in our interest in the social aspects of syphilis, is then the rapidity rather than the thoroughness of its action. It is a social asset to us because it protects us from the infected person, and it is an asset to the patient because it will set him on his feet, able to work and go about his business, in a fraction of the time that mercury can do it. The efficiency of salvarsan in the cure of syphilis in the early stages is due, first, to the large amount of it that can be introduced into the body without killing the patient, and second, to the promptness with which it gets to the source of trouble. In the old days, while we were laboriously getting enough mercury into the patient to help him to stop the invading infection, the germs marched on into his blood and through his body. With salvarsan, the first dose, given into the blood, reaches the germs forthwith and destroys them. There is enough of it and to spare. Twenty-four hours later scarcely a living germ remains. The few stragglers who escape the fate of the main army are picked up by subsequent doses of salvarsan and mercury, and a cure is assured. There is all the difference between stopping a charge with a machine gun and stopping it with a single-shot rifle, in the relative effectiveness of salvarsan and mercury at the beginning of a syphilitic infection. In syphilis affecting the central nervous system, salvarsan, modified in various ways, may be injected into the spinal canal in an effort to reach the trouble more directly. The method, which is known as _intradural therapy_, has had considerable vogue, but a growing experience with it seems to indicate that it has less value than was supposed, and is a last resort more often than anything else. It involves some risk, and is no substitute for efficient treatment by the more familiar methods. If necessary, a patient can have the benefit of both. The _luetin test_ was devised by Noguchi for the presence of syphilis, and is performed by injecting into the skin an emulsion of dead germs. A pustule forms if the test is positive. It is of practical value only in late syphilis, and a negative test is no proof of the absence of the disease. Positive tests are sometimes obtained when syphilis is not present. For these reasons the test is not as valuable as was at first thought. Chapter IX The Cure of Syphilis