These toxic substances are found, according to Gamaleia, Pfeiffer, and Sanarelli,[139] confined during the life of the microbe within its cellular envelope, and does not diffuse through this. Metchnikoff and Roux are of the contrary opinion,[140] however, and they have prepared a toxin almost insensitive to a temperature of 100° C., and precipitable from its solutions by ammonium sulphate or strong alcohol; the toxin is a toxalbumin. This toxin is quite toxic; one-third of a cubic centimeter suffices to kill 100 Gm. of guinea-pig in 18 hours; with larger doses, death is almost immediate. [139] _Annal. de l'Instit. Pasteur_, IX, p. 129. [140] _Ibid._, X, p. 257. By immunizing guinea-pigs, rabbits, and horses with this cholera toxin, Metchnikoff and Roux obtained a serum which is distinctly antitoxic for rabbits. Nothing absolutely certain has been found as to its action on man.[141] [141] HAFFKINE: _Compt. rend. de l'Acad. des Sciences_, 1892; METCHNIKOFF: _Annal. de l'Instit. Pasteur_, VII, p. 403; and ROUX: _Ibid._, X, p. 253. * * * * * We will not dwell longer here on the toxins of microbial origin. It appears evident, however, from what has been stated above, that the great majority, if not all, of the virulent microbes manifest their virulence by means of toxic secretions. Almost every one of these toxins has been the subject of study. They would otherwise not have interested us here, where our main object was but to dwell upon the general properties. CHAPTER V. THE VENOMS. =General Nature of Venoms.=--The venoms are more or less toxic products secreted by certain reptiles, batrachians, and fish; by a large number of invertebrates; by arachnids, apids, scorpionids, araneids, and a large number of other insects. The venoms are toxic principles very closely allied to the microbial toxins; like the latter, they form two classes, the one alkaloidal, the other proteid, possessing a true diastatic character. They closely resemble the microbial toxins, moreover, by the fact that they are capable of being transformed into vaccins by attenuation of their virulence, by the action of heat or chemical reagents, and of leading to habituation of use and the conference of immunity.[142] Moreover, like the various viruses, the serum of immunized animals is antivenomous, so that if injected into the veins or beneath the skin of non-immunized animals, the serum confers upon them an immunity against venom which lasts for some time. [142] _Annal. de l'Instit. Pasteur_, VIII, p. 281; _Journ. of Physiol._, VIII, p. 203; and _Soc. de Biol._, 1894, p. 111. These venoms, like the microbial toxins, possess but slight toxicity when absorbed via the stomach. Fraser, utilizing a method previously advocated, succeeded, by following this method, in vaccinating against serpent-venom by causing the absorption by animals of constantly increasing doses of venom.