As early as 1864 the _Gazette Hebdomadaire de Médecine et de Chirurgie_ published an account of urine made ammoniacal in the bladder, the author of which, Dr. Traube, makes the following observations:—“It was believed that, in consequence of the retention of the liquid, and the resulting distension of the bladder, that organ became irritated and produced a larger quantity of mucus, and that this mucus was the ferment which caused the decomposition of the urea, by virtue of a chemical action peculiar to itself. This belief can no longer be held in presence of M. Pasteur’s discoveries. This observer has demonstrated, in the most conclusive manner, that ammoniacal fermentation, like alcoholic and acetic, is produced by living beings, whose pre-existence in the fermentable liquid is a necessary condition of the process. The preceding fact offers a remarkable proof of M. Pasteur’s theory. In spite of the long duration of the retention, alkaline fermentation was not produced by an excessive secretion of the vesical mucus or of pus: it only began from the moment when the germs of vibrios passed from the outside into the bladder.”[28] Finally, we may conclude with perfect truth that the liquids of our economy, blood and urine, for instance, may afford a harbour to different ferments, even in the inmost parts of the organs, when external causes enable such ferments to find their way into those liquids, and that diseases of greater or less gravity result from this cause. On the other hand, it must be admitted that the bodies of animals in a state of health afford no means of entrance to these external germs. At the same time, direct experiment alone can convince the mind as to the truth of this latter assertion. Let us take some of the substances that are to be found inside living animals in perfect health, and expose them, in the same condition in which life has formed them, to contact with pure air. [Illustration: Fig. 5.] [Illustration: Fig. 6.] For this purpose we must provide ourselves with a glass flask, joined to a copper tap by means of an india-rubber tube, as shown in Fig. 5. The two branches of the tap should be about twelve centimetres long (about ½-in.), the one which is free tapering off like the end of a pipe. In order to destroy all germs which may exist in the flask, we must join the free end of the copper tube to a platinum tube kept at a very high temperature, after having carefully introduced into the flask a small quantity of water, and expelled all the air by converting the water into steam. Then as we allow the flask to cool, the air which re-enters it will necessarily pass through the hot tube (Fig. 6). [Illustration: Fig. 7.]