In the red sulphur bacteria the “bacteriopurpurin” which they contain appears to serve as a true respiratory pigment in a manner similar to the chlorophyl in green plants, except that these bacteria oxidize H₂S in the light as a source of energy instead of splitting up CO₂. The red pigment produced by certain bacteria has been shown to have a capacity for combining with O resembling that of hemoglobin, and some investigators have believed that such bacteria do store O in this way for use when the supply is diminished. With these few exceptions the pigments seem to be merely by-products of cell activity which are colored and have no known function. The red sulphur bacteria above mentioned and one or two other kinds retain the pigments formed within the cell. Such bacteria are called _chromophoric_ as distinguished from the _chromoparic_ bacteria whose pigment lies outside the cell. The chemical composition of no bacterial pigment has been determined up to the present. Some are soluble in water, as shown by the discoloration of the substances on which they grow. Others are not soluble in water but are in alcohol, or in some of the fat solvents as ether, chloroform, benzol, etc. These latter are probably closely related to the _lipochromes_ or “fat colors” of higher plants and animals. Attempts have been made to render the production of pigments a still more reliable means of identification of species of bacteria through a careful examination of the spectra of their solutions, but such study has not as yet led to any valuable practical results. The production of pigment depends on the same general factors which determine the growth of the organism but does _not necessarily run parallel_ with these. It is especially influenced by the oxygen supply (only a very few organisms are known which produce pigment anaërobically--_Spirillum rubrum_ is one); by the presence of certain food substances (starch, as in potato, for many bacteria producing yellow and red colors; certain mineral salts, as phosphates and sulphates, for others); by the temperature (many bacteria cease to produce color at all if grown at body temperature, 37°--_Erythrobacillus prodigiosus_--or if grown for a longer time at temperatures a few degrees higher). REDUCING ACTIONS.