At first sight, there appears to be a great difference between these alkaloidal bases, the ptomaines and leucomaines, and the albuminoid toxins proper. The toxic bases of the first two groups are quite definite chemical products which can be generally obtained quite pure, and frequently in crystalline form. The toxins proper, on the other hand, are highly complex albuminoid substances which greatly resemble the true diastases in all their properties. Nevertheless, between the toxic alkaloids, ptomaines and leucomaines, and the toxic albuminoids, or more properly toxins, there exists no absolutely sharp line of demarcation, but there is a gradual passage from the one to the other by every intermediary grade, as a result of the breaking down of the albuminoid molecule. We shall see, moreover, as we proceed, that these substances are formed under coexistent circumstances, and that they are, hence, found together, whether it be in virus or in snake venom. We will first consider the ptomaines, and then the leucomaines. =Ptomaines.= This name is more specially reserved to designate those alkaloidal substances, generally highly hydrogenized, that are formed outside the organism, from the fermentative action of anaerobic microbes on albuminoid substances. These bases are generally volatile, with an intense and tenacious purulent odor; often, however, they possess a floral odor (aubépine, syringa), and even like that of musk. They combine readily with acids and with the chlorides of the heavy metals, yielding crystallizable salts. The ptomaines afford no specific reaction whereby they may be readily identified; and their identification is effected only after a painstaking analysis. We must here call attention, however, to several of their more common properties, beginning with their basic character, their oxidizability by the air and consequently their well-defined reducing power--a property that led Selmi to propose a mixture of ferric chloride and potassium ferricyanide as a reagent for their detection.[6] They are precipitated by all the general reagents for the vegetable alkaloids. Selmi has given several reactions, such as those afforded by sulphuric, hydrochloric, and nitric acids, which appear, however, to apply much more to the impurities present than to the bases themselves. [6] Sulle ptomaïne od alcaloïdi cadaverici. Bologne, CLXXXVII, p. 11. The physiological action of these bases varies greatly; in some the action is an extremely toxic one, as in the case of neurine and muscarine, which are true ptomaines; there are others, such as cadaverine and putrescine, which are quite innocuous. The physiological action of these bases, like that of the true toxins, is studied by making hypodermic injections of solutions of the bases in healthy animals, such as guinea-pigs, rabbits, and dogs.