With regard to ozone, there remains something more to be said, namely, in regard to its application, in a far less diluted form than is possible when it is taken into the lungs, to the destruction of putrefying organic matter and putrefactive and disease-producing bacteria. It is now some five or six years since air containing a high percentage of ozone—produced by the action of the electric discharge—was used for the purification of the water-supply of large towns. It is a fact that river water into which such ozone containing air is pumped becomes pure in the highest degree, in consequence of the destruction by the ozone’s oxydising action, both of the bacterial germs always present in vast numbers in river water and of the organic matter on which the bacteria depend. This application of ozone is in use in several large towns for the purification of drinking-water, for which purpose it has very great advantages. It has also been successfully used by Dr. Allen, the director of the Plymouth Marine Laboratory, for keeping the water of the marine aquarium there in a state of purity and well charged with oxygen gas. A similar use has been made of oxygen containing a considerable percentage of ozone by enterprising surgeons for the cleansing of ulcer and abscess. It is clear that such a gas may present mechanical advantages over any liquid application. Ozone is not, apparently, in favour or fashion with the general body of medical practitioners at the present day, but possibly further examination and determination of its physiological properties may lead to its receiving more attention in medicine. Already the peroxide of hydrogen—which is more or less correctly described as “ozonised water,” and is used (under the name “Auricomous hair-wash”) to change dark hair by its oxydising action to a golden tint—is used by surgeons for washing out purulent wounds and abscess. Those who use the gas itself only go a step further. Some day we may see a more general use of ozone; on the other hand, it remains to be seen by direct and accurate experiment whether its properties are as valuable to man as we may hope they will prove to be. XXVII OXYGEN GAS FOR ATHLETES AND OTHERS