382 PATHOGENIC BACTERIA. ating power upon bouillon cultures of its respective organism. Loffler and Abel also prepared a colon serum which exerted a like specific action upon the colon bacillus, but was without effect upon the typhoid bacillus. The serum of immunized animals has been found to destroy the motility of the typhoid bacilli in a few mo- ments, and to cause them to group together. Widal found that the serum of convalescents and of individuals suffering from the acute disease possessed the same power, and suggested that this specific action might prove a val- uable adjunct in diagnosis. Wyatt Johnston1 and McTaggert worked upon the subject, and found that a drop of blood from a typhoid patient, dried upon paper and kept for some time, when moistened and brought in contact with a culture of the bacilli was still potent to bring about a characteristic effect. When such a preparation in the u hanging drop " is watched under the microscope the typhoid bacilli are found to be paralyzed in from one minute to half an hour, and subsequently to collect in masses—agglutina- tions. This reaction may occasionally be brought about by normal blood if insufficiently diluted, but is charac- teristic enough to be very useful in the diagnosis of ob- scure cases. In a later paper Johnston states that to ob- tain a satisfactory reaction an attenuated typhoid bacillus is more useful than a highly virulent one. My own experiments have satisfied me of the value of the test, both for making a diagnosis of the disease and for confirming the species of the bacillus in doubtful cases. It is now the opinion of all observers that cessation of motion and agglutination of the bacteria, resulting from the contact of typhoid bacilli and typhoid serum, are inconclusive for diagnostic purposes unless the reaction follows the combination of a suitable culture and a definite quantity of serum. 1 Montreal Med. Journal, March, 1897.