The observations of Dean and Opie upon the importance, in precipitin reactions, of the quantitative relationship between an immune serum and its antigen, also hold for antipneumococcus serum and pneumococcus soluble specific substance. Within the limits of pH 5 to 9, the reaction at which the precipitin test is performed in buffer mixture has little if any influence upon this relationship. In order to obtain the maximum precipitate from any given quantity of antipneumoccoccus serum, a definite, or optimum amount of soluble specific substance must be employed. The ratio between immune serum and reacting substance is practically constant; when an excess of reacting substance is present, the precipitate is not only decreased in amount, as determined by inspection and weight, but its appearance is altered.

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