It is chiefly for the diagnosis of tuberculosis in cattle, however, that tuberculin is valuable. Thanks to Nocard, the procedure has to-day become a common practice. The injection of a fairly large dose, 0.3 to 0.4 Gm., according to the size of the animal, causes, in about ten hours or so, if the animal is tuberculous, a strong febrile reaction with an elevation of temperature of 1.5 to 3° C., whereas if the animal is not tuberculous no such reaction takes place. Cases in which tuberculosis is far advanced, and in which the organism is impregnated with tuberculin, do not react after the injection of tuberculin.[109] [109] NOCARD and LECLAINCHE: _Les Maladies Microbiennes des Animaux_. Tuberculin does not confer immunity, and the bacillus retains all its virulence, even in injected tissues; nevertheless, the return to health of animals in which injections have been recently made may be due to the action of large doses of the serum; and on the other hand the tuberculin, in large quantities, may render the location unsuitable for the development of the tubercle bacilli. =Diphtheria Toxin.=--The most characteristic property of the diphtheria bacillus is the production, in culture media, of a special toxic substance which has been named _diphtheritic toxin_; this name, however, has come to be also extended to a liquid in which the bacilli have lived, and which has been sterilized by filtration or by any other suitable process. Roux and Yersin[110] were the first to affirm that diphtheria is an autointoxication caused by a very active poison formed by the microbe in the restricted locality where it develops. In order to obtain this toxin[111] a culture of the bacillus is first made in a mutton bouillon made strongly alkaline with sodium carbonate (10 grams per liter), and with the addition of 2 per cent. of peptone. At the end of about one month, the culture being kept at about 37° C., the liquid is filtered through porcelain. It is indispensable to employ a very virulent bacillus; it is hence frequently advantageous to increase the virulence and toxigenic power of the bacilli it is desired to use. [110] _Annal. de l'Instit. Pasteur_, II, p. 632, and VIII, p. 611. [111] See SPRONK: _Annal. de l'Instit. Pasteur_, IX, p. 785; _Ibid._, X, p. 333; MARTIN, _Ibid._, XII, p. 26; SPRONK, _Ibid._, XII, p. 711. The toxic liquid obtained is exceedingly powerful: 0.1 Cc. kills a rabbit in forty-eight hours. This toxin is very sensitive to the effects of heat. When heated to 65° C. it loses almost all its toxicity; at 70º C. it becomes innocuous; and it only requires to be heated to 100° C. for fifteen minutes in order to lose all immediate activity even in large doses. Nevertheless toxins thus weakened are capable of proving fatal to an animal even after five or six months. Light, oxygen, ozone and all oxidizers destroy the active principle of the diphtheria toxin, which is, moreover, rendered almost inactive by organic acids.