The passage from the organic into the inorganic world is best traced by following the line of Pasteur’s researches on ferments. How does the world escape being choked up by the accumulation of dead organic matter throughout innumerable ages? By what are called ferments, inducing processes of fermentation and putrefaction, by which the course of life is reversed, and the organic elements are taken to pieces and restored to the inorganic world. Pasteur proved, in opposition to the theories of Liebig and other older chemists, that this was not done directly by the oxygen of the air, but through the intermediate agency of living microbes, whose spores, floating in the air, took up their abode and multiplied wherever they found an appropriate habitation. Given an air purified from germs, or a temperature low enough to prevent them from germinating, and putrescible substances would keep sweet for ever. The practical realisation of this is seen in the enormous commerce in canned meats and fruits, and in the imports of frozen beef and mutton, causing a fall of rents and much lamentation among British landlords and farmers. But then the question was asked, How are your microscopic organisms disposed of? What are the ferments of your ferments? For even microscopic bacteria and vibrios would, in time, choke up the world by their residue if not got rid of. Pasteur answered that the ferments are destroyed by a new series of organisms—aerobes—living in the air, and these by other aerobes in succession until the ultimate products are oxidised. ‘Thus, in the destruction of what has lived, all is reduced to the simultaneous action of the three great natural phenomena—fermentation, putrefaction, and slow combustion. A living being, animal or vegetable, or the _débris_ of either, having just died, is exposed to the air. The life that has abandoned it is succeeded by life under other forms. In the superficial parts, accessible to the air, the germs of the infinitely little aerobes flourish and multiply. The carbon, hydrogen, and nitrogen of the organic matter are transformed by the oxygen of the air, and under the vital activity of the aerobes, into carbonic acid, the vapour of water, and ammonia. The combustion continues as long as organic matter and air are present together. At the same time the superficial combustion is going on, fermentation and putrefaction are performing their work in the midst of the mass by means of the developed germs of the original microbes, which, note, do not need oxygen to live, but which oxygen causes to perish. Gradually the phenomena of destruction are at last accomplished through the work of latent fermentation and slow combustion.’