The second essential principle is the keeping the fluid on the surface of the tissues at a constant degree of concentration. Up to the present, this principle has been completely ignored. As a rule, antiseptics are applied to wounds by means of absorbent gauze, and the liquid renewed once or twice in the twenty-four hours. It is certain, however, that, under these conditions, the bactericidal power of the substances employed rapidly vanishes. In fact, if a compress soaked in Dakin's solution 05 per cent. be applied to the surface of a wound, the result obtained is almost nil, because the concentration of the solution lessens very quickly, under the influence of dilution by the secretions of the wound, and the combination of hypochlorite of soda with the proteins of pus, of the tissues, and of blood. In a word, the degree of concentration of an antiseptic applied according to the usual surgical method at once becomes so feeble that no result can be hoped for. The only way to maintain at the needful strength, on the surface of a wound, a solution which is constantly being diluted and destroyed, is to keep on renewing it, unceasingly. For this reason we have used instillation, continuous or intermittent. The best method consists in allowing a current of the antiseptic liquid to flow very slowly over the whole surface of the wound. In the case of small wounds, and of those which can be filled with liquid like a cup, this is readily done. The antiseptic, supplied drop by drop, is slowly renewed, in contact with the tissues. When it escapes from the wound, it is absorbed by the dressing, and evaporates without wetting the patient. But when the wound is of large extent, and presents several openings, a considerable quantity of liquid would be needed to keep the whole raw surface continually moist. The amount which would escape from the wound would be too great to be absorbed by the dressing. One has to revert to the old process of continuous irrigation, which is complicated, and distressing to the patient.

On the other hand, experience has shown that if the liquid be applied over the surface of the wound every hour or every two hours, sterilisation is attained. This intermittent instillation is easy to apply. It is the procedure we are at present employing. It is far from being perfect, but it allows of the frequent contact of the surfaces of the wound with the antiseptic at a known degree of concentration. Better arrangements for keeping up both supply and strength of the antiseptic will doubtless be found. For example, if the liquid were to issue from numerous microscopic apertures in tubes inserted in all the cavities of the wound, the quantity needed would be smaller, and yet every part of the wound would be bathed incessantly by the antiseptic at the desired strength.

The degree of concentration of the antiseptic has been determined empirically. It is found that Dakin's solution, containing 0.45 to 0.5 per cent. of hypochlorite of soda, applied under the conditions just described, does no harm to tissues and sterilises wounds.