W.) No milk is free from bacteria, because the external orifices of the milk-ducts always contain them, but the forms present in the normal fluid are principally those which induce such changes as the souring or " turning " so frequently observed in standing milk (these were examined by Lord Lister as long ago as 1873-1877, though several other species are now known), and those which bring about the various changes and fermentations in butter and cheese made from it.
Promise of more light on these oxidation fermentations is afforded by the recent discovery that not only bacteria and fungi, but even the living cells of higher plants, contain peculiar enzymes which possess the remarkable property of " carrying " oxygen - much as it is carried in the sulphuric acid chamber - and which have therefore been termed oxydases.
Even when the light is not sufficiently intense, or the exposure is too short to kill the spores, the experiments show that attenuation of virulence, That bacterial fermentations are accompanied by the evolution of heat is an old experience; but the discovery that the " spontaneous " combustion of sterilized cotton-waste does not occur simply if moist and freely exposed to oxygen, philous bacteria.
These facts, and the further knowledge that many bacteria never observed as parasites, or as pathogenic forms, produce toxins or poisons as the result of their decompositions and fermentations of organic substances, have led to important results in the applications of bacteriology to medicine.
It is very probable that numerous symbiotic fermentations in the soil are due to this co-operation of oxygen-protecting species with anaerobic ones, e.g.