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is harmless because it is absorbed and desti-oyed by the blood-cells, and injurious because this does not happen; or at least that it becomes harmless if the destruction by the blood-cells takes place more rapidly and to a greater extent than the growth and multiplication of the bacillus, the converse being also time." If in the case of frogs, the temperature be raised, "thus favouring the growth of the bacilli and at the same time lowering the vitality of the cells, the bacilli grow and penetrate into the circulating blood."
If, however, the bacilli when introduced into the blood or into the tissues are not destroyed by the colourless blood-cells or otherwise, but are such as are able to live in the blood or tissues of living animals, what is the nature of the change which they produce? It appears that among the products which result from the decomposition of organised matter and are associated with the growth of bacteria, certain chemical poisons appear, which are capable of destroying animal life. We have long been familiar with the fact that the micrococcus or bacillus lacticus is the agent by which the lactic acid fermentation of the sugar contained in milk takes place, and that this lactic acid being neutralised with lime or other substance, will, after a time, through the agency of another micro-organism, probably the bacillus amylobacter, undergo a further change or fermentation, and be converted into butyric acid, with the simultaneous evolution of carbonic acid and hydrogen.
(OH 2C,H^-^ = C3H, . COOH + 2C0, + 2H,.
(COOH butyric acid lactic acid
And there are other fermentations of a similar character. But it is only in recent times that it has been discovered that, as
