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244 diamido-arseno-benzol, " 606 ") is sold as a yellow powder hermetically sealed in glass phials. It may be given intravenously, intramuscularly, subcutaneously, or by the mouth in alkaline solution. Of these, the intravenous and intramuscular methods are the ones to be recommended; the subcutaneous, and occasionally the intramuscular, injections are apt to be followed by such extensive sloughing as to require surgical interference. The intramuscular injection a suspension of the drug in oil—is generally made into the buttocks: the dose is 7-8 gr. (0.4-0.5 grm.) in the male; 4½ gr. (0.3 grm.) in the female, and ½ gr. (0.03-0.04 grm.) for each year of age in children is taken as the standard. For intravenous injection the glass phial containing 9 gr. (0.6 grm.) is taken, the contents carefully added to 50 c.c. physiological saline solution (made with freshly distilled water), and dissolved thoroughly by adding 18-23 drops of 15-per-cent. soda till a precipitate forms which redissolves on addition of more soda. The solution should be made up to 300 c.c. In a man the whole amount may be injected; in a woman, 200 c.c. or less. The vein should be exposed and the needle, attached to a funnel and a glass- jointed rubber tubing, inserted. Care should be taken that the funnel and tubing are filled with saline beforehand and all air excluded, the flow being controlled by a clip or a stopcock. A special apparatus has been devised and placed on the market by Martindale. It is advisable to run into the vein 100-200 c.c. of normal saline first, and then to add the salvarsan solution warmed to the temperature of the body. Neo-salvarsan is an oxidized product of salvarsan, possessing the advantage of being less toxic and more soluble; consequently it may be given in larger doses, 15 gr. being tolerated by an adult man. It is, therefore, preferable to the older product. It is easily soluble in normal saline, forming a neutral solution. According to Levaditi the most favourable time for the administration of salvarsan is during the "precritical " period of the spirochætal infection, when it causes a precocious crisis, thus enabling the organism to combat the infection by the usual processes—spirochæticidal bodies and phagocytosis. Serum-therapy.— Novy and Knapp have proved that active immunity follows recovery from spirochæte infection, and that this immunity can be increased to a remarkable degree by successive injections of spirochæte-infected blood. They have further shown that passive immunity can be imparted by the injection of recovered or of hyperimmunized blood, and that both active and passive immunity persist for