Pure distilled water has been shown to be relatively transparent to ultraviolet radiation. Sea-water, however, has been found to be considerably less transparent. The presence of soluble mineral salts was judged to be the cause. Measurements made by the author for the fresh waters of certain lakes and streams in the vicinity of Cleveland, indicate that although obviously carrying a far smaller mineral content, they also have a relatively low transmittance. Increased transmittance was found to result from passage of the samples through a Mandler filter cone of diatomaceous earth and also from the processes of coagulation and filtration to which the water of Lake Erie is submitted before being turned into the mains of the Cleveland Water System. It is concluded that the low transmittance of the fresh water cannot be ascribed wholly to dissolved mineral salts but must in part at least be due to scattering caused by the presence of fine particles.
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