“For nearly twenty years,” said Pasteur, “I have pursued, without finding it, a proof of life existing without an anterior and similar life. The consequences of such a discovery would be incalculable; natural science in general, and medicine and philosophy in particular, would receive therefrom an impulse which cannot be foreseen. Therefore, whenever I hear that this discovery has been made, I hasten to verify the assertions of my fortunate rival. It is true that I hasten towards him with some degree of mistrust, so many times have I experienced that, in the difficult art of experimenting, the very cleverest stagger at every step, and that the interpretation of facts is no less perilous.” Dr. Bastian operated on acid urine, boiled and neutralized by a solution of potash heated to a temperature of 120° C. If, after the flask of urine had cooled down, it was heated to a temperature of 50° C. in order to facilitate the development of germs, the liquid in ten hours’ time swarmed with bacteria. “Those facts prove spontaneous generation,” said Dr. Bastian. Pasteur invited him to replace his boiled solution of potash by a fragment of solid potash, after heating it to 110° C., in order to avoid the bacteria germs which might be contained in the aqueous solution. This question of the germs of inferior organisms possibly contained in water was--during the course of that protracted discussion--studied by Pasteur with the assistance of M. Joubert, Professor of Physics at the Collège Rollin. Such germs were to be found even in the distilled water of laboratories; it was sufficient that the water should be poured in a thin stream through the air to become contaminated. Spring water, if slowly filtered through a solid mass of ground, alone contained no germs. There was also the question of the urine and that of the recipient. The urine, collected by Dr. Bastian in a vase and placed into a retort, neither of which had been put through a flame, might contain spores of a bacillus called _bacillus subtilis_, which offer a great resistance to the action of heat. Those spores do not develop in notably acid liquids, but the liquid having been neutralized or rendered slightly alkaline by the potash, the development of germs took place. The thing therefore to be done was to collect the urine in a vase and introduce it into a retort both of which had been put through a flame. After that, no organisms were produced, as was stated in the thesis of M. Chamberland, then a curator at the laboratory, and who took an active part in these experiments.