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That sundry metals possess not merely a distinct inhibitory action upon the growth of moulds, bacteria, and other micro-organisms, but even possess germicidal properties has been known for a long period, and there have been numerous observations demonstrating this action. The earliest observation known to us is that by Raulin in the seventies, upon the remarkable inhibition produced by traces of silver upon the growth of theAspergillus niger . More recent work has in general followed the lines of the experiment employed by Miller, in his observations upon the bactericidal properties of gold and various alloys employed in dentistry; namely, plate cultures have been made of various micro-organisms, and into the inoculated culture media, while still fluid, small pieces or plates of the metals have been dropped, the presence or absence of a clear surrounding zone indicating arrest of growth of inoculated bacteria. As a result of these observations, it has been shown that certain metals possess inhibitory powers. More than this, according to Behring, reinoculation of the clear zone after the removal of the metal may lead to continued negative results. This would indicate that not merely is there inhibition, but an alteration of the gelatin, and suggests strongly the presence there of substances having a distinct germicidal effect. A full study of this nature was made by Bolton. Other observations (Picker, Von der Dois, Clarke and Gage) have been along the lines of either employing metal vessels to hold the inoculated fluids, or placing strips of the metal in glass vessels holding water containing known cultures of bacteria. In these experiments also, metals like copper, zinc, and silver have been noted to possess distinct bactericidal effects. But in the above observations there has been no adequate attempt to determine the means whereby the metals exert their bactericidal action. Leedham-Green, however, made the observa­tion that iron was inert or active according as to whether the conditions of the experiment prevented or permitted free oxidation of the metal. It was through oxidation that inhibitory action was brought about. The general opinion has been that there is a diffusion of the metal into the water or her medium, and that in some direct way the metal arrests the growth or actually brings about the death of the micro-organisms. Certain observations made by Prof. Barnes, and a recent paper by Barnes and Shearer, have opened up the possibility of gaining a more direct knowledge of the mechanism by which these metals act, and the present series of observations owes its origin to Prof. Barnes. Barnes and Shearer working with aluminium showed experimentally that aluminium foil in water containing dissolved oxygen generated peroxide of hydrogen according to the following hypothetical equation :—

The germicidal action of metals and its relation to the production of peroxide of hydrogen