344 PA THOGENIC BA CTERIA. The opinion of the writers is that '(the only trust- worthy difference between many of these varieties and the true cholera spirillum is the specific reaction with serum from animals immune from cholera, or by Pfeiffer's method of intraperitoneal testing in such animals.'' In discussing these spirilla of the Philadelphia waters Bergy1 says: 4 c The most important point with regard to the occur- rence of these organisms in the river water around Phil- adelphia, is the fact that similar organisms have been found in the surface-waters of the European cities in which there had recently been an epidemic of Asiatic cholera, notably at Hamburg and Altona. . . . The fore- most bacteriologists of Europe have been inclined to the opinion that the organisms which they found in the sur- face-waters of the European cities were the remains of the true cholera organism, and that the deviations in the morphologic and biologic characters from those of the cholera organism were brought about by their prolonged existence in water. No such explanation of the occur- rence of the organisms in Philadelphia waters can be given.'' 1 Jour, of the Amer. Med. Assoc., Oct. 23, 1897.