This toxin is capable of diffusing through animal membranes, a fact that is in agreement with the toxic effect seen in a subject attacked with diphtheria, and due to the toxin passing through the mucosa. In spite of this property, however, the diphtheritic poison may be taken into the stomach without any pernicious results. Roux and Yersin have shown that, like all the diastases, it may be precipitated from its solutions by the development, within these, of certain precipitates, particularly calcium phosphate. It is precipitated from its solutions by alcohol, as has been observed also in the case of diastatic solutions. All the toxic substance is contained in the albuminous precipitate thus obtained; but the prolonged action of alcohol, or repeated successive precipitations, alter it finally. Diphtheria toxin is likewise precipitated by the reagents for albumoses, particularly sodium sulphate in saturated solution. This procedure has been utilized by Brieger and Fraenkel for preparing the pure toxin, which they obtained in the form of very light, brilliant white, amorphous flocks, affording all the principal reactions of the soluble albumoses (biuret, xanthoproteic, Millon's), and which they characterized as a toxalbumin. On injecting into healthy animals this diphtheria toxin attenuated by sufficiently heating at 70° C, employing at first small doses, and gradually increasing, it is possible to immunize them against diphtheria, as was first demonstrated by Carl Fraenkel. Roux and Martin, who have specially studied this procedure,[112] have shown that a horse may be easily immunized by injecting into the animal the toxin diluted with a third of its volume of Gram's iodine solution, and in successively increasing doses. The initial dose is 0.25 Cc.; then, after two days, 0.5 Cc. of the same toxin is injected, and in like manner the dose is increased up to the eighteenth day, when the pure toxin is injected, at first in small doses, which are gradually increased so that at the end of two or three months injections of 80 Cc. of the pure toxin may be given without danger; the animal is then completely immunized. [112] Contribution à l'Étude de la Diphtérie. _Annal. de l'Instit. Pasteur_, VIII, p. 609; _Ibid._, p. 640. The serum of an animal rendered immune in this manner contains a diphtheria antitoxin which possesses high power. A guinea-pig which has received an injection of 0.01 Cc. of the antitoxin is perfectly capable of withstanding a lethal dose of 0.5 Cc. of the toxin. The antidiphtheria serum thus obtained, and in almost limitless quantities, from an immunized animal, is capable of saturating the therapeutic diphtheritic toxin, and has to-day taken rank in therapeutics as the most efficacious remedy in diphtheria. Injected in varying doses, it confers a temporary but immediate immunity.